Kirishimayama is a large group of more than 20 Quaternary volcanoes located N of Kagoshima Bay, Japan (figure 22). For the last 1,000 years, repeated eruptions have taken place at two locations in the complex: the Ohachi crater on the W flank of the Takachihomine stratovolcano, and the Shinmoedake stratovolcano 4 km NW of Ohachi. A single eruption was reported in 1768 from the Iwo-yama (Ebino Kogen) dome located on the NW flank of the Karakunidake stratovolcano, about 5 km NW of Shinmoedake.

Figure 22. Subfeatures of the Kirishimayama volcanic complex showing the three areas with activity discussed in this report: Ohachi, Shinmoedake, and Iwo-yama (Ebino Kogen). View is to the SE. Image taken by the Japan Maritime Self Defense Force on 7 October 2014. Courtesy of JMA (Volcanic activity report on Kirishimayama, October, Heisei 26 [2014]).

The last confirmed eruption at the Ohachi crater was in July 1923. Intermittent steam plumes have been observed since then, including in December 2003 (BGVN 33:09), but the Japan Meteorological Agency (JMA) noted that it had been quiet since 1 December 2007. Shinmoedake has been the site of several short-lived eruptive events since 2008. Most of the events were single-day explosions with ash emissions (BGVN 35:12). A more protracted event from January to September 2011 included numerous explosions with ash plumes, which produced ashfall tens of kilometers away, the growth of a lava dome, ejecta of large blocks, and small pyroclastic flows (BGVN 36:07). Shinmoedake remained quiet until seismicity increased on 23 September 2017, followed by several explosions during October 2017 (BGVN 43:01). Seismic unrest was first reported from the area around Iwo-yama in December 2013, and it has been regularly monitored since that time. This report covers activity from November 2017 through May 2018 and includes new explosive events at Shinmoedake during March-May 2018, an explosive event at Iwo-yama in April 2018, and a brief increase in seismicity at Ohachi in February 2018. Information is provided primarily by the JMA and the Tokyo Volcanic Ash Advisory Center (VAAC), with additional satellite data and news media reports.

Summary of activity during November 2017-May 2018. After steam plumes disappeared at Ohachi in mid-2006, only minor intermittent seismicity was reported through 2017. A sudden increase in earthquakes and tremor activity on 9 February 2018 led JMA to raise the 5-level Alert Level system from 1 (potential for increased activity) to 2 (do not approach the crater) for about a month. Activity diminished after the middle of February and Ohachi remained quiet through May 2018, with only a continuing modest thermal anomaly at the crater.

The latest eruptive episode at Shinmoedake, during 11-17 October 2017, generated an SO₂ plume recorded by NASA satellites, caused ashfall up to 100 km away, and created a new vent about 80 m in diameter on the E side of the crater. Intermittent earthquakes and tremors along with low-level steam plumes characterized activity during November 2017-February 2018. A new eruptive episode began on 1 March 2018 with near-constant explosive activity that lasted until 10 March. A new lava flow at the summit was first observed by JMA on 6 March and began to overflow the NW rim of the crater on 9 March. The Tokyo VAAC reported ash plumes over 6 km altitude on 10 March. An explosion on 5 April produced the largest ash plume of the period; it rose to 10.1 km altitude, was visible drifting E for 24 hours, and resulted in significant ashfall in the region. The lava flow had ceased advancing down the NW flank by the end of April. Another explosion on 14 May 2018 generated an ash plume that rose to 7.3 km altitude and caused ashfall 30 km S that covered the roadways.

An increase in seismicity at Iwo-yama in December 2013, followed by a 7-minute-period of tremor activity in August 2014 was the first recorded at the site since 1768. Thermal anomalies and weak fumarolic activity first appeared in December 2015. Seismicity, including intermittent tremor events and larger amplitude earthquakes, gradually increased during 2016 and 2017. Intermittent fumarolic activity and temperature anomalies began to increase measurably in mid-2017. Jets of sediment-laden hot water emerged from several vents early in 2017. A further increase in fumarolic activity and the temperature of the thermal anomalies in February 2018 led JMA to raise the Alert Level at Iwo-yama. Large amplitude earthquakes and a tremor event accompanied an ash-bearing explosion on 19 April 2018 from a vent on the S side of Iwo-yama. The following day, a vent opened 500 m to the W and produced vigorous steam emissions. On 26 April 2018 an explosion from the new vent sent ash 200 m high. Jets of hot water continued at the Iwo-yama vents through May 2018.

Activity at Ohachi during 2003-2018. JMA reported tremor activity with epicenters near Ohachi in mid-December 2003 (BGVN 33:09) that was followed by fumarolic activity for a few weeks. Intermittent steam plumes were observed during 2004; on 26 March 2004 a tremor event lasted for four hours and a steam plume rose 800 m above the crater (figure 23). A few periods of microtremor were recorded, and intermittent fumarolic activity was observed with webcams until March 2006, after which most activity ceased. JMA lowered the 5-level Alert Level from 2 (Do not approach the crater) to 1 (Potential for increased activity) on 22 May 2006. Fumarolic activity was not observed after July 2006, and no new thermal activity was reported during a field visit in October 2006. Minor seismicity was reported for a few days during July 2007, and small-amplitude, short-duration tremor activity was occasionally recorded during 2008-2014.

Figure 23. Steam plumes were visible on the NW side of the Ohachi crater at Kirishimayama on 31 March 2004. Courtesy of JMA (JMA Kirishimayama annual report, Heisei 16 (2004)).

Although earthquake activity increased slightly in July 2015, the warning level was not raised, and no surface fumarolic activity was observed during field visits in August and September 2015 (figure 24). Seismic activity remained elevated at Ohachi through February 2016 and then gradually decreased during March. Although tremors were recorded in May and December 2016, there was no change in condition at the site and seismicity continued to decrease; no tremors were recorded during 2017.

Figure 24. No fumarolic activity was visible at the Ohachi crater at Kirishimayama on 18 September 2015 during a site visit. View is to the NW. Courtesy of JMA (JMA Kirishimayama annual report, Heisei 27 (2015)).

Earthquake frequency on the SW side of Ohachi increased during 9-16 February 2018, resulting in 199 seismic events, and tremor activity was also recorded on 9 February. This activity led JMA to increase the Alert Level to 2 on 9 February 2018. In spite of the increased seismic activity, the thermal activity remained unchanged from previous months with continued minor thermal anomalies in the same areas as before (figure 25). Seismicity decreased significantly during March 2018 to only 13 volcanic earthquakes, and no microtremor activity was recorded. Inspections carried out on 11 and 14 March showed no surface changes (figure 26) and resulted in JMA lowering the Alert Level back to 1 on 15 March 2018. Ohachi remained quiet through May 2018.

Figure 25. Thermal anomalies at the Ohachi crater of Kirishimayama were unchanged compared with previous months when measured on 9 February 2018 in this view to the NW. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, February, Heisei 30 (2018)).

Figure 26. An overview looking W of the Ohachi crater at Kirishimayama on 2 March 2018 showed no surface activity after the increased seismicity of February. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

Activity at Shinmoedake during August 2008-October 2017. An explosion on 22 August 2008 lasted for about six hours and produced ashfall in Kobayashi City (10 km NE) (BGVN 33:09). Seismicity had increased rapidly a few days prior to the explosion, and then decreased gradually for the remainder of 2008. Other than a brief increase in seismicity in May the following year, only steam plumes rising about 100 m from the crater were reported for 2009.

Seven small ash-bearing explosive events were reported during March-July 2010. Small-amplitude tremor activity on 30 March 2010 was accompanied by a plume that rose 400 m above the crater rim; a small amount of ash fell 400 m to the W of the fumarole within the crater. The webcam on the S rim of the crater captured a grayish plume rising 300 m after a small explosion on 17 April 2010. Another small explosion on 27 May produced a grayish-white plume that rose 100 m above the crater rim and resulted in minor ashfall NE in Kobayashi City. Officials noted a new fumarole on the W flank after this event. Two more explosions on 27 and 28 June 2010 resulted in a small amount of ash deposited 10 km E of Shinmoedake. A small explosion was reported on 5 July. On 10 July, a grayish-white plume, observed in the webcam, rose 100 m above the crater rim after an explosion, and a small low-temperature pyroclastic surge flowed 300 m down the SW slope. GPS instruments recorded minor inflation from December 2009 through September 2010.

A new, more substantial, eruption began at Shinmoedake on 19 January 2011. Activity increased on 26 January with an explosion that released a large volume of ash and pumice and included the growth of a new lava dome (BGVN 35:12, 36:07). Thirteen additional explosions occurred through 1 March 2011. Activity became more intermittent after mid-February, and the last emission was reported on 7 September 2011. Seismicity declined significantly in March 2012 and had returned to background levels by May 2012. With no surface changes and very low seismicity, JMA reduced the Alert Level from 3 to 2 on 22 October 2013, and the only reported activity was steam plumes rising 50-200 m above the crater rim during 2013. The lava dome in the crater remained about 600 m in diameter. Inflation had slowed and stopped after December 2011 but began again around December 2013. Shallow, low-level seismicity during 2014 with epicenters near Shinmoedake was distributed within a few kilometers below the summit; there were no surface changes observed at the crater during several overflights conducted by the Japan Maritime Self Defense Force throughout the year.

Occasional steam plumes rising 400 m above the crater rim were reported during 2015. Volcanic earthquakes were intermittent, with brief increases in activity during March-May and October- December with roughly the same number as the previous year. Inflationary deformation that began around December 2013 ceased in January 2015. A very brief tremor on 1 March 2015 was the first recorded since 1 February 2012. During 2016, occasional steam plumes rose 300 m above the crater. In spite of a seismic swarm on 23 February 2016, and a general increase in seismicity throughout the year, no eruptions occurred, and no surface changes were observed. JMA kept the Alert Level at 2 throughout the year. A small tremor event on 17 September was the only recorded during 2016. Very little activity was reported from January to September 2017; occasional steam plumes were reported rising 400 m above the crater rim. JMA lowered the Alert Level from 2 to 1 on 26 May 2017.

A minor increase in seismicity was observed beginning in July 2017, and was followed by a marked increase on 23 September. After a further increase in frequency and amplitude of earthquakes on 4 October, JMA raised the Alert Level to 2 for Shinmoedake on 5 October 2017. This was followed by an eruption that began on 11 October 2017. A new vent was observed on the E side of the crater during an overflight that same day, and ashfall was reported in numerous communities as far as 90 km NE (BGVN 43:01). A significant SO₂ plume was measured by the OMI instrument on the Aura satellite the following day (figure 27). After raising the Alert Level to 3 on 11 October, JMA expanded the restricted area radius from 2-3 km during 15-31 October.

Figure 27. A significant SO2 plume from the explosion at the Shinmoedake crater of Kirishimayama was measured on 12 October 2017 by NASA's OMI instrument on the Aura satellite. Courtesy of NASA Goddard Space Flight Center.

Explosions on 14 October 2017 resulted in confirmed ashfall in Kagoshima city (50 km SW), Takahara Town (15 km E), Kobayashi city (25 km NE), Saito city (55 km NE), Hyuga city (90 km NE), and Misato town (75 km NE). Ongoing explosions continued until 17 October, after which persistent steam plumes were observed rising as high as 600 m above the crater. In an overflight conducted on 23 October JMA scientists noted the new vent was about 80 m in diameter, and ejecta from the vent had formed a small cone around the vent. (figure 28).

Figure 28. Two vents were visible on the E side of the crater in this view to the WNW taken on 23 October 2017 of Shinmoedake crater at Kirishimayama. The left vent (center front) had formed during the 2011 eruption, and the right vent formed during the 11-17 October 2017 eruption earlier in the month. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, October, Heisei 29 (2017)).

Activity at Shinmoedake during November 2017-March 2018. After the eruption of 11-17 October 2017 seismicity decreased significantly, and no morphological changes were observed for the remainder of the year. Steam plumes rose 300-500 m above the crater during November and December. Short-duration tremors were detected during 25-29 November, along with a slight increase in the number of volcanic earthquakes. A small earthquake swarm recorded during 2-4 December was the only significant seismic activity that month.

Infrequent, large-amplitude earthquakes were recorded during 15-17 January 2018, along with a few short-duration tremor events, the first since 29 November 2017. The earthquakes were located within a 1 km radius of Shinmoedake, around 2-4 km deep. Steam plumes at the crater rose no more than 100 m most days; occasional plumes rising as high as 200 m were noted. An earthquake swarm on 25 February was the first notable event of the month; the steam plumes remained under 100 m above the crater, except for a 500-m-high plume on 21 February. Thermal imaging surveys in late February indicated a modest increase in heat flow from fractures inside the crater and on the W slope compared with previous measurements.

Earthquakes with shallow epicenters below Shinmoedake increased in number early on 1 March 2018 and a new eruptive episode followed a few hours later, leading JMA to increase the restricted zone to 3 km around the crater (figure 29). SO₂ emissions also increased sharply. By the afternoon of 1 March an ash plume rose 1,500 m above the crater, emerging from the vent on the E side and drifting SE. Ashfall was confirmed on 1 March in the area up to 18 km E of the crater. Large blocks of ejecta were observed within the crater on 5 March.

Figure 29. A new eruptive episode at the Shinmoedake crater of Kirishimayama began around 1100 on 1 March 2018 with ash emissions emerging from the new vent on the E side of the crater. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, February, Heisei 30 (2018)).

During an overflight on 6 March 2018, JMA witnessed a new lava flow covering a large area on the E side of the crater floor (figure 30). Eighteen explosive eruptions occurred on 6 March and JMA reported that the ash plume rose 2,800 m above the crater (figure 31). Ashfall was confirmed SW of Shinmoedake in Shibushi city (50 km SSE), Tarumizu City (50 km SSW) and Aira City (30 km SW). NASA 's Aqua satellite captured a false color image of the eruption on 6 March showing the ash plume drifting SE and SW from Shinmoedake (figure 32). About 80 flights in and out of nearby Kagoshima airport were canceled.

Figure 30. Lava emerged from the new vent on the E side of the Shinmoedake crater at Kirishimayama on 6 March 2018 in this view to the W. Plumes of both ash and steam rose from the center and N sides of the crater. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, February, Heisei 30 (2018)).

Figure 31. Ash and steam rose from newly emergent lava inside the summit crater of Shinmoedake at Kirishimayama on 6 March 2018, and disrupted air traffic for most of the day. Courtesy of Kyodo News via AP.

Figure 32. NASA 's Aqua satellite captured a false color image of the eruption from Shinmoedake crater at Kirishimayama on 6 March 2018 with an ash plume drifting SE and SW. Courtesy of NASA Earth Observatory.

Tremor events occurred continuously over 1-8 March; forty-seven explosions were recorded between 6 and 8 March; they decreased in frequency after the middle of the month. The OMI instrument on the NASA Aura satellite recorded a significant SO₂ plume on 7 March 2018 (figure 33). Geospatial data that had shown a gradual inflation of the Kirishimayama complex since July 2017 showed a sharp deflation during 6-7 March 2018, after which inflation resumed.

Figure 33. An SO2 plume with a density of almost ten Dobson Units (DU) was recorded by the OMI instrument on the Aura satellite on 7 March 2018. Courtesy of NASA Goddard Space Flight Institute.

During an overflight on 9 March 2018, a staff member from the Geographical Survey Institute observed the lava flow beginning to overflow the NW side of the crater (figure 34). Explosions resulted in ejecta traveling 800 m from the crater on 9 March and an ash plume rising 3,200 m. An increase in the intensity of activity the following day sent ejecta 1,800 m from the vent and generated an ash plume that rose 4,500 m (figure 35); this led JMA to increase the restricted area around the crater to 4 km between 10 and 15 March.

Figure 34. The new lava flow began to overtop the NW side of Shinmoedake crater (left side of crater with steam) at Kirishimayama on 9 March 2018. Photographed by a staff member from the Geographical Survey Institute during a helicopter overflight by the Kyushu Regional Development Bureau. Courtesy of the Geographical Survey Institute (Correspondence on the eruption of Kirishimayama (Shinmoedake) in Heisei 30 (2018), 29 March 2018).

Figure 35. An increase in explosive activity at the Shinmoedake crater of Kirishimayama on 10 March 2018 sent an ash plume 4,500 m above the crater (left), and incandescent ejecta 1,800 m from the vent (right). Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

A thermal image taken on 11 March showed that the lava was moving very slowly down the NW flank, advancing only a few tens of meters since 9 March (figure 36). JMA confirmed during an overflight on 14 March that the lava flowing down the NW flank was about 200 m wide. Two explosions on 25 March produced plumes that rose 3,200 and 2,100 m, ejecta that traveled 800 m, and a small pyroclastic flow that advanced about 400 m down the W flank (figure 37). Although analysis of satellite data by Japan's Geographical Survey Institute suggested that the eruption of lava into the crater had ceased by 9 March, it continued to flow slowly down the NW flank for several weeks. The diameter of the flow inside the crater was about 700 m, and it had traveled about 85 m down the NW flank by 28 March (figure 38).

Figure 36. A thermal image taken on 11 March 2018 of the new lava flow in the Shinmoedake crater at Kirishimayama showed the slow movement of the flow over the NW rim and down the flank a few tens of meters in two days. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

Figure 37. Two explosions on 25 March 2018 from Shinmoedake crater at Kirishimayama produced plumes that rose 3,200 and 2,100 m, ejecta that traveled 800 m, and a small pyroclastic flow that advanced about 400 m down the W flank (foreground). Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

Figure 38. Lava was still slowly moving down the NW flank of the Shinmoedake crater at Kirishimayama on 26 March 2018, and gray ash covered much of the adjacent flank, possibly from a pyroclastic flow the previous day. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

The Tokyo VAAC issued multiple daily reports from 1-15 March 2018, and a few intermittent reports during the rest of the month. JMA usually reports plume heights in meters above the crater and the Tokyo VAAC reports them as altitudes above sea level; conversions are noted where the height or altitude of a plume is exceptional. They reported an ash plume drifting SE on 1 March at 1.5 km altitude; the plume had risen to 2.4 km by the end of the day. The following day a plume was visible in satellite images at 2.1 km altitude drifting E. Continuous emissions drifting NE above 2.4 km altitude were reported on 3 and 4 March. Several explosions generated plumes that were visible in satellite imagery during 5-7 March drifting S, SW, and W at altitudes between 3.0 and 4.6 km. Plumes from larger explosions during 9 and 10 March rose to altitudes between 4.3 and 6.1 km and drifted SE, finally dissipating after about 24 hours. Explosions on 12 and 13 March drifted NE and E at 3.4-4.9 km altitude, with continuous emissions visible in satellite imagery during those days. Two explosions on 24 March produced plumes that drifted SE at 3.7 and 4.9 km altitude, and were visible in satellite imagery until they dissipated the next day.

A strong MIROVA thermal anomaly signal appeared at the beginning of March and slowly tapered off into April. The signal is consistent with the reports of the eruption of lava from the summit of Shinmoedake and its gradual cooling (figure 39). The MODVOLC thermal alert signals also closely match the reports of the eruption of the lava. The first six alerts were issued on 6 March, four each on 9 and 10 March, three each on 11 and 12 March, and one each on 13, 14, 16, 23, and 30 March, matching a gradual cooling pattern for the lava after the main eruptive event.

Figure 39. A strong MIROVA thermal anomaly signal appeared at Kirishimayama at the beginning of March and slowly tapered off into April 2018. The signal is consistent with the reports of the eruption of lava from the summit of Shinmoedake, and its gradual cooling. A thermal image of the lava flow at Shinmoedake from 28 March 2018 (inset) shows significant cooling from two weeks earlier (see figure 36). Courtesy of MIROVA and JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

Activity at Shinmoedake during April and May 2018. A new explosion on 5 April 2018 generated a large ash plume that rose 5,000 m above the crater; a small pyroclastic flow traveled 400 m down the SE flank, and ejecta was thrown 1,100 m from the vent (figure 40). The Tokyo VAAC reported an explosion, and an ash plume at 6.7 km altitude drifting E visible in satellite imagery early in the day. A few hours later, the plume was visible at 10.1 km altitude, or more than 8,000 m above the crater. Incandescent tephra was ejected hundreds of meters high, and lightning was observed within the large ash plume (figures 41 and 42). The plume was observed continuously in satellite images for almost 24 hours before dissipating; a significant SO₂ plume was also recorded (figure 43).

Figure 40. Ejecta was thrown 1,100 m from the vent in an explosion at the Shinmoedake crater of Kirishimayma on 5 April 2018 (farthest right incandescence). A large ash plume (to the right of the main incandescence) eventually rose to over 8,000 m above the crater. View is to the N from the Inogishi webcam. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Figure 41. An explosion on 5 April 2018 from the Shinmoedake crater at Kirishimayama sent incandescent ejecta several hundred meters above the crater. Courtesy of Kyodo News via Reuters.

Figure 42. Significant lightning was reported in the large ash plume from the 5 April 2018 explosion at the Shinmoedake summit crater at Kirishimayama. Courtesy of Kyodo News via Reuters.

Figure 43. The OMPS instrument on the Suomi NPP satellite recorded an SO2 plume drifting SE after the 5 April 2018 explosion at the Shinmoedake crater of Kirishimayama. Courtesy of NASA Goddard Space Flight Center.

A large amount of ashfall was reported in parts of Kobayashi city and Takaharu (15 km E) (figures 44 and 45) on 5 April 2018. Ashfall reports also indicated that a wide area to the N of Shinmoedake including Hitoyoshi City (30 km N), to the NE including Kadogawa Town (95 km NE), and to the E including Miyazaki City (50 km E) were also affected. Another eruption took place the following day, on 6 April, but weather clouds obscured views of the summit. No eruptions were recorded after 6 April for the remainder of the month.

Figure 44. Ashfall was measured and sampled on 5 April 2018 in Kobayashi City (25 km NE) after an explosion with a large ash plume rose from the Shinmoedake crater at Kirishimayama. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

Figure 45. Ashfall covered major roadways and buildings in Takaharu, 15 km E of Kirishimayama, after an explosion from the Shinmoedake crater on 5 April 2018. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

In multiple flyovers, on 19, 20, and 21 April 2018, authorities observed lava continuing to flow down the NW flank (figure 46), along with residual high temperatures in the central part of the lava flow (figure 47). Additionally, fumarolic areas around the fractures on the W slope persisted. By the end of April, the flow on the NW flank of the crater was 150 m long. Seismicity had declined at the end of March, but increased again during the explosive period in early April. Occasional tremors were recorded during 5-14 April. Intermittent spikes of around 100 small earthquakes were also recorded on 14 and 21 April.

Figure 46. The lava flow down the NW flank of Shinmoedake crater at Kirishimayama was nearly stagnant by 21 April 2018, as seen in this view to the SW taken that same day by the Miyazaki Prefecture Disaster Preparedness Emergency Air Corps. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Figure 47. Residual high heat flow was still visible near the center of the Shinmoedake crater of Kirishimayama on 21 April 2018 but the lava flow had cooled significantly since March (compare with figure 36). Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Another spike in earthquakes with epicenters within 2 km of Shinmoedake occurred on 2 May 2018 with over 700 events recorded. A substantial explosion on 14 May generated an ash plume that rose 4.5 km above the crater according JMA (figure 48). The Tokyo VAAC reported the ash plume initially at 4.9 km altitude drifting SE based on webcam reports; when the plume appeared in satellite data a short time later it was drifting SE at 7.3 km altitude and was continuously visible in satellite imagery for about 24 hours before dissipating. Ashfall was confirmed in numerous areas of the Miyazaki prefecture to the E, and the Kagoshima prefecture to the S and W. Seismicity increased briefly after the explosion. Enough ash fell in Miyakonojo City (30 km S) that it covered the white lines on the roadways (figure 49). A thermal image taken on 15 May showed a new high-heat flow area on the E side of the new lava flow inside the crater that JMA concluded was likely the result of the explosive event of the previous day (figure 50).

Figure 48. A large explosion at the Shinmoedake crater of Kirishimayama on 14 May 2018 sent an ash plume to 4,500 m above the crater as seen in this view to the NE from the Inogishi webcam. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, May, Heisei 30 (2018)).

Figure 49. Enough ash fell in Miyakonojo City (30 km S) after an explosion at Shinmoedake crater of Kirishimayama on 14 May 2018, that it covered the white lines on the roadways. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, May, Heisei 30 (2018)).

Figure 50. The thermal signature at Shinmoedake crater at Kirishimayama on 15 May 2018 revealed a high-heat flow area that JMA concluded likely resulted from the explosion the previous day. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, May, Heisei 30 (2018)).

Activity at Iwo-yama during 2014-2017. An increase in seismicity around Iwo-yama, on the NW flank of the Karakunidake stratovolcano (figure 22) beginning in December 2013 was noted by JMA. The epicenters were distributed from 1-6 km below Iwo-yama. Satellite measurements suggested minor inflation in the area around Karakunidake beginning in December 2013, which lasted until January 2015. A 7-minute-long tremor event occurred near Iwo-yama on 20 August 2014. Although inspections of the area by JMA revealed no thermal or fumarolic activity, they listed the Iwo-yama area with an unofficial Alert Level of "Danger around the crater" on 24 October 2014, equivalent to the official Alert Level 2. They modified the warning during May 2015 to "Normal, keep in mind, it is an active volcano," the same as the official Alert Level 1. During the second half of 2015 there were occasional earthquakes and tremors reported in the area, but no surface or thermal activity was recorded (figure 51) until December. Thermal anomalies appeared in the area for the first time during the first week of December 2015; weak fumarolic activity accompanied by H₂S odors were first reported during 15-17 December 2015 on the SW side of the Iwo-yama crater (figure 52).

Figure 51. No surface activity, and very little thermal activity was present at the Iwo-yama (Ebino Kogen) area of Kirishimayama on 2 November 2015. View is to the N, taken from the N flank of Karakunidake. Courtesy of JMA (JMA Kirishimayama annual report, Heisei 27 (2015)).

Figure 52. Steam plumes and a thermal anomaly at the Iwo-yama area of Kirishimayama first appeared during December 2015 (images from 28 December 2015, view to the S). Courtesy of JMA (JMA Kirishimayama annual report, Heisei 27 (2015)).

Periods of intermittent microtremor activity occurred once in January, four times in February, and twice in December during 2016, with durations ranging from 40 seconds to 5 minutes. A seismic swarm on 28 February led JMA to raise the unofficial Alert Level to "danger around the crater" for the month of March (equivalent to the official Alert Level 2). A new thermal area with fumarolic activity appeared on 24 March 2016 on the SE side of the crater. Intermittent steam plumes were observed throughout 2016; the highest rose 200 m on 11 October. Thermal anomalies also persisted throughout the year on the S and SW areas of the crater. Alert Level 1 (Note that it is an active volcano) was formally assigned to Iwo-yama on 6 December 2016. The Alert Level was raised to 2 on 12 December after a seismic swarm, tremor, and the observation of inflation in the inclination data in the previous days.

Fumarolic activity decreased in January 2017 after a brief increase at the end of December 2016; JMA lowered the Alert Level back to 1 on 13 January and steam plumes generally rose only 30 m high during the month. The thermal anomalies persisted in the same areas of the SW and W portions of the crater as before, though new fumarolic activity appeared in those areas during February 2017. During March field surveys, observers identified hot water emerging from the fumaroles in the SW and S areas of the crater. The inclinometer detected inflation beginning on 25 April 2017, but it leveled off during August. An increase in the number of fumaroles in the area of the thermal anomaly at the SW side of the crater was confirmed by a JMA field inspection in late April. When the University of Tokyo Earthquake Research Institute visited the site on 8 May 2017, they observed sediment-laden water deposits that had been dispersed on the SW side within the crater, and ejecta around the SW edge. This led JMA to increase the Alert Level to 2.

Fumarolic activity increased during mid-to-late July 2017 and steam plumes were reported at 300 m above the crater for a brief period. On 27 July visitors confirmed dead and discolored plants on the NE side of the crater, and audible fumarolic activity. A new thermal anomaly zone with fumaroles was visible on the SW flank outside the crater during a site visit on 31 August. Low levels of seismicity were intermittent throughout 2017, but no tremor events were recorded. A large amplitude earthquake with its epicenter under Iwo-yama occurred on 5 September 2017; no sudden changes were observed at the site a few days later, although thermal images taken on 9 September revealed an increase in temperature from two years prior (figure 53, compared with figure 52). JMA lowered the warning level to 1 at the end of October. During November and December 2017, steam plumes generally rose 100-200 m above the crater.

Figure 53. Steam plumes and a thermal anomaly persisted into September 2017 at the Iwo-yama crater of Kirishimayama. Emissions of the plume on the left were audible during the July visit. Compare with the lower temperatures measured in December 2015, figure 52. Image taken on 9 September 2017 from the Iwomayama South webcam on the S side of the area. Courtesy of JMA (JMA Kirishimayama annual report, Heisei 29 (2017)).

Activity at Iwo-yama during January-May 2018. An analysis of nearby hot-spring waters indicated a significant jump in Cl/SO₄ ratios characteristic of high-temperature volcanic gas beginning in November 2017. The first tremor since 12 December 2016 was recorded on 19 January 2018 and coincided with a brief period of inflation in the vicinity of Iwo-yama. Regional inflation of the area had begun again in July 2017 and continued into 2018. Low-frequency, small-amplitude earthquakes were intermittent during January 2018 and steam plumes rose 100-200 m. Increases in seismicity, fumarolic activity, and the temperatures of the thermal anomalies during mid-February 2018 prompted JMA to raise the Alert Level on 20 February 2018 at Iwo-yama to 2. Steam plume heights increased to 200-300 m after 20 February. Seismicity decreased during March 2018, however observations from the webcam revealed an increase in fumarolic and thermal activity (figure 54).

Figure 54. Fumarolic activity and heatflow increased at the Iwo-yama crater of Kirishimayama during March 2018, with steam plumes at the central vent rising several hundred meters. Images taken on 23 March 2018. View is to the N from the Iwo-yama south webcam. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, March, Heisei 30 (2018)).

The infrared imaging webcam recorded a burst of heat from a vent on the SW side of the crater on 7 April; the amplitude of seismic vibrations also increased. A field visit on 9 April revealed a hot water pool several meters in diameter on the SW side of the crater with sediment-laden water flowing from it and a 10-m-high steam plume. Local inflation recorded at Iwo-yama turned to deflation on 19 April; large-amplitude earthquakes were also reported. A tremor that day was followed by an explosion a few minutes later from a new vent on the S side of Iwo-yama. The plume rose 500 m and ejecta was scattered 200-300 m from the vent to the SE. During an overflight on 19 April JMA noted ash deposits around the vent; ash emission from the vent continued until the following morning (figure 55). The Tokyo VAAC reported a small ash emission on 19 April from Kirishimayama that rose to 1.8 km altitude and drifted E, but it was not visible in satellite imagery. On the evening of 20 April, another new vent with a vigorous steam plume appeared 500 m W of Iwo-yama (figure 56). Sediment-laden water was observed around the vent the following day. Increased seismicity at Iwo-yama lasted for about 20 days; additional tremor activity was reported on 20 and 24 April.

Figure 55. An explosion sent steam and ash 500 m high, and ejecta 200-300 m SE from a new vent on the S side of Iwo-yama on 19 April 2018 at Kirishimayama. Ash emission continued until the following morning. N is to the left, fresh ash deposits cover the area SE of the new vent (upper right). Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Figure 56. A new fumarole with a vigorous steam plume appeared about 500 m W of Iwo-yama during the evening of 20 April 2018. N is to the left. Miyazaki Prefecture Disaster Preparedness Emergency Air Corps Photograph taken from a helicopter on 21 April 2018. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

A brief explosion that lasted about ten minutes occurred from this new vent around 1815 on 26 April 2018 sending a plume of ash about 200 m above the vent (figure 57). A small ash emission from Kirishimayama was reported by the Tokyo VAAC on 26 April that rose to 1.5 km altitude. In a site visit on 30 April, JMA noted active fumaroles and small explosions around both vent areas (figure 58). After the explosion of 19 April, steam plumes rose as high as 700 m from the vent on the S side of the crater, and intermittent spouts a few meters high of sediment-laden water were also observed. Steam plumes rose as high as 500 m from the vent located 500 m to the W.

Figure 57. An explosion from the new vent located 500 m W of Iwo-yama at Kirishimayama on 26 April 2018 sent ash 200 m above the vent. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Figure 58. Vigorous steam plumes rose from both the S side vent at Iwo-yama (background) and the new vent 500 m W (foreground) on 30 April 2018 at the Kirishimayama complex. North is to the left. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Fumarolic activity continued at Iwo-yama during May 2018, but no new explosions nor ash emissions were reported. Shallow seismic events were intermittent, but significantly decreased from April. No tremors were recorded. JMA lowered the Alert Level on 1 May 2018 from 3 to 2. Steam plumes rose 300-500 m from the vents, and thermal anomalies persisted at the crater and the adjacent new vent to the W throughout the month. Jets of sediment-laden hot water rising several meters continued from the vent on the S side of Iwo-yama (figure 59).

Figure 59. Jets of sediment-laden hot water (gray spout at center) rose several meters from the S vent at Iwo-Yama at Kirishimayama during May 2018. Image taken on 15 May 2018. Courtesy of JMA (Volcanic activity commentary on Kirishimayama, April, Heisei 30 (2018)).

Geologic Background. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located, 1700-m-high Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Geographical Survey Institute, Geospatial Information Authority of Japan, Ministry of Land, Infrastructure, Transport and Tourism, No. 1 North Town, Tsukuba city, Ibaraki Prefecture 305-0811 Japan Tel: 029-864-1111 (Representative) Fax: 029-864-1807 (URL: http://www.gsi.go.jp/index.html); Kyodo News (URL: https://www.kyodonews.jp/english/); Associated Press (URL: http://www.ap.org/ ); Reuters (http://www.reuters.com/).

Although tephrochronology has dated activity at Sabancaya back several thousand years, renewed activity that began in 1986 was the first recorded in over 200 years. Intermittent activity since then has produced significant ashfall deposits, seismic unrest, and fumarolic emissions. A renewed period of explosive activity began in early November 2016 and continued through 2017. It was characterized by continuing pulses of ash emissions with plume heights exceeding 10 km altitude, thermal anomalies, and numerous significant SO₂ plumes (BGVN 42:12). Details of the continuing eruptive activity from December 2017 to May 2018 in this report come from the two Peruvian observatories that monitor the volcano: Instituto Geofisico del Peru - Observatoria Vulcanologico del Sur (IGP-OVS), and Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico) (OVI-INGEMMET). Aviation notices come from the Buenos Aires Volcanic Ash Advisory Center (VAAC), and satellite data is reported from several sources.

Sabancaya continued with its explosive eruption that began on 6 November 2016 during December 2017-May 2018. Around 100 aviation notices were issued each month by the Buenos Aires VAAC; tens of daily explosions were reported, fluctuating from highs in the 60s per day in December 2017 to lows in the teens per day during February-April 2018. Ash plumes heights varied at 3-5 km above the summit; altitudes mentioned in the VAAC reports were between 7.3 and 8.5 km altitude most days, although plume heights over 9.1 km were observed a number of times. MIROVA thermal anomalies were recorded every week; MODVOLC thermal alerts occurred every month. A significant number of SO₂ anomalies greater than two Dobson Units were measured by NASA's Goddard Space Flight Center each month (table 2).

Table 2. Eruptive Activity at Sabancaya, December 2017-May 2018. Compiled using data from IGP-OVS, OVI-INGEMMET, Buenos Aires VAAC, HIGP - MODVOLC Thermal Alerts System, and NASA Goddard Space Flight Center.

Activity during December 2017-February 2018. The Buenos Aires VAAC issued 120 aviation alerts during December 2017; webcam and satellite imagery revealed continuous emissions of water vapor and gas, accompanied by sporadic puffs of ash, throughout the month. When visible in satellite imagery, plumes rose to 7.3-8.2 km altitude (figure 46); a few plumes were reported to 9.1 km altitude. According to OVI-INGEMMET, about 1,800 explosions took place in December. During the third week, ashfall was reported in Huambo (28 km WNW). There were two MODVOLC thermal alerts issued, on 3 and 10 December.

Figure 46. Webcam photo of an ash plume at Sabancaya on 16 December 2017. The Buenos Aires VAAC reported a plume that day to 8.2 km altitude. Courtesy of OVI-INGEMMET (RSSAB-51-2017/OVI-INGEMMET & IGP Semana del 11 al 17 de diciembre de 2017).

The number of explosions reported by OVI-INGEMMET dropped slightly to about 1,400 during January 2018. The number of VAAC reports was similar to December; when weather clouds prevented observations of emissions, seismic activity showed intermittent peaks that suggested puffs of ash. Plume descriptions by the Buenos Aires VAAC ranged from intermittent plumes that rose to 7.0-7.6 km altitude early in the month to persistent puffs of ash that rose to 7.9-8.2 km altitude during the last two weeks of January. The prevailing winds were directed SW and NW, and ash plumes often drifted as far as 50 km. NASA Goddard Space Flight Center recorded at least 13 days with SO₂ emissions greater than two Dobson Units (DU) (figure 47). HIGP issued two MODVOLC thermal alerts on 4 and 20 January.

Figure 47. SO2 emissions at Sabancaya were significant throughout the report period. Most months, NASA-GSFC measured 10 or more days where the Dobson Unit (DU) values exceeded two. Dobson Units are a measure of the molecular density of SO2 in the atmosphere. The larger plumes shown here are from 6 January 2018 (top left), 23 February 2018 (top right), 18 March 2018 (bottom left), and 28 April 2018 (bottom right). Courtesy of NASA Goddard Space Flight Center.

OVI-INGEMMET reported ash plume heights during February 2018 at 2,500-4,500 m above the summit. They also noted that deflation was measured during the middle two weeks of the month. The number of daily explosions decreased significantly from the previous few months, with about 500 total explosions recorded in February. The Buenos Aires VAAC noted that the webcam showed continuous emissions of gases with sporadic puffs of ash every day that the summit was visible. Ash plumes were only visible in satellite imagery a few times during the month; during 8-10 February, intermittent emissions were seen moving SE between 7.9 and 8.5 km altitude. During 17-24 February, weak, thin ash plumes drifted several different directions at 7.3-7.9 km altitude (figure 48), and on 28 February a plume was visible drifting NW at 7.6 km altitude. Only a single MODVOLC thermal alert was issued on 18 February.

Figure 48. A strong pulse of ash rose from the summit of Sabancaya early in the morning of 21 February 2018. Courtesy of OVI-INGEMMET (RSSAB-08-2018/OVI-INGEMMET & IGP Semana del 19 al 25 de febrero de 2018).

Activity during March-May 2018. Three MODVOLC thermal alerts were issued in March 2018, two on 14 March and one on 27 March. Sporadic ash explosions continued, but with the lowest number per day of the reporting period. About 450 explosions were recorded during March. In spite of the smaller number of explosions, some of the tallest ash plumes of the period occurred this month. The Buenos Aires VAAC reported a diffuse ash plume drifting NW in satellite imagery on 2 March at 8.8 km altitude. The following week, several ash plumes were spotted in satellite imagery at altitudes of 7.3-8.2 km drifting either SW or NW. On 11 March, cloudy weather prevented visual satellite imagery observations, but multispectral imagery and the webcam revealed intermittent pulses of ash moving SW at 7.6 km altitude. The following day sporadic strong pulses of ash were observed in the webcam, and there was a pilot report of an ash plume at 9.1 km altitude. During the second half of March, ash plumes were noted in satellite imagery most days at altitudes of 6.4-8.2 km; a few pulses produced short-lived ash plumes that rose over 9.1 km, including on 14, 22, 24, and during 27-30 March (figure 49). The highest plume was observed in visible imagery drifting E on 28 March at 10.1 km altitude. A lahar was also reported on 28 March descending the SE flank, towards the Sallalli River; no damage was reported.

Figure 49. An ash plume at Sabancaya on 30 March 2018 can be seen rising from the summit and above the meteorological cloud in this webcam image. The Buenos Aires VAAC reported ash plumes on 30 March that rose to 9.1 and 9.5 km and drifted NE. Courtesy of OVI-INGEMMET (RSSAB-13-2018/OVI-INGEMMET & IGP Semana del 26 de marzo al 01 de abril de 2018).

The number of explosions during April 2018 increased slightly from March to about 540. The maximum plume heights ranged from 2,000 to 3,200 m above the summit according to OVI-INGEMMET. The webcam showed continuous emissions of water vapor and gas and sporadic pulses of ash throughout the month. Ashfall was reported during the first week in Achoma (23 km NE), Chivay (33 km NE), and Huanca. During the second week, the prevailing winds brought ashfall to the W and NW to Huambo (28 km W) and Cabanaconde (22 km NW). The Buenos Aires VAAC reported faint ash plumes visible in satellite imagery nearly every day; plume heights consistently ranged from 7.0 to 8.2 km altitude. Three MODVOLC thermal alerts were issued during the month, one on 13 April and two on 17 April.

Activity increased in many ways during May 2018. The Buenos Aires VAAC issued 132 aviation alerts, the most of any month during the period. The numbers of daily explosions increased compared to April, resulting in a monthly total of around 900. OVI-INGEMMET reported plume heights up to 4,300 m above the summit. MODVOLC thermal alerts were issued on 8, 19, 24, and 26 May. In addition to ash plumes visible in satellite imagery every day at altitudes of 7.3-8.2 km altitude (figure 50), a significant number of ash plumes were reported to altitudes greater than 9.1 km during the month, resulting in more VONA's (Volcanic Observatory Notice to Aviation) issued than in previous months. Sporadic strong puffs of ash were observed in the webcam on the days that satellite imagery measurements of ash plume heights exceeded 9.1 km including on 4, 5, 10, 14, 19, 21, 22, 25, 28, and 31 May. The highest plumes reached 10.4 km altitude on 19 May and 10.1 km altitude on 25 May. Hotspots were also reported on 20, 24, and 27 May. As in previous months, the webcam showed constant emissions of steam and gas, with intermittent pulses of volcanic ash throughout the month.

Figure 50. An IGP webcam at Sabancaya recorded the plume height above the summit at 2,800 m on 27 May 2018. Courtesy of OVI-INGEMMET (RSSAB-22-2018/OVI-INGEMMET & IGP Semana del 28 de mayo al 3 de junio del 2018).

Geologic Background. Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

Information Contacts: Observatorio Volcanologico del INGEMMET, (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Instituto Geofisico del Peru, Observatoria Vulcanologico del Sur (IGP-OVS), Arequipa Regional Office, Urb La Marina B-19, Cayma, Arequipa, Peru (URL: http://ovs.igp.gob.pe/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Nicaragua's Volcan Masaya has an intermittent lava lake that has attracted visitors since the time of the Spanish Conquistadores; tephrochronology has dated eruptions back several thousand years. The unusual basaltic caldera has had historical explosive eruptions in addition to lava flows and actively circulating magma at the lava lake. An explosion in 2012 ejected ash to several hundred meters above the volcano, bombs as large as 60 cm fell around the crater, and ash fell to a thickness of 2 mm in some areas of the park. Brief incandescence and thermal anomalies of uncertain origin in April 2013 were followed by very little activity until the reemergence of the lava lake inside Santiago crater was reported in December 2015. By late March 2016 the lava lake had grown and intensified enough to generate a significant thermal anomaly signature (BGVN 41:08, figure 49) which persisted at a constant power level through April 2017 (BGVN 42:09, figure 53) with an increase in the number of thermal anomalies from November 2016 through April 2017. Although the MIROVA thermal anomaly signal decreased slightly in intensity during May 2017, INETER scientists reported continued strong convection at the lava lake. Similar activity continued throughout July 2017-April 2018 and is covered in this report with information provided by the Instituto Nicareguense de Estudios Territoriales (INETER) and satellite thermal data.

A persistent thermal signature in the MIROVA data during July 2017-April 2018 supported the visual observations of the active lava lake at the summit throughout this period (figure 58). MODVOLC thermal alerts were also issued every month, with the number of alerts ranging from a high of 17 in November 2017 to a low of six in April 2018.

Figure 58. MIROVA thermal data for Masaya for the year ending on 11 May 2018 showed a persistent and steady level of heat flow consistent with the observations of the active lava lake inside Santiago crater. Courtesy of MIROVA.

INETER made regular visits to the summit most months in coordination with specialists from several universities to gather SO₂ data; CO₂, H₂S and gravity measurements were also taken during specific site visits. Thermal measurements around the lava lake inside Santiago crater taken on 24 February 2018 indicated temperatures ranging from 210-389°C. Seismicity remained very low throughout the period. The lava lake was actively convecting each time it was visited, and Pele's hair was abundant around the summit area (figures 59-64).

Figure 59. The lava lake at Masaya was actively convecting on 22 August 2017 when observed by INETER scientists. Courtesy of INETER (Boletín Sismos y Volcanes de Nicaragua. Agosto, 2017).

Figure 60. Pele's hair near the summit of Masaya on 22 August 2017. Scale is likely a few tens of centimeters. Courtesy of INETER (Boletín Sismos y Volcanes de Nicaragua. Agosto, 2017).

Figure 61. The summit crater (Santiago) of Masaya with an active lava lake and fumarole plume (white circle) during 8-16 January 2018. Courtesy of INETER (Boletín Sismos y Volcanes de Nicaragua. Enero, 2018).

Figure 62. Thermal measurements of the lava lake inside Santiago crater at the summit of Masaya on 24 February 2018 indicated temperatures in the 210-389°C range. Courtesy of INETER (Boletín Sismos y Volcanes de Nicaragua. Febrero, 2018).

Figure 63. Nindiri plateau, the broad, flat area inside the summit crater of Masaya, was covered with Pele's hair and basaltic tephra on 6 March 2018. Courtesy of Carsten ten Brink.

Figure 64. The lava lake inside Santiago crater at Masaya was actively convecting on 1 April 2018. Courtesy of Alexander Schimmeck.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Alexander Schimmeck, flickr (URL: https://www.flickr.com/photos/alschim/), photo used under Creative Commons license Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0) (URL: https://creativecommons.org/licenses/by-nc-nd/2.0/); Carsten ten Brink, flickr (URL: https://www.flickr.com/photos/carsten_tb/), photo used under Creative Commons license Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0) (URL: https://creativecommons.org/licenses/by-nc-nd/2.0/).

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes constructed in the Chilean Central Andes. The Nuevo and Arrau craters are adjacent vents on the NW flank of the cone of the large stratovolcano referred to as Volcán Viejo. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater on the E flank of Nuevo. Explosions continued through September 2017 with ash plumes rising several kilometers and Strombolian activity sending ejecta hundreds of meters (BGVN 42:10). This report covers continuing activity from September 2017-May 2018. Information for this report is provided by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), Oficina Nacional de Emergencia-Ministerio del Interior (ONEMI), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

About 150 ash-bearing explosions were recorded during September and October 2017, with plumes rising almost 2 km above the summit. Activity decreased during the second half of October, and no ash plumes were recorded during November. A significant increase in activity in early December led to over 200 explosions with ash emissions. An overflight on 21 December 2017 produced images of a fissure at the bottom of the new crater. The presence of a growing lava dome in the crater was confirmed in early January 2018. Frequent Strombolian explosions produced nighttime incandescence at the summit and down the flanks. Hundreds of ash-bearing explosions occurred during February 2018; the largest plume rose 2.5 km above the summit, and many smaller pulses produced ash and steam that rose 1.5 km. Sporadic incandescence at night and continued explosions of magmatic gases were typical during March 2018. A large explosion on 31 March coincided with the first appearance of a low-level MODIS thermal anomaly in the MIROVA data, and incandescence from explosions at night indicated that the dome continued to grow during April and May. SERNAGEOMIN reported that the top of the lava dome was visible from the E flank for the first time at the end of May 2018.

Activity during September-December 2017. SERNAGEOMIN reported 117 ash-bearing explosions between 16 and 30 September 2017 (figure 17). The one that released the most energy occurred on 19 September. The plumes of steam and ash rose up to 1,800 m above the crater. The Buenos Aires VAAC observed a narrow plume of ash in satellite imagery moving N at 3.9 km altitude and dissipating rapidly on 15 September, and a similar plume moving SE near the summit on 26 September 2017.

Figure 17. Over 100 ash-bearing explosions were reported at Nevados de Chillán during late September 2017, including ones on 15 September (upper left), 20 September (upper right), 23 September (lower left) and 24 September (lower right). Courtesy of SERNAGEOMIN.

During the first two weeks of October 2017 there were 30 ash-bearing explosions recorded. The Buenos Aires VAAC reported small sporadic puffs of ash on 6 October 2017 that were visible in the webcam (figure 18), but not in satellite data, and a similar dense but short-lived plume on 14 October. SERNAGEOMIN reported a series of pulsating low-energy explosions visible in the webcam that drifted SW on 11 and 12 October 2017, and rose no more than 1 km above the summit.. Only two ash-bearing explosions were recorded during the second half of the month. The volcano was much quieter during November; plumes of steam were observed rising only 100 m above the summit throughout the month, with no ash-bearing plumes reported.

Figure 18. Ash plumes at Nevados de Chillán on 6 (left) and 11 (right) October 2017 were two of the 30 plumes recorded during the first half of October. Courtesy of SERNAGEOMIN.

A significant increase in activity in early December 2017 resulted in 245 explosions associated with ash emissions during the first two weeks, some rising as high as 3,000 m above the summit. The Buenos Aires VAAC reported a puff of ash on 1 December that rose to 3.7 km altitude and drifted S, dissipating rapidly. The next day another plume rose slightly higher, to 4.3 km. A dense emission on 4 December rose to 4.9 km and drifted SE before dissipating in a few hours and was not visible in satellite data. On 11 and 14 December, short-lived emissions rose to 4.3 km (figure 19). A yellow cloud of sulfur formed on 11 December within 300 m of the active crater. The webcams also recorded sporadic nighttime incandescence during increased explosions in the early morning of 14 December. Continuous steam emissions with pulses of minor ash were first noted on 16 December; they were visible in satellite imagery the next day at 3.9-4.3 km altitude drifting NE, and by 18 December, consisted only of water vapor.

Figure 19. An increase in explosive activity at Nevados de Chillán in December 2017 resulted in numerous explosions with ash plumes including on 1 December (upper left), 2 December (upper right), 4 December (lower left), and 11 December (lower right). Courtesy of SERNAGEOMIN.

In a special report released on 19 December, OVDAS-SERNAGEOMIN reported an increase in surface activity over the previous three days, recording minor explosions averaging four per hour, and seismic pulses lasting 5-10 minutes; they also noted harmonic tremor with the increase in explosion frequency. A detailed review of images taken during an overflight on 21 December revealed a fissure 30-40 m long trending NW at the bottom of the crater. Incandescence at night was regularly observed after 20 December (figure 20), and ash emissions rose to 3,000 m above the summit during the second half of the month.

Figure 20. Phreatic explosions with steam and minor ash were common at Nevados de Chillán during the last two weeks of December 2017. Ash emissions and pyroclastic flows (top image) were noted during 12-19 December, and numerous incandescent blocks accompanied the explosions on 28 December (bottom image). Courtesy of SERNAGEOMIN.

Activity during January-April 2018. SERNAGEOMIN volcanologists identified a growing lava dome within the new crater during two overflights on 9 and 12 January 2018 (figures 21); it was emerging from the fissure first identified on 21 December. During the first two weeks of January SERNAGEOMIN reported 1,027 pulsating explosions associated primarily with magmatic gases, and very little ash that rose up to 1,000 m above the summit. Confirmed ash emissions were reported on 11 January at 4.3 km altitude faintly visible moving SE in satellite imagery, according to the Buenos Aires VAAC. Nighttime incandescence from the growing dome was periodically observed (figure 22). Based on the overflight data and satellite imagery, they calculated a growth rate for the dome of 1,360 m³ per day. They estimated the size at 37,000 m³ by mid-month.

Figure 21. During an overflight at Nevados de Chillán on 9 January 2018, SERNAGEOMIN scientists observed the growing dome within the crater. Courtesy of SERNAGEOMIN.

Figure 22. Incandescence at night increased from the growing dome at Nevados de Chillán on 13 January 2018. Courtesy of SERNAGEOMIN.

Overflights on 23 and 31 January measured temperatures of 305-480°C over the surface of the dome, with the highest values at the fissure. The growth rate calculated after these overflights was 2,540 m³ per day. The webcam revealed emissions of ash and water vapor during the second half of the month that rose less than 1,000 m above the summit crater.

An explosion on 2 February 2018 sent an ash plume to 2,500 m above the summit (figure 23). Vibrations from the explosion were reported in Las Trancas (10 km) and at the Gran Hotel Termas de Chillan (5 km). SERNAGEOMIN began referring to the active crater as Nicanor, and the dome was named Gil-Cruz. During the first two weeks of February, 840 explosions associated with plumes of magmatic gases were reported. The plumes generally rose as high as 1,500 m above the summit and were often accompanied by incandescence at night. Two overflights on 7 and 14 February recorded temperatures of 500 and 550°C. SERNAGEOMIN determined a dome growth rate of 1,389 m³ per day, and a total volume of 82,500 m³ by mid-month. At least four explosions on 14 February were characterized by two simultaneous plumes, one of white steam and the other darker with a higher ash content according to SERNAGEOMIN. The highest plume that day reached 1,200 m above the summit crater. The Buenos Aires VAAC also reported a small pulse of ash on 14 February that rose to 4.6 km altitude and drifted SE. The dome continued to grow slowly during the rest of February, with a small increase in size noted during a 22 February flyover. Plumes of mostly water vapor with minor ash rose a maximum of 1,080 m above the summit during the hundreds of small explosions that took place.

Figure 23. A substantial explosion on 2 February 2018 at Nevados de Chillán sent an ash plume 2,500 m above the summit and generated vibrations that were felt 10 km from the summit. Courtesy of SERNAGEOMIN.

Sporadic incandescence at night and continued explosions of magmatic gases were typical during March 2018, with plume heights reaching 2,000 m over the Nicanor crater. During an overflight on 11 March, a temperature of 330°C was measured around the Gil-Cruz dome, which had grown to a volume of about 100,000 m³ but still remained below the crater rim. Morphological changes in the still-slowly growing dome included fracture lines and unstable large vertical blocks. A significant decrease in seismic energy was noted beginning on 24 March that ended when two larger explosions occurred on 30 and 31 March (figure 24).

Figure 24. A substantial explosion on 31 March 2018 at Nevados de Chillán generated distinct ash and steam plumes (top) and sent several large blocks down the flanks (bottom). Courtesy of SERNAGEOMIN.

During an overflight on 3 April 2018, scientists observed energetic pulses of steam and minor ash from the central NW-SE trending fissure inside the crater. They noted that lapilli from explosions had been ejected as far as 1 km from the fissure, and that the Gil-Cruz dome had increased in volume since 11 March; they also observed an area of subsidence on the top of the growing dome (figure 25). The dome was expanding toward the E side of the crater, and the top of the dome rose above the crater rim. They measured a maximum temperature of 670°C on the surface of the dome. The decrease in daily seismicity, the larger explosions of the previous days, and the increased size of the dome with greater risk of collapse, pyroclastic flows, and lahars, all led SERNAGEOMIN to raise the alert level at Chillan to Orange on 5 April 2018.

Figure 25. The growing lava dome at Nevados de Chillán, referred to as Gil-Cruz, had an active steam plume at the center when photographed by SERNAGEOMIN during an overflight on 3 April 2018. Courtesy of SERNAGEOMIN.

The Buenos Aires VAAC reported continuous emissions of steam and gas with minor ash along with a small pulse of ash on 2 April 2018. Low-altitude plumes of mostly water vapor were common throughout April 2018. Incandescence from explosions was visible on clear nights during the month, and ejecta rose as high as 250 m above the crater and was scattered around the crater rim. Seismicity remained constant at moderate levels related to the repeated explosions and the growth of the dome. A faint ash plume could be seen in visible satellite imagery on 18 April at 3.7 km altitude drifting E.

Observations reported on 1 May 2018 from the previous flyover indicated that the rate of growth of the dome had slowed to about 690 m³ per day, and the estimated volume had grown to about 150,000 m³. Activity remained at similar levels throughout May 2018. Seismic instruments recorded long-period seismicity and tremor episodes similar to previous months that corresponded with surface explosions and the extrusion of the lava dome. Seismic energy levels were moderate but fluctuated at times. Plumes of predominantly water vapor with minor gas rose a few hundred meters above the summit drifting generally S or SE before dissipating. Incandescence was often observed on clear nights, accompanied by ejection of incandescent blocks that were observed generally 100 to 150 m above the active crater. A larger explosive event took place on 7 May. Occasional plumes with minor ash were reported on 11 May. SERNAGEOMIN reported on 24 May 2018 that the top of the lava dome was visible from the E flank.

Geologic Background. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The largest stratovolcano, dominantly andesitic, Cerro Blanco (Volcán Nevado), is located at the NW end of the group. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The new Volcán Nuevo lava-dome complex formed between 1906 and 1945 between the two volcanoes and grew to exceed Volcán Viejo in elevation. The Volcán Arrau dome complex was constructed SE of Volcán Nuevo between 1973 and 1986 and eventually exceeded its height.

Information Contacts: Servicio Nacional de Geología y Minería, (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637/1671, Santiago, Chile (URL: http://www.onemi.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

The Marapi volcano on Sumatra (not to be confused with the better known Merapi volcano on Java) previously erupted on 4 June 2017, generating dense ash-and-steam plumes that rose as high as 700 m above the crater and caused minor ashfall in a nearby district (BGVN 42:10). The volcano is monitored by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Centre for Volcanology and Geological Hazard Mitigation or CVGHM).

On 27 April 2018, a phreatic explosion produced an ash plume that rose 300 m above the crater rim (figure 8); a thin ash deposit was reported in the Cubadak area (Tanah Datar Regency), about 12 km SE. Another explosion at 0703 on 2 May 2018 (figure 9) produced a voluminous dense gray ash plume that rose 4 km above the crater rim and drifted SE; seismic data recorded by PVMBG indicated that the event lasted just over 8 minutes (485 seconds).

The Alert Level has remained at 2 (on a scale of 1-4), where it has been since August 2011. Residents and visitors have been advised not to enter an area within 3 km of the summit.

Figure 8. Ash plume from a phreatic explosion at Marapi on 27 April 2018. Courtesy of Sutopo Purwo Nugroho (BNPB).

Figure 9. An explosion from Marapi on 2 May 2018 sent an ash plume to a height of 4 km. Courtesy of PVMBG.

Geologic Background. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2000 m above the Bukittinggi plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in historical time.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/).

As has been the case since at least 1971, the active lava lake in the summit crater of Nyiragongo was present during a tourist visit in June 2017, and seismicity was recorded in the crater in October 2017 (BGVN 42:11). Thermal data from satellite-based instruments shows that an open lava lake remained through 23 May 2018. MIROVA analysis of MODIS satellite thermal data (figure 64) shows nearly daily strong thermal anomalies. Similarly, MODVOLC alerts for the same time period shows a consistently frequent number of anomalies (figure 65).

Figure 64. Thermal anomaly MIROVA plot of log radiative power at Nyiragongo for the year ending 23 May 2018. Courtesy of MIROVA.

Figure 65. Map showing MODVOLC alert pixels at Nyiragongo, reflecting MODIS satellite thermal data, for the year ending 23 May 2018. Each pixel shows a thermal alert for a ground area of about 1.5 km2. Nyiragongo (many pixels) is in the center of the map, and Nyamuragira volcano (fewer pixels) is about 13 km to the NNW. Courtesy of HIGP - MODVOLC Thermal Alerts System.

Geologic Background. One of Africa's most notable volcanoes, Nyiragongo contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. In contrast to the low profile of its neighboring shield volcano, Nyamuragira, 3470-m-high Nyiragongo displays the steep slopes of a stratovolcano. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

The most recent eruption at Ebeko, a remote volcano in the Kuril Islands, began in October 2016 (BGVN 42:08) with explosive eruptions accompanied by ashfall. Frequent ash explosions were observed through November 2017 and the eruption remained ongoing at that time (BGVN 43:03). Activity consisting of explosive eruptions, ash plumes, and ashfalls continued during December 2017 through May 2018 (table 6). Eruptions were observed by residents in Severo-Kurilsk (about 7 km E), by volcanologists, and based on satellite imagery. The Kamchatkan Volcanic Eruption Response Team (KVERT) is responsible for monitoring Ebeko, and is the primary source of information. The Aviation Color Code (ACC) remained at Orange throughout this reporting period. This color is the second highest level of the four color scale.

Table 6. Summary of activity at Ebeko volcano from December 2017 to May 2018. Aviation Color Code (ACC) is a 4-color scale. Data courtesy of KVERT

Minor ash explosions were reported throughout the period from December 2017 through May 2018 (figure 17). Minor amounts of ash fell in Severo-Kurilisk at the end of 2017 and into 2018. Ash was reported on 2-4, and 7 December 2017; 15, 16, 18, and 29 January 2018; 8, 17, and18 February; 17 and 21 March; and 6 April. Ash plume altitudes during this reporting period ranged from 1.5 to 3.5 km (table 6); the summit is at 1.1 km.

Figure 17. Explosions from Ebeko sent ash up to an altitude of 1.5 km, or about 400 m above the summit, on 6 February 2018. Courtesy of T. Kotenko (IVS FEB RAS).

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/).

Langila, one of the most active volcanoes of New Britain (figure 7), has been intermittently ejecting ash since April 2016 (BGVN 42:09). Volcanic ash warnings continue to be issued by the Darwin Volcanic Ash Advisory Centre (VAAC). Recent ash plume altitudes (table 5) are in the range of 1.5-2.5 km, but several in mid-April to mid-May 2018 reached up to twice that level. Thermal anomaly data acquired by satellite-based MODIS instruments showed a gradual decrease in power level and occurrence through mid- to late-2017, followed by significantly fewer alerts and anomalies in the first half of 2018. Rabaul Volcano Observatory (RVO) data indicates the activity during 2017 was primarily located in Crater 2 (northern-most crater).

Figure 7. Satellite imagery showing Langila volcano at the far NW end of New Britain island. The brown color of recent lava flows and other volcanic deposits are easily noticeable compared to green vegetated areas. The volcano is about 9 km due south of the community labeled Poini. Imagery in this view is from sources listed on the image; courtesy of Google Earth.

Table 5. Reported data by Darwin Volcanic Ash Advisory Centre (VAAC) on ash plume altitude and drift from Langila based on analyses of satellite imagery and wind model data between 21 June 2017 and 28 May 2018.

MIROVA analysis of thermal anomalies measured by MODIS satellite sensors show a gradual decline of radiative power from early June 2017 to the end of the year (figure 8). Sporadic low-power anomalies occurred in January, April, and May 2018.

Figure 8. Thermal anomalies from MODIS data analyzed by MIROVA, plotted as log radiative power vs time for the year ending 6 June 2018. Courtesy of MIROVA.

Thermal alerts from MODVOLC analyses were concentrated between early June 2017 and late September 2017 (figure 9), with only one pixel being measured in 2018 through early June, that alert being on 5 January 2018.

Figure 9. Map showing thermal anomalies from MODIS data analyzed by MODVOLC for the year ending 6 June 2018. Courtesy of HIGP - MODVOLC Thermal Alerts System.

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower eastern flank of the extinct Talawe volcano. Talawe is the highest volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila volcano was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the north and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit of Langila. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea.

Extensive lava flows, bomb-laden Strombolian explosions, and ash plumes emerging from MacKenney crater have characterized persistent activity at Pacaya since 1961. The latest eruptive episode began with intermittent ash plumes and incandescence in June 2015; the growth of a new pyroclastic cone inside the summit crater was confirmed in mid-December 2015. Strombolian activity from the cone continued during 2016 and it grew sporadically through September 2017 (BGVN 42:12). Lava flows first emerged from fissures around the summit during January-April 2017. Explosions from the cone summit caused growth and destruction of the top of the cone; by the end of September it was about 10 m above the elevation of the crater rim. This report describes the continued growth of the pyroclastic cone and the increasing emergence of lava flows around the summit during October 2017-March 2018. Information was provided primarily by the Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) and satellite thermal data.

Thermal activity was relatively quiet at Pacaya during October and November 2017. The pyroclastic cone inside MacKenney crater continued to grow as material from Strombolian explosions sent ejecta a few tens of meters above the cone and onto its flanks, slowly filling the area within the crater. In late November, small lava flows began to emerge from the crater. Material flowed from the 2010 fissure on the NW side of the crater, and also appeared from new lateral fissures on the W and SW flanks. Multiple small short-lived lava flows traveled a few hundred meters down the flanks with increasing frequency during January through March 2018. Strombolian activity from the summit of the cone occasionally reached over 100 m; by the end of March, the summit of the cone remained about 25 m above the crater rim, and much of the crater was filled with ejecta (figure 84).

Figure 84. A satellite image of Pacaya dated 7 March 2018 shows MacKenney crater at the summit nearly full of ejecta from the growing pyroclastic cone, and at least two small steam plumes on the SW flank from fissures that show dark traces of recent fresh lava. Courtesy of DigitalGlobe and Google Earth.

Activity during October-December 2017. Activity during October 2017 consisted primarily of degassing with small plumes of steam and gas rising 100 m above the summit, and weak Strombolian explosions. . By the end of the month, the cone inside MacKenney crater rose about 10 m above the crater rim. At night, incandescent ejecta could be seen 25-100 m above the summit of the cone. During the last week of October strong winds dispersed the plumes SW and SE, and ashfall was reported 2 km from the crater in El Rodeo.

Steam and gas plumes generally rose no more than 25 m above the summit for the first 20 days of November 2017. Beginning on 21 November, more substantial steam and gas plumes, rising 500 m, were observed in the webcam (figure 85). An increase in tremor activity on 28 November coincided with an increase in explosive activity, a gray ash plume, and the appearance of a small lava flow on the NW flank that extended about 30 m. By the end of the month the cone had reached about 25 m above the rim of MacKenney crater and continued to grow from the accumulation of tephra fragments ranging in size from one millimeter to 50 cm that were ejected 25-100 m above the summit (figure 86). Explosions could be heard up to 1 km from the cone.

Figure 85. A steam plumes rises about 500 m above the summit of Pacaya on 21 November 2017. Courtesy of Michigan Technological University and INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, novembre 2017).

Figure 86. The pyroclastic cone at Pacaya had nearly filled MacKenney Crater by 17 November 2017 (upper photo). An explosion from the summit of the cone with ash and ejecta was captured by the thermal camera on 17 November (lower image). Courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, novembre 2017).

Strombolian explosions rising to 25 m continued in early December. On 10 December 2017, INSIVUMEH noted that there were two lava flows, one flowing on the SE flank with a length of 50-75 m and a second flowing NW towards Cerro Chino for 75-100 m. Strombolian explosions were reported 100 m above the summit of the cone on 15 December, and 25-50 m high on 25 December. The flow on the NW flank was about 100 m long on 26 December.

Activity during January-March 2018. Weak Strombolian activity continued from the cone during January 2018 with ejecta reaching 50 m above the summit. Small lava flows on the NW flank, generally only a few tens of meters long, were visible as incandescence at night (figure 87). While the height of the cone inside MacKenney crater remained about 25 m above the crater rim, material from the continuing low-level explosions had filled a large area of the crater by the end of the month. Blocks up to 1 m in diameter were also dislodged by the tremors and flow activity on the SW flank of MacKenney crater (figure 88). An increase in explosive activity beginning on 20 January resulted in audible explosions heard 2 km from the cone and fine ash deposited on the flanks. A new, larger flow also emerged from the crater early on 20 January and descended about 400 m down the SW flank, with material spalling off the front as it cooled. The following day, low-level Strombolian activity continued, and the flow remained active 200 m down the SW flank. During the last few days of January, the flow rate decreased, and the active flow was only 25 m long (figure 89).

Figure 87. Incandescence from the summit of Pacaya on 8 January 2018, viewed from the SW flank, was caused by Strombolian activity and lava flows. Photo by Instagram user @cesiasocoy, courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, enero 2018).

Figure 88. Low-level Strombolian activity sent ejecta up to 50 m above the summit of the cone at MacKenney crater on Pacaya during most of January 2018. The top of the cone inside the crater was just visible above the crater rim at the summit in this view from the NW flank taken on 17 January 2018. White blocks at the base of the SW slope on the right of the image are recently dislodged, 1-m-diameter blocks. Courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, enero 2018).

Figure 89. Lava flows on the SW flank of Pacaya on 25 January 2018, photographed by Instagram user @Carolinegod1. Courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, enero 2018).

Low-level steam and occasional gas plumes rising up to 300 m above the summit were typical during February 2018 (figure 90). In addition, intermittent lava flows continued to travel tens to a few hundred meters down the S, SW, and W flanks. A 25-m-long flow was observed on the SW flank on 2 February. On 8 February, a 150-m-long flow was noted, also on the SW flank. INSIVUMEH reported a 300-m-long lava flow from the NW area of crater on 9 February in the region of the 2010 fissure; it traveled NW towards Cerro Chino crater. A flow 75-100 m long was observed on the SW flank on 10 February; the next day 150-m-long flows were visible on both the SW and W flanks. Flows on both flanks were 100 m long on 12 February. A 30-m-long flow appeared on the SW flank on 13 February. The flow on the NW flank that began on 9 February was 20-m-wide and only 50 m long during the afternoon of 14 February. A flow was also visible on 14 February extending 250 m down the SW flank (figure 91).

Figure 90. A vigorous steam plume rose 300 m from the summit of the pyroclastic cone inside MacKenney crater at Pacaya during February 2018. The top of the cone was just visible above the crater rim in this view from the NW. Courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, febrero 2018).

Figure 91. Steaming lava flowed on the SW flank of Pacaya on 14 February 2018 and dislodged loose debris on the slope. Courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, febrero 2018).

Multiple lava flows on the SW flank ranged from 50-200 m long during 15-20 February. A flow on the W flank grew from 25 to 150 m during 17-23 February (figure 92). A flow reached 500 m down the SW flank on 25 February and after flow-front collapses was still 300 m long by the end of the month. A new surge of lava on 27 February emerged from the fissure on the NW flank of MacKenney crater and traveled 150 m towards Cerro Chino crater. Explosive activity remained constant; weak explosions, generally 3-5 times per hour, scattered ejecta on the flanks of the cone and created incandescence at night that often reached 15-35 m above the cone. The explosions also generated weak avalanches that sent material up to 1 m in diameter down the S and SW flanks to an area frequented by park visitors. Explosions were sometimes heard up to 3 km from the crater. Strombolian explosions increased in height towards the end of the month; they were reported at 150 m above the summit on 26 February.

Figure 92. A lava flow emerged from a fissure on the W flank of Pacaya on 18 February 2018 and was imaged with a thermal camera as it traveled 150 m down the flank. Courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, febrero 2018).

Strombolian activity and persistent lava flows throughout March 2018 resulted in continued growth of the pyroclastic cone within the MacKenney crater. Low-level steam and gas plumes generally rose a few tens of meters above the summit; occasional plumes rose as high as 500 m. Small lateral fissures near the crater rim produced repeated small lava flows that generally flowed less than 250 m SW and W. Weak explosions averaging 3-5 per hour sent ejecta 10-50 m above the pyroclastic cone.

During the first week of March, flows on the SW flank were active as far as 500 m down the flank. A flow on 4 March was 65 m long, and one on 5 March ranged from 50-200 m long (figure 93). During the second week, two flows were active to 300 m down the W flank, and two others on the SW flank were 150-200 m long. A flow was reported 200 m down the E flank on 16 March. Multiple lava flows were visible during 17-23 March; one traveled 250 m down the SW flank, two others went 150 m down the W flank and remained active through the end of the month.

Figure 93. Landsat satellite imagery from 5 March 2018 shows a thermal anomaly from a SW-directed lava flow at Pacaya, about 250 m long. Landsat 8 image processed by Rudiger Escobar (Michigan Technological University), courtesy of INSIVUMEH (Departamento de Investigación y servicios Geofísicos, Informe mensual de la actividad volcánica, marzo 2018).

Geologic Background. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Google Earth (URL: https://www.google.com/earth/).

Historical records of eruptions at Ecuador's Volcán El Reventador date back to 1541 and include numerous lava flows and explosive events (figure 74). The largest historical eruption took place in November 2002 and generated a 17-km-high eruption cloud, pyroclastic flows that traveled 8 km, and several lava flows. Eruptive activity has been continuous since 2008. Persistent ash emissions and incandescent block avalanches characterized activity during January-September 2017 with large pyroclastic and lava flows during June and August (BGVN 43:01). Explosions that produced ash plumes and incandescent blocks continued throughout October 2017-March 2018. Information is provided primarily by the Instituto Geofisico-Escuela Politecnicia Nacional (IG-EPN) of Ecuador, the Washington Volcanic Ash Advisory Center (VAAC), and also from satellite-based MODIS infrared data.

Figure 74. Aerial image of Reventador's inner caldera with its pyroclastic cone emitting a plume of steam and ash. View is looking to the W. Photograph taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Persistent, near-daily ash emissions were typical for Reventador during October 2017-March 2018 (figure 75). In general, the plumes drifted W and NW over sparsely populated nearby areas, but occasional wind-direction changes resulted in ashfall in larger communities within 30 km to the S and SW. The plume heights were commonly 1,000 m above the summit, with the highest plume rising 5 km (to 8.5 km altitude) in October. Most days that the summit and slopes were not obscured by weather clouds, there were observations of incandescent blocks falling at least 300-500 m down the flanks. Larger explosions generated Strombolian fountains and incandescent blocks that traveled 800 m down the flanks every week, even farther on occasion (figure 76). Heavy rains caused one lahar in late November; no damage was reported. Small pyroclastic flows on the flanks were observed once or twice each month (figure 77). The lava flows of June and August 2017 continued to cool on the flanks (figure 78). Thermal activity was somewhat higher during October 2017 with 19 MODVOLC thermal alerts issued, but it remained constant throughout the rest of the period with 8-11 alerts each month. The MIROVA radiative power data showed a similar pattern of moderate, ongoing activity during this time.

Figure 75. A dense ash plume rose from Reventador during the first week of December 2017, viewed from a shelter 3.5 km E of the summit. Photograph taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Figure 76. Incandescent blocks rolled hundreds of meters down the flanks of Reventador during the first week of December 2017. Photograph taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Figure 77. A small pyroclastic flow traveled down the flank of Reventador during the first week of December 2017 while an ash plume rose about 1 km above the summit. Photograph taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Figure 78. The lava flows from June and August 2017 were still cooling on the flanks of Reventador during the first week of December 2017. Photograph taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Activity during October-December 2017. The Washington VAAC issued ash advisories every day but one during October 2017. IGEPN reported near-daily emissions of ash, with plumes rising over 1,000 m many days of the month and rising to 500-800 m the other days. Plume drift directions were generally W or NW. Incandescence at the summit crater was visible on most nights, and incandescent block avalanches were seen rolling 400-800 m down the flanks during 15 nights of the month. Explosive activity intensified for several days near the end of the month (figure 79). A possible pyroclastic flow traveled down the SE flank in the morning of 24 October.

Figure 79. Strombolian explosions from two vents at the summit and incandescence on the SE flank of Reventador were captured on 24 October 2017 by B. Bernard. Photo taken from the Hosteria Reventador, 7.2 km SE from the summit. Courtesy of IGEPN (Informe Especial del Volcán El Reventador – 2017 – No. 5, Actualización de la actividad del volcán, 30 de octubre del 2017).

IGEPN scientists in the field during 23-25 October 2017 noted a high level of explosive activity with loud noises and vibrations felt in the vicinity of Hostería Reventador, about 7.2 km SE of the volcano. Thermal imaging data gathered during their trip indicated that the maximum temperatures of the explosions were over 500°C and that the lava flows of June and August were much cooler with temperatures ranging between 100 and 150°C (figure 80). A dense ash plume rose to more than 2,800 m above the summit and drifted N and E on 25 October (figure 81).

Figure 80. Thermal imaging at Reventador on 24 October 2017 indicated that the temperatures of explosions were over 500°C, and that the lava flows of June and August 2017 were much cooler, around 100-150°C. Image taken by M. Almeida from the Hosteria Reventador, 7.2 km SE from the summit. Courtesy of IGEPN (Informe Especial del Volcán El Reventador – 2017 – No. 5, Actualización de la actividad del volcán, 30 de octubre del 2017).

Figure 81. A dense ash plume rose at least 2,800 m above the summit of Reventador on 25 October 2017 and drifted NE. Photo by B Bernard, courtesy of IGEPN (Informe Especial del Volcán El Reventador – 2017 – No. 5, Actualización de la actividad del volcán, 30 de octubre del 2017).

The Washington VAAC reported numerous ash emissions during 24-26 October 2017 at altitudes of 5.8-6.1 km, drifting N and NE from the summit about 35 km. IGEPN reported continuing ash emissions beginning on 27 October that lasted for several days, including observations that day of a plume that rose to 4,900 m above the summit. The Washington VAAC reported the plume at 8.5 km altitude, the highest for the period of this report. During the last few days of October, the wind changed to the S, resulting in reports of moderate ashfall in Napo province in the towns of San Luis, San Carlos (9 km S), El Salado (14 km S), El Chaco (33 km SW), and Gonzalo Díaz de Pineda (El Bombón, 26 km SW).

Persistent ash emissions continued during November 2017 along with observations of incandescence at the summit crater. Plumes of steam, gas, and ash were reported over 600 m above the summit throughout the month; the Washington VAAC issued multiple daily aviation alerts with plume heights averaging 4.3-4.9 km altitude, usually drifting W. Higher altitude plumes over 6.0 km were reported a few times with the highest during 11-12 November rising to 6.7 km. There were reports in the morning of 1 November of ashfall in Borja and San Louis (SE) and on 4 November of minor ashfall in the communities adjacent to the volcano. Incandescent blocks were seen rolling 300 m down the flanks during 7-9 November. Heavy rains on 20 November resulted in a lahar on the E flank. During 22-27 November blocks rolled as far as 800 m down all the flanks, with many on the S and SE flanks (figure 82).

Figure 82. Steam, gas, and ash plumes, and incandescent blocks rolling down the flanks were common occurrences at Reventador throughout November 2017. Top: An ash and steam plume on 22 November 2017 rose over 600 m and drifted W. Bottom: Incandescent blocks rolled as far as 800 m down the flanks on 23 November 2017, mostly on the S and SE flanks. Courtesy of IGEPN webcam (Informe Diario del Estado del Volcán Reventador Nos. 2017-326, and 2017-327).

Although multiple daily aviation alerts continued throughout December 2017 from the Washington VAAC, weather clouds often prevented satellite observations of the ash plumes. When visible, plume heights were generally 4-5 km altitude, drifting W or NW; the highest plume on 17 December reached 5.5 km and drifted WNW before dissipating. IGEPN noted incandescence at the summit on almost all nights it was visible; incandescent blocks traveled as far as 900 m down all the flanks on 11 December, and 400-800 m most nights. They also reported ash plumes rising more than 600 m above the summit 24 days of the month. A video of typical activity at Reventador was taken by Martin Rietze during 1-7 December 2017, along with numerous excellent photographs (figures 83-85).

Figure 83. Strombolian explosions at Reventador during the first week of December 2017 sent showers of incandescent debris skyward (upper photo) before sending larger incandescent blocks hundreds of meters down the flanks of the cone (lower photo) while a dense ash plume rose from the summit area. Photographs taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Figure 84. Lightning strikes were photographed within the dense ash plumes that rose from the summit of Reventador during the first week of December 2017. Photograph copyright by Martin Rietze and used with permission.

Figure 85. Explosions at Reventador during the first week of December 2017 produced dense ash plumes and small pyroclastic flows down multiple flanks. The flanks were bare at the beginning of the ash emission event (upper photo) but small pyroclastic flows can be seen descending the flanks a few moments later (lower photo). Photograph taken during 1-7 December 2017, copyright by Martin Rietze and used with permission.

Activity during January-March 2018. Except for several cloudy days during the third week of January 2018 when no observations were possible, IGEPN reported recurring emissions of steam, gas, and ash rising over 600 m and drifting mostly W or NW throughout the month. During 11-12 January ash plumes briefly drifted E. Incandescent block avalanches were reported most often traveling 200-400 m down the S and SE flanks; a few times they travelled up to 800 m down all the flanks. Other than the cloudy days of 20-24 January, the Washington VAAC issued multiple daily aviation alerts. When ash plumes were visible in satellite imagery, plume altitudes ranged from 4.3-4.9 km, except for 30-31 January when they were reported at 5.2 km (figure 86).

Figure 86. Ash plumes and incandescent blocks were reported numerous times at Reventador during January 2018. Top left: Steam, gas, and ash were reported rising over 600 m and drifting NW and E on 2 January. Top right: on 3 January, the drift directions of the steam, gas, and ash plumes were W and NE. Lower left: Incandescent blocks were reported travelling 800 m down all the flanks on 12 January. Lower right: Ash plumes on 30 January were reported by the Washington VAAC at 5.2 km altitude, the highest during the month; they drifted N and W. Courtesy of IGEPN (Informe Diario del Estado del Volcán Reventador, Nos. 2018-2, 2018-3, 2018-12, and 2018-30).

Multiple daily aviation alerts continued from the Washington VAAC throughout February 2018. While daily plume heights mostly averaged 4.3-4.9 km altitude, there were a greater number of higher-altitude ash plumes than during recent months. A plume on 5 February was reported at 6.1 km drifting 15 km N and a plume the following day drifted 30 km ENE at 7.6 km altitude. A plume on 16 February rose to 5.5 km and drifted 55 km NW; one on 22 February rose to 7.0 km and drifted almost 100 km SE before dissipating. The next day, a plume rose to 5.5 km and drifted 35 km SE. Two separate plumes were observed in satellite imagery drifting NE on 25 February, the first rose to 5.5 km and drifted 110 km and the second rose to 6.4 km and drifted 45 km before dissipating. IGEPN reported a plume of steam, gas, and ash on 27 February that rose over 1,000 m above the summit and drifted NE. Although IGEPN only reported incandescent avalanche blocks on 11 days in February, more likely occurred because the view was obscured by weather clouds for 14 days of the month.

Minor ashfall in the vicinity of the volcano was reported by IGEPN on 1 March 2018. They also noted steam and gas plumes containing moderate amounts of ash that rose over 2,000 m above the summit and drifted SW and S that day (figure 87). IGEPN reported ash emissions around 600 m or higher above the summit on 21 days during the month. In addition to persistent incandescent activity at the summit, avalanche blocks rolled down all the flanks 800 m numerous times. A pyroclastic flow was reported 400 m down the S flank on 13 March (figure 88). Incandescent blocks rolled 1,000 m down all the flanks on 22 March. Other than a plume reported in satellite imagery at 5.8 km moving E on 26 March, all of the ash plumes reported by the Washington VAAC during March ranged from 3.9-4.9 km altitude and generally drifted NW or W.

Figure 87. A plume of steam, gas, and ash rose from Reventador on 1 March 2018; IGEPN reported it as rising over 2,000 m above the summit and drifting SW and S. A small pyroclastic flow also appeared to descend the flank. Courtesy of IGEPN (Informe Diario del Estado del Volcán Reventador, No. 2018-60).

Figure 88. Continued explosions at Reventador during March 2018 produced abundant incandescent avalanche blocks, ash plumes, and a few pyroclastic flows. Top: Abundant incandescent blocks rolled 800 m down all the flanks on 6 March 2018. Bottom: An ash plume rose over 600 m above the summit and drifted NW while a pyroclastic flow traveled 400 m down the S flank on 13 March 2018. Courtesy of IGEPN (Informe Diario del Estado del Volcán Reventador, Nos. 2018-65 and 2018-72).

Geologic Background. Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic Volcán El Reventador stratovolcano rises to 3562 m above the jungles of the western Amazon basin. A 4-km-wide caldera widely breached to the east was formed by edifice collapse and is partially filled by a young, unvegetated stratovolcano that rises about 1300 m above the caldera floor to a height comparable to the caldera rim. It has been the source of numerous lava flows as well as explosive eruptions that were visible from Quito in historical time. Frequent lahars in this region of heavy rainfall have constructed a debris plain on the eastern floor of the caldera. The largest historical eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IGEPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Martin Rietze (URL: http://mrietze.com/web16/Ecuador17.htm).

The dacitic Santiaguito lava-dome complex on the W flank of Guatemala's Santa María volcano has been growing since 1922. The youngest of the four vents in the complex, Caliente, has been actively erupting with ash explosions, pyroclastic, and lava flows for more than 40 years. During January-October 2017 (BGVN 42:12), daily weak ash emissions sent ash plumes to altitudes around 3.3 km, and ashfall was frequent in villages and farms within 12 km S and SW. The lava dome that appeared within the summit crater of Caliente in October 2016 continued to grow, increasing the frequency of block avalanches moving down the flanks. Several lahars affected the major drainages during May-October. Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) and the Washington VAAC (Volcanic Ash Advisory Center) provided regular updates on the continuing activity during the time period of this report from November 2017-April 2018.

Activity at Santa Maria was very consistent with little variation during November 2017-April 2018. Plumes of steam with minor magmatic gases rose continuously from the Caliente crater 300-500 m above the summit, drifting SW or SE before dissipating. In addition, tens of daily explosions with varying amounts of ash rose to altitudes of around 3.5-4.0 km and usually traveled short distances of 20-30 km before dissipating. The longest-lived plume, on 22 March 2018 drifted 100 km before dispersing. Almost all of the plumes drifted SW or SE; minor ashfall occurred in the mountains and was reported at the fincas up to 15 km away in those directions several times each month. Continued growth of the lava dome at Caliente resulted in block avalanches descending its flanks every day. The MIROVA plot of thermal energy during this time shows a consistent level of heat flow with minor variations. The spike of strongest heat flow in late March 2018 corresponds with the largest ash plume reported (figure 70) for the period.

Figure 70. MIROVA plot of thermal energy from Santa Maria for the year ending 12 July 2018 shows persistent low levels of heat flow. The spike at the end of March 2018 corresponds to the largest reported ash plume for the period. Courtesy of MIROVA.

Activity during November 2017-January 2018. During November 2017, persistent steam plumes rose 100-500 m above the summit crater at Caliente, and generally drifted SE. Tens of weak explosions daily created ash plumes that rose to about 3.2 km altitude and drifted usually SE. These resulted in ashfall reported near Finca San José on 9, 26, and 28 November, and in the mountains around Finca la Florida on 27 November. The Washington VAAC reported an ash emission seen in satellite imagery on 18 November drifting S about 15 km from the summit at 4.3 km altitude. Block avalanches were reported daily, they usually extended down the SE flank, occasionally making it to the base of the dome.

Characteristic steam plumes rising 100-500 m continued daily throughout December 2017. Numerous daily weak to moderate explosions generated ash plumes that rose to around 3.0-3.3 km altitude and drifted most often to the SW. Weak to moderate, and occasionally strong block avalanches descended the SE flank of the dome most days.

The Caliente dome maintained constant degassing with mostly steam plumes and occasional magmatic gas throughout January 2018 (figure 71). The plumes rose 50-300 m above the dome; most plumes came from the crater, but a few rose from fissures on the flanks. Explosions with ash plumes rose to 2.8-3.5 km altitude and generally drifted W or SW (figure 72). The seismic station registered 15-21 weak to moderate explosions per day. Ash generally drifted to the E or SE and caused ashfall in the regions around the fincas of San José, Patzulin, La Quina and others. Finca San José reported ashfall in the vicinity on 6, 7, and 9 January, and El Faro noted nearby ashfall on 9 January. A small plume with minor ash content was noted in satellite imagery by the Washington VAAC on 10 January drifting E at 4.3 km altitude. Ash emissions extended about 35 km SW before dissipating on 12 January, also at 4.3 km. Weak and moderate-size block avalanches occurred daily with blocks generally descending the SE or E flank of the dome.

Figure 71. A typical plume of steam and magmatic gas rose from the Caliente vent at Santa Maria on 8 January 2018. Courtesy of INSIVUMEH (Informe mensual de actividad volcánica enero 2018, Volcán Santiaguito, 1402-03).

Figure 72. An explosion at the Caliente dome of Santa Maria on 7 January 2018 sent ash a few hundred meters above the summit crater. Courtesy of INSIVUMEH (Informe mensual de actividad volcánica enero 2018, Volcán Santiaguito, 1402-03).

Activity during February-April 2018. Plumes of steam and gas continued rising daily to a few hundred meters above Caliente during February 2018. Weak and moderate explosions with steam and ash rose to 2.6-3.2 km altitude and drifted variably S, SE, W, or SW during the month (figure 73). Explosions averaged about 14 per day. Ashfall was reported in the fincas to the E and SE during the first week, including at Finca San José on 5 February, and la Florida on 10 February; they occurred in the mountainous areas W and SW during the rest of the month. Ashfall was also reported around the perimeter of the volcano several times during the last week of the month. The Washington VAAC reported an ash plume at 4.6 km altitude on 12 February drifting rapidly W, and a thin veil of gas and minor ash on 28 February extending about 15 km SW from the summit at 4.3 km altitude. Observations of repeated block avalanches down the SE flank throughout the month concurred with thermal measurements on 28 February that showed the hottest areas of the dome at the summit and on the SE flank (figure 74).

Figure 73. An explosion of steam and ash rose from Caliente at Santa Maria on 18 February 2018. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 17 al 23 de febrero de 2018).

Figure 74. Material inside the summit crater of Caliente at Santa Maria measured about 140°C on 28 February 2018, and showed the warmest region on the SE flank where most of the block avalanches occurred. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo:, Volcán Santiaguito (1402-03), Semana del 24 de febrero al 02 de marzo de 2018).

Block avalanches down the SE and S flanks of Caliente from the growing summit dome persisted at weak to moderate levels throughout March 2018 (figure 75). Ten to twenty daily ash-bearing explosions usually rose to about 3.2 km altitude and drifted SW or SE causing ashfall around the perimeter. Ashfall was reported in the mountains around Finca San José on 4-6, 9, 20, and 23 March, and in the Palajunoj area on 11 March. Steam plumes rising from the summit of Caliente to 2.9-3.1 km altitude drifting SE or SW were a daily feature of activity (figure 76). The Washington VAAC reported an ash plume on 5 March that rose to 4.6 km altitude and drifted SW before dissipating within 15 km of the summit. On 21 March, an emission was observed in satellite imagery that extended about 35 km SW from the summit at 4.6 km altitude. Another ash plume the following day also rose to 4.6 km altitude and extended almost 100 km SW before dissipating. That same day, 22 March, MODVOLC issued four thermal alerts for Santiaguito, and the MIROVA system showed a spike in thermal activity as well (figure 70).

Figure 75. Block avalanches descended the SE flank of Caliente at Santa Maria on 6 March 2018. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo:, Volcán Santiaguito (1402-03), Semana del 03 al 09 de marzo de 2018).

Figure 76. A typical steam plume rose from Caliente summit during the last week of March 2018. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 17 al 23 de marzo de 2018).

Multiple daily explosions with ash rose up to 3.2 km altitude during April 2018. The plumes drifted SW or SE, spreading fine-grained ash over the nearby hills. Finca San José reported ashfall on 2 April and the Palajunoj area reported ashfall on 10, 13, 15, and 17 April. Abundant degassing of mostly steam plumes at the Caliente crater continued throughout the month, as did the constant descent of block avalanches down the SE flank.

Geologic Background. Symmetrical, forest-covered Santa María volcano is one of the most prominent of a chain of large stratovolcanoes that rises dramatically above the Pacific coastal plain of Guatemala. The 3772-m-high stratovolcano has a sharp-topped, conical profile that is cut on the SW flank by a large, 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank and was formed during a catastrophic eruption in 1902. The renowned plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four westward-younging vents, the most recent of which is Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).

An eruption at Sheveluch has been ongoing since 1999, and volcanic activity was previously described through January 2018 (BGVN 43:02). Ongoing activity has consisted of pyroclastic flows, explosions, and lava dome growth with a viscous lava flow in the N. According to the Kamchatka Volcanic Eruption Response Team (KVERT), moderate emissions of gas-and-steam have continued, and ash explosions up to 10-15 km in altitude could occur at any time. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) throughout this reporting period from February through April 2018.

KVERT reported continuous moderate gas-and-steam plumes from Sheveluch during February-April 2018 (figure 49). Satellite imagery interpreted by KVERT showed a thermal anomaly over the volcano on 13 days during February, 21 days in March, and 15 days in April. Cloud cover obscured satellite imagery the remainder of the time during this reporting period.

Figure 49. Photo of the lava dome at Sheveluch on 25 March 2018. Courtesy of Yu. Demyanchuk (IVS FEB RAS, KVERT).

The MIROVA system detected intermittent low-power thermal anomalies from February through April 2018. Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were not detected during this period.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far East Division, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

No explosions . . . were recorded in May or June, but plume emission continued. The highest plume in May rose 1800 m on the 19th. Ash accumulation in May was 112 g/m² at the observatory. No earthquake swarms were recorded by the nearest seismometer, 2.3 km NW of the crater.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: JMA.

A cooperative study of Arenal by the OVSICORI and the SI, assisted by Earthwatch and Smithsonian Research Expedition volunteers, has completed eight periods of continuous day/night monitoring, generally of 10-14 days each, in the past 2 years. Most of the observations were made from the Arenal Volcanological and Biological Observatory, 2.7 km S of the summit, on the Marigold Genis macademia plantation. The following is excerpted from a report by W. Melson. A more detailed version will be published in Boletín de Vulcanología [see Further Reference, below].

"Over the past 2 years, Arenal's eruptions include the infrequent emission of lava flows and a variety of frequent pyroclastic eruptions that can be classified into three overlapping and sometimes sequential event types. Sounds were recorded at the Observatory using a standard cassette recorder and directional microphone. We also used a sound-level meter and a strip recorder to obtain time-sound intensity records of eruptions. Only rarely is Arenal's summit visible. Thus, we normally must classify eruptions by their sound characteristics (figure 20).

Figure 20. Arenal eruption sound sequence at 0407 on 3 April 1989, beginning with an explosion (type 1) and grading through type 2 to type 3. Sound level intensities were made from a tape recording and are thus only relative. The predominance of low-frequency components in the sequence is shown by comparing the unfiltered sequence (solid line) with the low-frequency filtered (<100 Hz) sequence (dotted line).

1. Explosions are intense, brief, energy releases, usually <2 seconds long, accompanied by ballistic ejection of blocks and bombs and rarely by small pyroclastic flows (figure 21). They may initiate a sequence of all three types of pyroclastic events. Typical eruption columns are 300-1,000 m high and tephra volumes are small. Blocks rarely reach ~15 m across and may impact 1.5 km from the active crater, although typical ranges are <800 m. The sound is intense, and sound levels of 125 decibels (dB) have been recorded at the Observatory, but values between 96 and 115 dB are more common. Most but not all explosions are associated with a small, brief, high-frequency seismic signal. Less commonly, they are associated with a long-duration signal that may grade into harmonic tremor. Sound intensity and tephra volume are not clearly related; explosions with relatively low sound levels can involve large volumes of tephra. Intense explosions commonly follow long repose periods and are thus particularly dangerous. One such explosive event killed a tourist who ascended to near the crater on 6 July 1988. On the rare clear nights at Arenal, explosions are the most visually spectacular types of eruptions.

Figure 21. Explosion plume and impacting blocks, photographed from the Observatory, 2.7 km S of the new summit crater, on 15 April 1989 about 25 seconds after the onset of the explosion at 0759. Plume drift and tephra fall are to the W, the normal direction of trade winds at Arenal.

2. Long-duration eruptions of blocks, bombs, and tephra may occur singly, or, more typically, in a series of varying loudness and ejecta volume. They are commonly associated with an intense, sometimes harmonic seismic event lasting >30 seconds. Pyroclastic flows associated with this type of eruption are of the fallback type, where tephra of low ejection velocity falls on the crater rim and coalesces into coherent flows. We have observed three pyroclastic flows over the past 2 years that descended >1 km from the crater; all were associated with a low-intensity sound signal but with a strong and sustained seismic signal. The sonic signatures are rich in low-frequency components (<100 Hz) that are not as loud and are of longer duration than for the explosions. Tephra volume is not clearly related to sound level; barely audible events of this type have produced large volumes of tephra. Although they might be termed lava fountaining, they should not be confused with the activity generated by magmas of much lower viscosity that are involved in lava fountaining during basaltic eruptions. Arenal's magmas are now highly viscous basaltic andesites containing >50 volume % crystals with compositionally evolved matrix glasses that are mainly dacitic.

3. A sequence of rhythmic gas emissions with or without ejection of small amounts of tephra. Frequencies are typically about 0.75-1.5 Hz between separate events. Within a given eruptive sequence, these are the highest-frequency, lowest sound-intensity, events.

"The frequency of eruptions varies widely with time. We have found no clear-cut cyclicity nor other obvious patterns in these data (figure 22). Over the past 2 years, the seven periods of close monitoring suggest a decline in the frequency of pyroclastic eruptions followed by a slight increase. During the April 1989 observations, the number of explosions (type 1) particularly increased. Small lava flows moving down the S slope also led to an increase in recorded rockslides. However, during the past 2 years, most of the lava flows have moved down the N slopes, many of them in the headwaters of the Río Tabacón; rockslides associated with their advance are not audible from the Observatory.

Figure 22. Average number of eruptions at Arenal per hour during each 10-14-day period of observation, 28 April 1987-April 1989.

"The number of pyroclastic events decreased dramatically after about 15 April 1989, reaching the lowest level in the past 2 years. Only one explosion occurred during 5 days of close monitoring 30 June-4 July. During that time, intense lava fountaining in the summit crater was visible at night and at least two wide but thin flows were active on the N flank, in the headwaters of the Río Tabacón, with advancing flow fronts ~1,200 m below the new crater, now at ~1,600 m elevation. This is the second period of low pyroclastic activity associated with a high level of lava flow production. The first was recorded 9-19 February 1988, when an active lava flow had reached ~1,200 m elevation in the headwaters of the Río Tabacón. The rate of magma emission is far greater during times of strong lava emission than during even high levels of pyroclastic activity. It is likely that during periods of high rates of lava production, the conduit is essentially open, preventing formation of a plug by cooling and degassing, and hence the buildup of vapor pressure and attendant pyroclastic events.

"We find no consistent relationship between tremor levels and eruption frequency or type during our last two periods of close monitoring, except for Type 2 eruptions, which were most common at high tremor levels during both periods. Notably, explosions (Type 1 eruptions) occurred at minimal levels during tremor-free periods during the February expedition, but at maximum frequency during periods of maximum tremor in February."

The ICE reported that seismicity declined to a moderate level in June, with a mean of only three recorded volcanic earthquakes/day. However, there was an increase in the number of harmonic tremor episodes, related to lava degassing.

Further Reference. Melson, W., 1989, Las erupciones del Volcán Arenal, 1 al 13 de Abril de 1989: Boletín de Vulcanología (Univ Nacional, Costa Rica), no. 20, p. 15-22 (in Spanish).

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: W. Melson, SI; V. Barboza, J. Barquero, E. Fernández, and R. Saenz, OVSICORI; R. Barquero and G. Alvarado, ICE.

After a small ash ejection 5 April, tephra emission continued at a relatively high rate in May and June. On 8 May at 1000, a vent (1 m in diameter) on the Naka-dake crater floor ejected ash to ~10 m. At 1132, an M 3.3 shock (3 on the JMA Intensity Scale) occurred beneath the crater and was felt at AWS. Five (felt) aftershocks were recorded on 8 May (at 1120, 1147, 1216, 1417, and 2039), and 1 (not felt) was recorded the next day (at 0057) by a seismograph 0.8 km W of the crater. A 1-km area around the crater was closed to tourists by the Aso Disaster Authority. During a field survey at 1910, no ash ejection was observed.

On 16 May, ash rose ~100 m above the crater rim at 0810, and ~200 m at 1030. About 20% of the crater floor was covered by a rainwater pool, from which mud and water were continuously ejected to 3 m. During a field survey on 20 May at 1150, a strong rumbling noise was audible, but no ash ejection was seen.

Ash rose ~200 m above the crater rim on 22 May from 0740 to 0800, and 20 m above the crater floor at 0820. Activity declined, stopping by 1000. Two days later at 1000, ash was ejected to 200 m above the crater rim, and 5 g/m² of ash was deposited at AWS. Ash had not fallen there since 28 June 1985. Red glow at the vent and in cracks on the crater floor was observed at night through May. During the night of 27 May, red glow emanated from 40-50% of the crater floor. On 28 May, ash rose about 50 m from the N portion of the vent.

In June, a vent on the NW floor of Crater 1 emitted an ash-laden steam plume a few hundred meters above the crater rim. During a 6 June field survey, the vent had enlarged and was emitting a 300-m ash plume. Flames from burning volcanic gases were occasionally observed rising 3-4 m above the crater floor during night visits. Ash accumulation at AWS was 9 g/m² on the 7th, and 2 g/m² on the 8th. The Crater 1 vent was buried by ash during rainfall 8-9 June. A new vent (named "891") about 18 m in diameter opened in the center of the crater floor on 10 June, and was the largest new vent since "853" formed 6 May 1985. The highest plumes of the month reached 1,000 m above the crater rim on 7 and 20 June.

Isolated volcanic tremor remained high (200-400 events/day) in May and June (figure 11) with a total of 5,760 events in May and 6,752 in June (compared to 5,821 in April). The amplitude of continuous tremor was generally unchanged in May but increased slightly in June.

Figure 11. Daily number of isolated tremor episodes at Aso, January-June 1989. Courtesy of JMA.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

"Bagana is currently the most active volcano in Papua New Guinea. Unfortunately, civil disturbance on Bougainville Island Island prevents proper monitoring. The observer reported fluctuating night glows from the summit and from the new (blocky) lava flow on the S flank. Incandescent rockfalls were frequent on all flanks, accompanied by rumbling sounds. Explosions and incandescent projections over the crater were reported 10 and 12-15 June. The thick, white to brown plume . . . produced occasional light ashfalls downwind."

Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

Information Contacts: P. de Saint-Ours and B. Talai, RVO.

From the beginning of 1985 until the end of 1988, activity . . . was characterized by a generally deflationary trend, but uplift then resumed and a maximum uplift of 7.2 cm was measured in June.

The surveillance network operated by OV consists of eight seismic stations, five tide gauges (four in the Gulf of Pozzuoli, one in Naples for comparison), and four electronic tiltmeters (figure 16). Periodic levelling measurements are made on an extended line and distance measurements are performed twice a year. Radon content and water temperature are monitored in four water wells. Periodic measurements of S/C ratio and water vapor content of fumarolic emissions are made at Solfatara Crater.

Figure 16. Levelling network and tide gauges at Campi Flegrei.

Deformation. Vertical motion recorded by the tide gauge in Pozzuoli harbor showed steady deflation until mid-1987 (figure 15). The record then became more oscillatory and some uplift episodes were observed in the general deflationary trend. Figure 15 also shows vertical motion recorded on the levelling line at benchmark 25 (the site of maximum vertical deformation). A steady trend with an average rate of -12.7 mm/month was observed until mid-1987. From then until the beginning of 1989 a decrease in the subsidence rate was observed, and a net uplift of 7.2 cm was measured January-June 1989. Since the end of 1988, four tilt stations have been installed at Campi Flegrei. They are 2-component horizontal pendulum systems with resolutions of 6.9 and 14.5x10-9 rad for the radial and tangential components, respectively. One tiltmeter is in Baia Castle (on the W side of the bay), the other three along an abandoned tunnel roughly 2.5-3.5 km N of Pozzuoli pier. Different trends were observed December 1987-June 1989, showing complex local movement still not fully understood. Two periods of inclination toward the SE were observed, 10 December 1987-12 February 1988 and 22 March-7 April 1988, compatible with deflation of the area of maximum vertical deformation. In other periods the trends were less compatible with this feature, as if the source of deformation had changed its center. Particularly notable was the rotation of the vector after March 1989, indicating an inclination toward the ENE.

Seismicity. No seismic events were observed from 1985 through the beginning of 1987. Since April 1987, several swarms have been observed (figures 17 and 18): 10 April 1987, 50 events, maximum M 2, W sector of Solfatara; 4 November 1987, 26 events, maximum M 1.1, E sector of Solfatara; March 1989, 15 events, Solfatara area; 3 April 1989, 82 events, maximum M 2.2, Solfatara; May 1989, 33 events, maximum M 2.2, Solfatara; 1-13 June 1989, 45 events, maximum M 2.7. Most notable was the occurrence of several low-frequency events, the first time that such events have been observed. They were generally shallow and on the E border of Solfatara crater.

Figure 17. Seismic stations (large squares) and March-June 1989 earthquake epicenters (diamonds) in the Campi Flegrei area.

Figure 18. Number of local earthquakes recorded in the Campi Flegrei area, January 1987-June 1989.

Chemistry. The Costagliola well near Monte Nuovo has shown a clear increase in average radon content superimposed on annual variations. A similar trend is apparent for radon contents measured in water wells in different parts of Campi Flegrei. Both the S/C ratio and the water vapor content of a fumarole at Solfatara showed a steady increase starting in mid-1986.

Geologists noted that "All of these data seem to indicate a progressive change in the style of activity . . . , and it seems that the steady deflationary trend has come to an end. We still do not know if the picture we have described is the precursor of a new prolonged uplift phase, or if it represents the restoration of a trend similar to that after the 1970-72 uplift episode, characterized by oscillatory activity until 1982. It is notable, however, that Campi Flegrei is displaying in each new episode of unrest a new phenomenon that was not observed in the previous one. In 1970-72 there was a major uplift without significant seismic activity, and in 1982-84 there was uplift accompanied by seismic activity. In this case, although we still do not know if a sustained uplift will occur, there is the occurrence of low-frequency seismic events."

Further Reference. Tedesco, D., Bottiglieri, L., and Pece,R., 1988, 10th of April 1987 seismic swarm; correlation with geochemical parameters in Campi Flegrei Caldera (southern Italy): Geophysical Research Letters, v. 15, p. 661-664.

Geologic Background. Campi Flegrei is a large 13-km-wide caldera on the outskirts of Naples that contains numerous phreatic tuff rings and pyroclastic cones. The caldera margins are poorly defined, and on the south lie beneath the Gulf of Pozzuoli. Episodes of dramatic uplift and subsidence within the dominantly trachytic caldera have occurred since Roman times. The earliest known eruptive products are dated 47,000 yrs BP. The caldera formed following two large explosive eruptions, the massive Campanian ignimbrite about 36,000 BP, and the over 40 km3 Neapolitan Yellow Tuff (NYT) about 15,000 BP. Following eruption of the NYT a large number of eruptions have taken place from widely scattered subaerial and submarine vents. Most activity occurred during three intervals: 15,000-9500, 8600-8200, and 4800-3800 BP. Two eruptions have occurred in historical time, one in 1158 at Solfatara and the other in 1538 that formed the Monte Nuovo cinder cone.

Information Contacts: G. Luongo, C. Del Gaudio, F. Obrizzo, G. Ricciardi, and D. Tedesco, OV; R. Pece and R. Scandone, Univ di Napoli.

When Julián Flores Díaz and José Angel Cortés visited Colima 13-14 and 25-26 May, the summit area consisted of a dome on the N side, a semicircular depression on the SE side, and an irregular platform (figure 3). Fumaroles were concentrated in three areas on the dome (figure 4). On 14 May, gas emission, dominated by SO₂, had increased and the gas was light-brown in color, but it had substantially diminished by 25-26 May.

Figure 3. Sketch of Colima's summit, May 1989. Courtesy of J.F. Díaz.

Figure 4. Map (top) and cross-section (bottom) of Colima's summit area, showing positions of the dome, fumarolic activity, and the summit depression. Courtesy of J.F. Díaz.

The depression that formed 2 July [1987] after a phreatic explosion and avalanche from the summit was 100-150 in diameter and 30-40 m deep (from the high point in the middle of the summit area) [but see 15:12]. The area was warm but fumaroles observed during a November 1988 overflight had disappeared. Altered fragmented rocks and sand were present on the depression's floor. The remainder of the summit area, an irregular platform, was composed of blocks of many shapes and sizes. Warm gases containing SO₂ were emitted, and blocks were altered and covered with sulfur. On the SW flank, a talus slope of scoria and sand had developed. Thermometric equipment was not available to the team.

A group from CICBAS, Universidad de Colima (Guillermo Castellanos, Carlos Ariel Ramírez-Vázquez, and Juan Reyes-Gómez) visited the volcano 23-25 May. Average temperatures adjacent to fumaroles were 167°C, a decrease from 216°C measured in May 1988. Emissions were dense, dark-gray in color, and had a pH of 2-3. New fractures were observed near the fumaroles. Rockfall avalanches, persisting for much of the past year, were last seen 14-15 April on the W flank (observed 20 km from the volcano). Three avalanche paths were visible, on the W, E, and N flanks.

Two digital high-gain 3-component seismographs and one analog single-component seismic station were installed near the volcano (figure 5). The seismographs collected data continuously for about 40 hours and recorded an average of 30 events/day. Preliminary analysis of the data by Reyes and Ramírez showed that most of the activity was tectonic with long separation between P- and S-wave arrivals. On 1, 14, and 22 June, the operators of the Red Sismologica Telemetrizada de Colima (a network that will consist of eight short-period, vertical seismograph stations; figure 6) installed three telemetric stations. Data are telemetered to CICBAS in the city of Colima. No deformation data are available, but changes in Colima's shape are visible and geodetic studies would be welcomed.

Figure 5. Location of digital high-gain 3-component seismographs (SS2, SS3) and an analog single-component (SS1) seismograph installed near Colima. Courtesy of G. Castellanos.

Figure 6. Distribution of instruments for the planned Colima Telemetric Seismological Network (RESCO). Courtesy of G. Castellanos.

Geologic Background. The Colima volcanic complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the 4320 m high point of the complex) on the north and the 3850-m-high historically active Volcán de Colima at the south. A group of cinder cones of late-Pleistocene age is located on the floor of the Colima graben west and east of the Colima complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide caldera, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, and have produced a thick apron of debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions (most recently in 1913) have destroyed the summit and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Julián Flores-Díaz, Instituto de Geografía y Estadística, Univ de Guadalajara; Guillermo Castellanos, Gilberto Ornelas-Arciniega, C. Ariel Ramírez-Vázquez, G.A. Reyes-Dávila, and Hector Tamez, CICBAS, Universidad de Colima.

The following, from IIV, describes activity May-June 1989.

Summit activity. (S. Calvari, M. Coltelli, and M. Pompilio.) Vigorous activity at the two central crater vents (Bocca Nuova and La Voragine) continued in May. On the 4th, La Voragine ejected bombs and lapilli that fell as far as the rim of Cratere del Piano (roughly 300 m away), choking the crater bottom with tephra. In late May, explosive activity diminished and continued at a normal level throughout June. Discontinuous effusive activity was observed in May within Bocca Nuova, and bombs accumulated in the crater to ~ 100 m from the rim. From late May through most of June, many bombs, some of considerable size, fell outside the crater. This activity suddenly stopped in late June, when the small cone inside the crater collapsed, and was succeeded by sporadic scoria ejection from two vents. Mild Strombolian activity at Southeast Crater in May slightly eroded the scoria cone that had formed in April (14:05). Strombolian activity continued at a medium-low level in June, with occasional pulses ejecting small numbers of bombs over wide areas. The vent on Northeast Crater's floor continued to degas through May and June.

Seismicity. (V. Longo, A. Montaldo, M. Patané, E. Privitera, and S. Spampinato.) The frequency of tectonic seismicity in May and June was generally similar to that of the past year, with occasional seismic swarms. During the last two days in May, low-energy events were detected ~ 10 km below the volcano's central area. A seismic swarm, recorded 19-24 June on the W flank, was 13-15 km deep and included the largest events (M 3.1-3.2) of the month. One of the earthquakes (on the 24th at 0230) was felt by area residents. On 28 June, a small mainshock-aftershock sequence (11 events) was recorded, with the largest earthquake located near the S portion of the Valle del Bove at <5 km depth. From late June to 1 July, events with M 2.5-3.0 occurred 10-15 km beneath the summit. No significant variations in the volcanic tremor pattern were observed during May or June.

Ground deformation. (O. Campisi, G. Falzone, B. Puglisi, G. Puglisi, and R. Velardita.) Ground deformation measured at the Serra Pizzuta Calvarina borehole tilt station showed no significant variation in May or June. Measurements in May using the S trilateration network showed little deformation since l June 1988.

SO₂ emissions. (T. Caltabiano and R. Romano.) The average value of SO₂ flux in May 1989 was the lowest of the past year, but moderately high values returned in June. SO₂ flux was measured 3, ll, 17, and 24 May and 1, 7, 15, 22, and 29 June. Emissions fluctuated in May, with high values on the 3rd and 17th and low values on the 11th and 24th, reaching only 2,500 t/d on the latter date.

Tephra composition. (S. Calvari, M. Coltelli, and M. Pompilio.) January 1989 activity produced hawaiite tephra, with petrography and chemical composition similar to tephra from the previous year. Tephra emitted from Southeast Crater during 1988 had relatively more evolved compositions, but early 1989 tephra was less differentiated than material emitted by the other summit craters.

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: R. Santacroce, IIV.

After a 2-week earthquake swarm, a brief submarine eruption built a small cone on the sea bottom a few kilometers off the coast of the Izu Peninsula. [See 14:7 for a more detailed report from JMA.]

Earthquakes began 30 June, and by 9 July, more than 19,000 had been recorded. Many were at depths of 4-5 km in a zone roughly 3-7 km NE of Ito, a city of 72,000 about 100 km SW of Tokyo and 40 km NW of Oshima volcano. The swarm included a pair of strong events that occurred within a minute of each other on 9 July at 1109; the first was of M 5.5, the second slightly weaker. At least 18 people were injured by these shocks, and landslides were reported at 16 sites. A year earlier, more than 17,000 events centered farther from the coast were recorded during a month of seismicity that began in late July 1988. Previous swarms had occurred SE of the 1989 epicentral area in 1984 and 1985, and numerous other 1984-86 events occurred in a zone separating the 1984 and 1985 swarm epicenters.

The eruption began on 13 July. A JMA seismometer started to record microseismicity at 1829. The captain of the RV Takuyo (Hydrographic Dept, JMSA), carrying out a bathymetric survey in the area, reported hearing an explosion sound from the sea bottom and a 30-second vibration at 1833. One minute later, the JMA seismometer was saturated by seismic events and remained saturated for the next 10 minutes or more. At 1840, the crew of the RV Takuyo saw the sea surface dome upward about 500 m from the vessel, then a gray-black plume rose from the same area. Five more plumes, ~30 m high and 100 m across, were observed in the next 5 minutes. The ejection of each plume was accompanied by violent shaking and vibration of the ship. No more eruptive activity was reported. Seismographs were again saturated at 1902, and another seismic sequence, of different frequency, was recorded at 1907. Another 15 minutes of volcanic microseismicity began at 2130. No detailed reports were available for the next few days, but strong seismicity stopped after 16 July.

After the eruption, a bathymetric survey using an unmanned vessel detected a new cone in about 100 m of water at the eruption site. The cone was about 450 m wide, with a summit crater 200 m in diameter, but rose only ~10 m above the sea bottom. The eruption occurred in a region of Recent monogenetic volcanism that has built numerous subaerial and submarine cones (figure 1). One nearby pyroclastic flow (Kawagodiara) on the Izu Peninsula has been dated at about 3,250 BP. No ages are available for the submarine edifices, although very fresh pillow lavas were found downslope during work in a submersible.

Figure 1. Topographic and bathymetric map of the E-central Izu Peninsula and nearby waters, after Ishii and others (1988). The 13 July eruption site is labeled with a star. Young submarine cones are labeled with letters and open triangles. Pillow lavas were found in the outlined area labeled D173, 174 Tanaka.

Reference. Ishii, T., Watanabe, M., Ishizuka, T., Ohta, S., Sakai, H., Haramura, H., Shikazono, N., Togashi, K., Minai, Y., Tominaga, T., Chinzei, K., Horikoshi, M., and Matsumoto, E., 1988, Geological Study with the "Shinkai 2000" in the West Sagami Bay including Calyptogena Colonies; Technical Reports of the Japan Marine Science and Technology Center, 1988, p. 189-218.

Geologic Background. The Izu-Tobu volcano group (Higashi-Izu volcano group) is scattered over a broad, plateau-like area of more than 400 km2 on the E side of the Izu Peninsula. Construction of several stratovolcanoes continued throughout much of the Pleistocene and overlapped with growth of smaller monogenetic volcanoes beginning about 300,000 years ago. About 70 subaerial monogenetic volcanoes formed during the last 140,000 years, and chemically similar submarine cones are located offshore. These volcanoes are located on a basement of late-Tertiary volcanic rocks and related sediments and on the flanks of three Quaternary stratovolcanoes: Amagi, Tenshi, and Usami. Some eruptive vents are controlled by fissure systems trending NW-SE or NE-SW. Thirteen eruptive episodes have been documented during the past 32,000 years. Kawagodaira maar produced pyroclastic flows during the largest Holocene eruption about 3000 years ago. The latest eruption occurred in 1989, when a small submarine crater was formed NE of Ito City.

Information Contacts: T. Ishii, SI; S. Aramaki, Earthquake Research Institute, Univ of Tokyo; JMA; Hydrographic Dept, JMSA; Asahi Shinbun News, Tokyo.

A M 6.1 S-flank earthquake on 25 June triggered collapse of the coastal lava bench, but apparently had little effect on the continuing eruption. Lava flows that emerged from the tube system on the lower flanks reached the sea at two new sites, after destroying structures near the coast.

Surface lava flows that broke from the W tube system in April and destroyed houses . . . in May advanced S towards the coast in June. Lower elevation lava breakouts from the W tube, which had moved SW around the Royal Gardens kipuka in May, also continued to advance. Lava flows moving W along the Chain of Craters road destroyed a maintenance area on 21 June. The two flow fronts merged the next day, destroying the National Park Service Wahaula Visitor Center (figure 61). By 25 June, the flow front had advanced another 100 m W along Chain of Craters road. A lava front that had moved to within 30 m of the coast in mid-May, stagnated, reactivated in mid-June, and entered the sea on 22 June in a new area at Kupapau Point. The Kupapau flow (intermittently active) had stagnated by 30 June, but resumed activity in early July. On 23 June, lava began entering the ocean at Poupou (just E of the Wahaula residential area). Lava also continued to enter the ocean E of Kupapau Point.

Figure 61. Map of the coastal area affected by the recent activity of Kupaianaha, as of September 1989. Dashed lines indicate roads buried in June and July; filled squares represent structures destroyed during the same period (VC = Visitor Center). Lava contacts from lower Royal Gardens subdivision to the Wahaula area are preliminary. The four "entries" are places where the lava was entering the ocean in July. Lava contacts from lower Royal Gardens subdivision to the Wahaula area are preliminary. Courtesy of Christina Heliker.

The M 6.1 earthquake on 25 June at 1727 was centered on the SE coast, W of Kalapana, at 19.36°N, 155.08°W, 9 km depth (figure 62). Preliminary assessment of the data suggests that the main shock caused seaward movement of Kilauea's S flank along a subhorizontal plane at the bottom of the volcanic pile near the ocean floor. Aftershock focal depths indicate rupture from near the surface to slightly more than 10 km depth. The motion was similar to the M 7.2 earthquake that struck the same region on 29 November 1975 and most of the strong S flank earthquakes (M>5.5) commonly occur in the mainshock area. Significant earthquakes also were located in this area in March 1954 and September 1979.

Figure 62. Locations of the M 6.1 earthquake and associated aftershocks, 25 June-6 July, 1989. Courtesy of R. Koyanagi.

The earthquake caused almost total collapse of the seacoast lava bench, but apparently did not significantly disrupt the lava tube system. The next morning, geologists noted that the level of the Kupaianaha lava pond had dropped by ~1 m. Lava flow activity at the coast declined 27-28 June, accompanied by a slight decrease in tremor 26-28 June. On the 28th, tremor near the vent gradually rose to normal as the level of Kupaianaha lava pond rose ~1.5 m. By the next day, activity at the coast returned to the pre-earthquake level. An active lava pond in Pu`u `O`o was visible on 28 June.

During the last few days of June, tremor amplitude was relatively steady beneath the East rift zone near Pu`u `O`o and Kupaianaha. Low-amplitude tremor signals associated with ocean front activity near Kupapau Point also resumed. The 25 June earthquake saturated seismographs, masking signals from the associated lava bench collapse. The number of shallow microearthquakes was about average in the summit region and above average in the East rift zone. Intermediate-depth long-period events in the summit region continued at a moderate rate . . . .

Geologic Background. Kilauea, which overlaps the E flank of the massive Mauna Loa shield volcano, has been Hawaii's most active volcano during historical time. Eruptions are prominent in Polynesian legends; written documentation extending back to only 1820 records frequent summit and flank lava flow eruptions that were interspersed with periods of long-term lava lake activity that lasted until 1924 at Halemaumau crater, within the summit caldera. The 3 x 5 km caldera was formed in several stages about 1500 years ago and during the 18th century; eruptions have also originated from the lengthy East and SW rift zones, which extend to the sea on both sides of the volcano. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1100 years old; 70% of the volcano's surface is younger than 600 years. A long-term eruption from the East rift zone that began in 1983 has produced lava flows covering more than 100 km2, destroying nearly 200 houses and adding new coastline to the island.

Information Contacts: C. Heliker and R. Koyanagi, HVO.

"Langila returned to very subdued activity in June. Crater 2 released moderate white-grey emissions, accompanied by occasional rumbling noises. Explosions were heard on 1, 2, 6, 24, and 30 June, and a weak red glow was seen above this crater on the night of the 14th.

"When the volcano was inspected on 10 June, Crater 2 had enlarged and deepened since the last field inspection in October 1985 (10:10). The flat, [40]-m-wide, annular platform that formerly surrounded the crater had caved in, resulting in an estimated [130]-m wide crater with a narrow ledge. The crater now has a composite funnel shape produced by the sinking of the former magma plug in two successive steps. The top of the active plug (responsible for the occasional night glow) is now at ~1,045 m altitude (the crater rim is at 1,100-1,120 m) and clogged by debris from sub-continuous rocksliding.

"Crater 3 . . . remains inactive. The crater is sealed at ~900 m asl by a flat muddy floor from wash-outs of the walls (the crater rim is at 1,045-1,080 m altitude). The source of white vapour occasionally observed from the observatory is an active fumarole at the base of the sub-vertical S wall."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower eastern flank of the extinct Talawe volcano. Talawe is the highest volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila volcano was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the north and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit of Langila. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: P. de Saint-Ours and B. Talai, RVO.

A lava dome has been growing in the active summit crater, site of occasional tephra emission since 1986. Observations and pictures from Stephen Foot (MINSAL, Ltda.), who climbed the volcano on 18 April 1989, confirm Paul King's February 1989 report of a steaming lava dome (14:3). The photographs clearly show a dome growing in the W crater of the eastern of Lascar's two andesite cones (figure 1). Until early 1986, this crater was empty, with only solfataric and fumarolic activity. Foot's photographs show that by April 1989 the dome had reached an estimated 200 m in diameter and 50 m height. The dome had steep sides and a blocky, steamy, dark brown surface. Steam emissions of different intensities were still being continuously released in late June, and glow was visible from Toconao (~30 km away) on one occasion.

Figure 1. Photograph of the growing lava dome in Lascar's summit crater, 18 April 1989, by Stephen Foot. Courtesy of M. Gardeweg.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: M. Gardeweg, SERNAGEOMIN, Santiago; S. Foot, MINSAL Ltda., Santiago.

On 12 January, when Michael Peterson led a field party to the volcano's summit, no liquid lava was visible in the crater. Steam was emitted from vents T4/T7, T8, and T9, as well as from areas along the saddle. Intermittent rumbling sounds originated from near H4 (W of T5). During an overflight in late May, Steve Cunningham witnessed bubbling lava on the SE side of the crater, near T10.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, Kenyatta Univ; Thad Peterson, Arusha, Tanzania.

An earthquake swarm began 4 May under the SSW flank of Mammoth Mountain, just outside the SW caldera rim (figure 7). The number of events increased through early June, with 44 recorded on the 11th. Seismicity was continuing as of 10 July, and totaled 712 recorded events (magnitude greater than or equal to 0.3) (figure 8). Most were small (M <1); the largest, M 3.1, occurred on 21 June at 0058. As the swarm continued, most of the events remained centered beneath the SW flank of Mammoth Mountain, on strike with the Inyo chain, at depths ranging from 2 to 9 km. Focal depths during previous swarms have generally been around 6 km. Most of the shallower earthquakes showed less high-frequency energy in their spectra, probably because of attenuation effects, but had clear S-waves and were therefore not considered low-frequency events. However, seven low-frequency events were recorded on 11 June. Several mixed-frequency events had high-frequency P and S-waves superimposed on 1-2-Hz waves, suggesting possible resonance of a fluid-filled cavity. Possible spasmodic tremor was recorded for 2-3 minutes on 2 and 26 June, and 6 July.

Figure 7. Representative epicenters (26-31 May) of the May-July 1989 earthquake swarm at Long Valley. Mammoth Mountain is shown by the solid triangle. Events S of the caldera are in the Sierra Nevada. Courtesy of Stephen McNutt.

Figure 8. Number of local earthquakes per day recorded by the California Division of Mines and Geology NEWT system, 5 May-30 September. Courtesy of Stephen McNutt.

The Devils Postpile dilatometer, near the W foot of Mammoth Mountain, recorded 0.05 microstrain of deformation during the swarm's most active day, 11 June. No significant changes to existing trends were reported from other instruments a few kilometers away.

The May-July swarm is the largest near Mammoth Mountain in 3.5 years; a small swarm occurred there in January 1987. During the past 4 years, virtually all of the other seismic swarms in the Mammoth Lakes area have lasted only a few days. The largest recent swarm, 393 recorded events in the caldera's E moat, began 22 November 1988 and ended after 3 days.

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento.

The eruption was continuing as of late June. Explosive activity remained relatively weak (VEI 1) through much of May, with occasional more violent pulses (VEI 2) as on 1-3 and 16-25 May. Hugo Moreno flew over the area on 30 May. Strong WNW winds carried the plume directly over Lonquimay village (~20 km ESE of Navidad Crater; figure 12). The lava flow continued to advance very slowly at the front in the Lolco River valley (~9.5 km from the crater) and more vigorously at the Laguna Verde front (~4 km from the crater). Lava volume was estimated at 160 x 10⁶ m³.

Figure 12. Approximate ashfall thicknesses in the Lonquimay area, as of mid-May 1989, courtesy of O. González-Ferrán. The lava flow is shown in black.

As of mid-June, hundreds of cattle and horses had died of osteofluorosis caused by 300-400 ppm fluorine on grass in an 80,000 hectare (800 km²) area. Some dogs have also recently died after suffering from nervous, renal, digestive, and breathing problems. Concentration of very fine ash has at times been at levels 10 times those considered safe for breathing. Mid-June medical checks of 260 people revealed neurological damage with associated reflex loss in 45 adults and children.

A report (quoted in the 24 June El Mercurio) from Maximino Beltrán, Regional Secretary of Health, to the national Subsecretary of Health, detailed numerous neurological and blood chemistry abnormalities discovered in varying proportions of area residents. An autopsy on a 64-year-old woodcutter, exposed to ashfall for more than 8 hours daily, revealed evidence of acute hemorrhagic colitis and massive bilateral lung hemorrhaging, plus central nervous system lesions. Similar lesions (plus lung, liver, and heart problems) were seen in seven dogs (one sick and six outwardly healthy) studied in the eruption area. The report recommended prompt evacuation of the most affected people, the 800 inhabitants of the Bernardo Nanco area, and the evacuation or relocation of ~3,800 persons judged moderately affected, in the town of Lonquimay. Evacuations had apparently begun by early July.

Geologic Background. Lonquimay is a small, flat-topped, symmetrical stratovolcano of late-Pleistocene to dominantly Holocene age immediately SE of Tolguaca volcano. A glacier fills its summit crater and spills down the S flank. It is dominantly andesitic, but basalt and dacite are also found. There is an E-W fissure, although the prominent NE-SW Cordón Fissural Oriental fissure zone cuts across the entire volcano, that produced a series of NE-flank vents and cinder cones, some of which have been the source of voluminous lava flows, including those during 1887-90 and 1988-90 that traveled up to 10 km.

Information Contacts: H. Moreno, Univ de Chile; O. González-Ferrán, Univ de Chile; Pedro Riffo, Univ de la Frontera; El Mercurio, Santiago.

"Activity was at a very low level throughout June. Southern Crater released white to grey vapour [and ash] in weak to moderate amounts. Weak deep rumbling noises were occasionally heard. Main Crater released weak emissions of white vapour. The seismicity fluctuated at a somewhat lower level than 'normal' inter-eruptive rates, between 500 and 1,100 minor events/day. Tilt readings also fluctuated, although continuing on a slow deflationary trend since early March."

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical 1807-m-high basaltic-andesitic stratovolcano to its lower flanks. These "avalanche valleys" channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most historical eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent historical eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: P. de Saint-Ours and B. Talai, RVO.

The February-March lava lake in Santiago Crater (14:02) probably froze over in early March, and degassing from the lake vent had apparently ceased by 12 March. Other vents remained open through April, with occasional strong degassing episodes. Beginning around 11 May, collapses from the W, S, and N sides of the main crater blocked all vents. Little, if any, gas emission was evident until 22 May when park rangers reported more collapses and a plume visible from the Masaya road (6 km from the crater).

On 25 May, geologists found fresh scoria and lithic fragments scattered from Plaza Sapper to the San Pedro crater (figure 7, top). Ten-cm fragments were found to 20 m from the edge of Santiago, 5-cm fragments to 50 m, and fragments

Figure 7. Sketch of the summit complex at Masaya, May-June 1989 (top) and Santiago Crater, 3 June 1989 (bottom). Courtesy of B. van Wyk de Vries and O. Castellón.

A 3 June visit revealed small amounts of fresh scoria up to 5 cm in diameter as far as 50 m SW of the crater. The tephra was probably erupted on 2 June when inhabitants reported a "brown cloud". Crater geometry was similar to that in February. The lava lake vent and the "cannon" (3rd vent in 14:02) were blocked by collapse debris, but vent No. 2 (glowing vent in 14:02) had enlarged and was thought to be the source of the eruptions. On 25 May the vent was oval and about 4 m across, oriented vertically, rather than horizontally as in February. On the 26th it had enlarged by 1 m, and by 3 June it was 7 x 3 m and rectangular. There appeared to be a considerably larger chamber beneath the vent. The cannon (3rd) deepened slightly between 25 May and 3 June.

Periodic fumarolic activity on the W wall and from a fault on the N side (figure 7, bottom) was also observed. Weak fumaroles along the trend of the fault (on the Nindirí crater floor below La Cruz) had temperatures

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Information Contacts: B. van Wyk de Vries and O. Castellón, INETER.

During the first 12 days of May, activity remained similar to that at the end of April. Gas emission was intense, and ejections of mud and lithic ash fed plumes that reached maximum heights of 1.5-2 km above the crater floor. Individual ash ejections lasted for more than an hour. Trade winds generally carried ash clouds toward the WSW. Various towns reported ashfalls, including Atenas, 32 km SW (on 9 May). Some ashfalls also occurred on the ENE and S flanks. On the rim, roughly 300 m W of the center of the crater (point F on figure 18) 5 mm of new ash was measured on 7 May and 60 mm on the 12th. The ash was composed of hydrothermally altered lithic fragments, soluble mud, and sulfur. The maximum measured grain size was 1.5 mm, and 80% of the ash volume was composed of fragments between 0.075 and 0.25 mm.

Figure 18. Sketch map of Poás showing ash isopachs as of 12 May. Grid spacing is 1 km. Thicknesses of ash at each collection point: A, 3 mm; B, 5 mm; C, 13-20 mm; D, 42 mm; E, 50 mm; F, 60 mm. Courtesy of Gerardo Soto.

Gases were dominated by water vapor from the aquifer beneath the crater, and included SO₂, H₂S, and (possibly) hydrogen. Sulfur sublimates were deposited around fumarolic vents, and some of the sulfur burned, forming SO₂. Flames from the combustion of sulfur (and perhaps hydrogen) were intense above some vents. In the center of the area formerly occupied by the crater lake, two primary pyroclastic mud cones (and various smaller neighboring cones) had been growing since mid-April, reaching maximum heights of 25 m despite frequent collapses. In the SE part of the crater, there was a molten, bubbling, sulfur lake and sulfur had flowed across the muddy crater floor. Fumaroles emitted sulfurous gases and a mud-sulfur cone was growing. The crater's NE quadrant included a vigorous fumarole that emitted sulfur-rich gas with a jet-aircraft sound, and deposited sulfur sublimates.

With the onset of the rainy season in mid-May, water started to accumulate in the former crater lake, reaching a depth of about 2 m by early June. Eruptive activity began to decline noticeably on 13 May. By the last week of May, the central cones had collapsed and been reworked by convective bubbling. Nevertheless, emission of water vapor and sulfur gases, some burning, continued at the end of the month. Bubbling was vigorous in the muddy zones on the crater floor, but no mud columns were ejected nor were there ash eruptions. The former site of the sulfur lake was occupied by a muddy area and a fumarole producing sulfur sublimates that burned with red-orange flames. Bubbling mud and intense evaporation were found in the active zone in the NE part of the crater. A zone of weak fumaroles and sulfur sublimates was present on the wall and NE side of the inner crater. Activity on the remnants of the 1953-55 [dome] remained stable through June, with low-temperature fumaroles depositing sulfur, gypsum, other minerals, and clays.

Intense gas emission (dominated by water vapor, with SO₂ and H₂S) continued in June from the crater lake. The lake remained about 2 m deep through the month. Its inner zone was muddy and showed continuous convective bubbling, while its periphery was emerald green with a pH <= 0.5, fed by multiple surface springs of about pH 2.0. There were five principal hot areas in the lake's inner zone, three in the N area, one in the center, and one to the SE. The NE site showed intense fumarolic activity and had constructed a small mud-sulfur cone that contained an orange-brown lake of molten sulfur and boiling mud. The central N site included small cones with mud/sulfur spines. Fumarolic activity and a mud rampart had developed at the SE site. At the other hot areas, intense convection of muddy water generated waves. Small emissions of muddy ash occurred within the crater, including one on 23 June at 1845 that produced a column hundreds of meters high. Other explosions occurred between 28 June and 2 July.

Substantial changes have been noted in volcanic seismicity. The characteristic B-type shallow (<500 m depth) signals declined in May but increased again in June.

During the first 30 days of May, 2,247 seismic events were recorded, a daily mean of about 75 (figure 19), down from 141/day in April. June's average was similar (1,904 events in the first 27 days, a mean of 71/day) but the number of earthquakes increased sharply after lower activity during the month's first week. Geologists noted that tremor or volcanic noise has become common at Poás, probably resulting from continuous degassing in a partially open conduit. Origins looked like those of B-type signals and the activity could represent continuous trains of B-type events. A-type shocks, of volcano-tectonic origin, had preliminary locations near the crater, with magnitudes <1.

Figure 19. Number of seismic events/day at Poás, 1-30 May and 1-27 June, 1989. Courtesy of Mario Fernández.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: Gerardo J. Soto, Guillermo E. Alvarado, Mario Fernández, and Héctor Flores, UCR.

"Activity remained at background levels throughout June. There were 152 small earthquakes recorded in the caldera. The daily count fluctuated between 0 and 15. Only two events were large enough to be accurately located, originating 1 km under Greet Harbour. Monthly levelling measurements to Matupit Island show a steady (or slightly subsiding) trend since December 1988. Neither tilt nor EDM data have shown any significant trend."

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the 688-m-high asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: P. de Saint-Ours and B. Talai, RVO.

Seismic energy release has been at increased levels since about February 1988. A sharp increase in seismicity began on 24 June 1989 with a felt earthquake (M 3.1) in Arenas crater. The next day, a shallow swarm of high-frequency events (also in Arenas crater) began at 1130 and continued for 1 hour. From 0100 to 1100 on the 26th, another high-frequency swarm was centered at 4 km depth, 3 km W and SW of Olleta crater (Olleta is roughly 5 km W of Arenas crater). Late that evening, a shallow high-frequency swarm began in Arenas crater, followed by strong tremor associated with a small ash emission that deposited 1 mm of ash, 4 km from the crater. The press reported that the civil aeronautics board issued a warning to airline pilots to avoid a 60-km area around the volcano. Tremor gradually diminished, disappearing on 28 June. SO₂ emission was moderate during June. Dry and electronic tilt did not show significant changes. As of 10 July, a yellow alert remained in effect for population within a 10-km radius of the volcano.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales; Reuters.

Previously unobserved fumarolic activity on the SE spur of Casita (at sites 150 m and 0.5-1 km below the communications complex on the summit) was noticed on 8 June. Area residents report that the activity has been present for some time. Emissions appear to originate from a N-S fissure (figure 1). Casita was last reported active in the l6th century.

Figure 1. Oblique sketch of Casita, its fumaroles, and neighboring volcanic features, 8 June 1989. Courtesy of B. van Wyk de Vries and O. Castellón.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: B. van Wyk de Vries and O. Castellón, INETER, Managua.

Santiaguito's most recent (7th) period of rapid block lava extrusion began in June 1986 and had declined about February 1988. A small lobe that descended slowly toward the W margin of the lava field was 1.3 km from the dome's Caliente vent in November 1988. Very slow extrusion continued until the onset of a new period of vigorous lava production around 14 February. Observations 23-24 March revealed that the new lava flow, about 70 m wide and 20 m thick, was overriding the June 1986-February 1988 lava (figure 9) and its oversteepened front had reached about 1,470 m altitude. Moderate pyroclastic avalanches generated by collapse of the flow at the altitude of maximum slope (2,000-1,800 m) and at its oversteepened front partially filled canyons in the headwaters of the Río Nimá II and the tributary E of the lava flow. Brief observations 3 May about 1 km from the flow (at El Mirador) showed no substantial changes.

Figure 9. Map of Santiaguito Dome, showing the ages of its lobes. Succesive fronts of 1986-89 lava flows are shown. Modified from Rose and others (1987). Courtesy of Otoniel Matías.

During September and October 1988, seismic instruments 2.6 km S and 5 km NNW of Santiaguito recorded 8-28 explosions and 130-330 avalanches/day. After the beginning of November, the number of explosions declined to 4-16 daily and the number of avalanches to 60-120 (figure 10), remaining at similar low to moderate levels through late February. More violent explosions began on 25 February and continued through 13 March, stronger than any since the start of vigorous block lava extrusion in June 1986. Some dense ash columns rose at least 3 km above the crater and were visible from the summit of Fuego, 75 km away. Ash columns during this period easily exceeded the height of Santa María's summit (3,772 m), more than 1,200 m above the vent, forming mushroom-shaped clouds 1 km in diameter. Ash reached parts of Quetzaltenango, 12 km NE, within 15 minutes. During this period, 8-26 explosions were recorded daily. The strongest produced acoustic waves that moved suspended objects 7 km to the S (at Finca El Faro). Sounds similar to a jet turbine continued for up to 4 minutes, alternating with the phreatomagmatic explosions. Winds 24-25 February were dominantly from the N-NE at 20-30 km/hour; fine ashfall was reported to 28 km S-SW (in the El Palmar, San Felipe, and Retalhuleu regions). From 26 February through 13 March, winds were generally from the S-SW, calm in the morning and reaching 18-30 km/hour in the afternoon. Fine ash was carried 7-25 km NW and NE; losses from vegetation damage were reported in Llanos del Pinal, Almolonga, and Quetzaltenango (7, 12, and 14 km N-NE).

Figure 10. Number of daily explosions (bottom) and an extrapolation of the number of daily avalanche events (top) recorded by seismic stations 2.6 km S and 5 km NNW of Santiaguito, November 1988-April 1989. Courtesy of Otoniel Matías.

A brief decline was evident 14-16 March, with only 6-10 small explosions daily generating clouds <=1 km high. Activity increased again 17 March, dominated by degassing that produced dense whitish clouds with little ash and moderate to strong jet turbine sounds. Between 14 and 24 explosions/day were recorded through 31 March. The number of explosions grew gradually in early April, reaching 34 on the 18th (the most recorded in a single day since June 1988) then fell to 14-26/day after the 21st. Avalanches from the dome, the central area of the lava flow (2,000-1,700 m elevation), and its oversteepened front ranged from 150 to 300/day.

Weak to moderate fumarolic emissions persisted from the N and S margins of the Caliente vent area. The E fumarole was more active and acted as a secondary crater during some explosions, feeding columns that were similar to or smaller than those from the main vent. The E fumarole may have been the source of the jet turbine sounds as it underwent high-pressure degassing. After some explosions, its emissions increased, often persisting for several hours as sustained columns rose tens of meters to 1 km. Very weak fumarolic emissions occurred throughout the summit area of the dome complex, frequently linked with increased activity from Caliente vent.

At press time, we learned that Santiaguito erupted an ash column to 4 km above the dome on 19 July at 0915 [see also 14:07]. A pyroclastic flow traveled 5 km down the Río Nimá II, reaching 2 km from Finca La Florida. Ash was 1 cm thick at Finca Monte Bello (6 km WSW) and fell as far as the Mexican border. Thirty two Central American volcanologists, attending a course in El Palmar (12 km SSW of the volcano), witnessed the eruption during good viewing conditions, took photographs, and made a videotape. The eruption was followed by two smaller explosions within 1/2 hour, and another at 1600. Prelimimary observations by volcanologists suggest that the eruption may have been associated with partial collapse around the vent. There were no reports of death or damage.

Geologic Background. Symmetrical, forest-covered Santa María volcano is one of the most prominent of a chain of large stratovolcanoes that rises dramatically above the Pacific coastal plain of Guatemala. The 3772-m-high stratovolcano has a sharp-topped, conical profile that is cut on the SW flank by a large, 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank and was formed during a catastrophic eruption in 1902. The renowned plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four westward-younging vents, the most recent of which is Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Otoniel Matías and Jorge Girón, INSIVUMEH; W. Rose, Michigan Technological Univ.

. . . March-April activity is summarized in table 2. No explosions were observed in May, but several tens of explosions 22-23 June were accompanied by detonations and air shocks. Ash fell on the S part of the small island volcano, in the only inhabited area.

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit of the volcano is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed forming a large debris avalanche and creating the horseshoe-shaped Sakuchi caldera, which extends to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: JMA.

A visit to the volcano on 5 June revealed two small brown crater lakes, 10 m across (figure 2). A number of large collapses had occurred, covering much of the crater floor with blocks. Fumarolic activity was vigorous (particularly from a vent on the SE side) and produced a continuous plume over the crater. No eruptive activity has been reported since December 1987.

Figure 2. Sketch of the active crater of Telica, 5 June 1989. Courtesy of B. van Wyk de Vries and O. Castellón.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: B. van Wyk de Vries and O. Castellón, INETER, Apartado 1761, Managua, Nicaragua.

Tephra produced by the phreatomagmatic explosions that began 19 December contained a little fresh magma (scoria and blocks) of basaltic andesite composition similar to that of the 1926 and 1962 ejecta. Some of the pyroclastic flows and surges melted snow and fed small lahars. A detailed description of this eruption can be found in Katsui (1989).

No eruptive activity has occurred since a brief explosion from crater 62-2 on 5 March. A continuous steam plume, which often contained ash in May but was white in June, was observed from Tokachi-dake Observatory. Plume heights reached 800 m above the crater rim in May and 100-600 m in June. A seismograph 4.5 km NNW of the crater recorded only five volcanic earthquakes and no volcanic tremor in May, but seismicity increased in late June (figure 5). A total of 25 volcanic earthquakes was recorded in June, and seismicity remained elevated as of early July.

Figure 5. Daily number of local seismic events, 1 January-9 July 1989 (top) and number of small earthquakes recorded by a seismograph ~2 km NW of the volcano, 11 June-9 July 1989 (bottom). Courtesy of JMA.

Reference. Katsui, Y., ed., 1989, The 1988 eruption of Tokachi-dake, its sequence, mechanism, and influence on community: Report of Natural Disaster Scientific Research no. B-63-5, March 1989, 108 pp (8 papers).

Geologic Background. Tokachidake volcano consists of a group of dominantly andesitic stratovolcanoes and lava domes arranged on a NE-SW line above a plateau of welded Pleistocene tuffs in central Hokkaido. Numerous explosion craters and cinder cones are located on the upper flanks of the small stratovolcanoes, with the youngest Holocene centers located at the NW end of the chain. Frequent historical eruptions, consisting mostly of mild-to-moderate phreatic explosions, have been recorded since the mid-19th century. Two larger eruptions occurred in 1926 and 1962. Partial cone collapse of the western flank during the 1926 eruption produced a disastrous debris avalanche and mudflow.

Information Contacts: JMA.

"The level of activity has shown a continuous decrease since the mild phreatic unrest in March. Throughout the month, the terminal crater was releasing a plume of white vapour, while the seismicity was steadily decreasing . . . "

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the north coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: P. de Saint-Ours and B. Talai, RVO.

Eruptions of ash and blocks continued from R.F. Crater and Donald Duck vent in May and June. On 10 May, when R. Fleming visited White Island, R.F. Crater was erupting dark gray coarse ash, most of which fell into the crater. Donald Duck vent was emitting minor amounts of gas. A small (3 m diameter) new vent had opened 20-30 m NNE of Donald Duck, discharging gas and ash. On 1 June, Fleming observed similar conditions.

During geological fieldwork on 23 June, the main crater floor was covered with fine gray ash that thickened toward Donald Duck vent. Block-ejecting explosions (the largest yet from Donald Duck) had apparently also occurred since the 1 June visit. Fresh new impact craters and lithic blocks (up to 1 m in diameter) were abundant to ~200 m SW of Donald Duck, which had enlarged to 100 m in diameter and >200 m in depth. No fresh magma has been detected in the Donald Duck tephra. The new vent NNE of Donald Duck vent was no longer active. The pits that had formed in late January (SEAN 14:01) and the 1980 pits (W of Donald Duck) were quiet, but had recently collapsed (probably due to recent heavy rainfalls) and were deeper, with vertical walls.

Large scoria bombs (1 m) and blocks (>5 m in diameter near the 1978 Crater rim) had been erupted from R.F. Crater, which was emitting a dilute, green-brown ash column and a few small blocks. Coarse ash fell back into the crater. A total of 450 mm of ash had accumulated on the 1978 Crater rim since 26 April. Rare, vesiculated, brown glass was the only indication of fresh magma in the tephra. Hitchhiker vent (in Congress Crater) was slightly enlarged, but had not collapsed, suggesting reinforcement by local intrusions. Recent heavy rainfalls had triggered several debris flows of saturated ash from the 1978 Crater walls. The largest had flowed across the 1978 Crater floor and over the rims of R.F. and Congress Craters.

Fumarole temperatures in the Donald Mound area had dropped since 26 April, and tephra (ejected from Donald Duck) covered the vents. Deflation of the area had accelerated, with the W portion subsiding 21 mm and the NW portion >40 mm since 16 March. The area near the rim of 1978 Crater had subsided 300 mm since the small eruptions in early 1984 (09:02).

Intermittent seismic data after 26 April showed that seismicity had not significantly changed, other than an increase in E-type events (14 in May and 4 in June before transmission ceased). A- and B-type events were recorded most days, with maximum daily totals of 12 and 15 events respectively. Microearthquakes were recorded 26-31 April and 20-21 May, with 10 events/minute on 27 April.

Vegetation studies indicate that the post-l976 eruption is stronger than any in the last several hundred years at White Island (White Island 1976-82 Eruption [appendix by Clarkson and others]: New Zealand Geological Survey Bulletin, in press).

Geologic Background. Uninhabited 2 x 2.4 km White Island, one of New Zealand's most active volcanoes, is the emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes; the summit crater appears to be breached to the SE, because the shoreline corresponds to the level of several notches in the SE crater wall. Volckner Rocks, four sea stacks that are remnants of a lava dome, lie 5 km NNE. Intermittent moderate phreatomagmatic and strombolian eruptions have occurred throughout the short historical period beginning in 1826, but its activity also forms a prominent part of Maori legends. Formation of many new vents during the 19th and 20th centuries has produced rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project.

Information Contacts: I. Nairn and B. Scott, NZGS Rotorua; P. Otway, NZGS Wairakei.

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).