Explosions . . . on 30 November at 0830 and 1504 . . . occurred after 57 days of quiescence. The ash plume from the morning explosion was the highest of the month, rising more than 4,000 m. Strong winds carried substantial quantities of ejecta southward. Lapilli/block fall 3 km S of the crater (at Arimura) broke four house windowpanes and 13 car windshields. Weather clouds obscured the plume from the afternoon explosion, but tephra was again blown southward by strong winds, breaking five more house windowpanes and seven car windshields at Arimura, and two car windshields at Tarumizu City, 8 km SE of the crater. The year's previous damage from Sakura-jima's explosions was on 1 May (windows broken by an air shock) and 28 August (two car windshields broken by lapilli). At total of 107 grams/m² of ash were deposited [at the KLMO], down slightly from . . . October.

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 total of 158 recorded earthquakes in November represented a decline from 202 the previous month. Steam emission appeared unchanged, with plumes reaching 400 m height. Seismic activity was at similar levels in early December.

Geologic Background. Akan is a 13 x 24 km caldera located immediately SW of Kussharo caldera. The elongated, irregular outline of the caldera rim reflects its incremental formation during major explosive eruptions from the early to mid-Pleistocene. Growth of four post-caldera stratovolcanoes, three at the SW end of the caldera and the other at the NE side, has restricted the size of the caldera lake. Conical Oakandake was frequently active during the Holocene. The 1-km-wide Nakamachineshiri crater of Meakandake was formed during a major pumice-and-scoria eruption about 13,500 years ago. Within the Akan volcanic complex, only the Meakandake group, east of Lake Akan, has been historically active, producing mild phreatic eruptions since the beginning of the 19th century. Meakandake is composed of nine overlapping cones. The main cone of Meakandake proper has a triple crater at its summit. Historical eruptions at Meakandake have consisted of minor phreatic explosions, but four major magmatic eruptions including pyroclastic flows have occurred during the Holocene.

Information Contacts: JMA.

The Vanuatu arc was visited by an ORSTOM mission 5-18 September. The following is modified from their report in the LAVE Bulletin.

Thick puffs of ash rose several hundred meters, and scattered blocks were ejected, from a vent 200 m below the rim of Niri Tamo, which formed adjacent to Mbuelesu crater in 1989 (14:10). The approach to the crater was sprinkled with blocks 10 cm to 1 m in diameter. One block, 2 m in diameter, was located near a possible new crater (Niri Taten) that was S of Mbuelesu, near the source of the 1988 lava flows and [~500 m] from Niri Tamo. A zone of intense degassing, with temperatures of at least 625°C, occurred within Niri Taten. Mbuelesu appeared more elongate to the NE than represented on 1989 maps and no longer contained a lava lake. One vent, sounding like a reactor, violently emitted ash, gas, lava blocks, and fragments. The plume rose vertically at 30 m/s, and projectiles frequently landed beyond the rim of the crater. Benbow crater emitted a strong bluish plume, suggesting a significant SO₂ content.

Further Reference. Eissen, J.P., Monzier, M., Robin, C., Picard, C., and Douglas, C., 1990, Report on the volcanological field work on Ambrym and Tanna Islands (Vanuatu) from 2 to 25 September 1990: Rapport Missions Sci Terre Geologie-Geophysique – ORSTOM (Noumea), no. 22, p. 1-22.

Geologic Background. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides arc. A thick, almost exclusively pyroclastic sequence, initially dacitic, then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major plinian eruption with dacitic pyroclastic flows about 1900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the caldera floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: M. Lardy, ORSTOM, New Caledonia; B. Marty, CNRS, France; LAVE.

The 13 October blast of steam and mud roared continuously (like a geyser) for 10-20 minutes, decreasing in intensity following the initial explosion (reported by a survivor to be around 20 seconds long). No seismic signals were recorded before or during the event by seismometers 4 and 30 km away. A portable seismometer, operated for a few days following the blast, also recorded no signals.

The 30-m-diameter, 15-m-deep crater produced by the blast was partially filled by a continuously boiling muddy lake during a 27 October visit. A sulfurous "rotten egg" smell was noted. Prior to the blast, the site was an area of steaming ground, with two small hot springs (1-2 m across) and 1 mudpot (1 m across) much smaller than the present crater.

The blast was laterally oblique to the N and its effects abruptly ended at a maximum of 130 m. Damage included downed trees and limbs, collapsed walls of buildings, and missing roofs. The massive, non-sorted deposits were clay-rich and composed of light-colored highly altered rock fragments. Deposits were thickest to the N where they ranged from 1 m on the crater rim to 30 cm at 20 m from the rim. The death toll increased to 26 after 13 people died in hospitals.

Geologic Background. The Apaneca Range (also known as the Cuyanausul Range) consists of an elongated group of roughly E-W-trending basaltic-to-andesitic Pleistocene and Holocene stratovolcanoes in western El Salvador between the Santa Ana complex and the Guatemala border. The 5 x 3.5 km wide Pleistocene dacitic-rhyolitic Concepción de Ataco caldera lies at the western end of the complex, along with post-caldera late-Pleistocene to Holocene andesitic-dacitic lava domes. The post-caldera cones of Cerro el Aguila (the highest peak of the complex) and Cerro los Naranjos at the eastern end of the chain were mapped as Holocene by Weber and Weisemann (1978). Young craters on basaltic Laguna Verde stratovolcano may also have been active during the Holocene. Numerous fumarole fields are located on the N flank of the range, and the Ahuachapán geothermal field has been producing since 1975. Several small hydrothermal explosions have occurred in historical time, including one in October 1990 at the Agua Shuca thermal area in which 26 people were killed.

Information Contacts: C. Dan Miller, USGS.

Strombolian activity, with small explosions, lava extrusion, and voluminous gas emission, continued during November. The following is a report by W. Melson.

"The volcano was continuously monitored, seismically and by direct observations from the Arenal Observatory 24-27 November. During this period, lava flow emission was continuous down the S and SW slopes, none reaching farther than 300 m below the summit (~1,300 m elevation). The volcano was usually obscured by clouds, but changing rates of flow advance were evidenced by the frequency of audible and sometimes visible avalanches that ranged from ~1-20/hour from the lava fronts on the oversteepened near-crater slopes. We did not determine whether flows were simultaneously active on the N and NW slopes. No significant pyroclastic events, such as explosions, occurred during this interval.

"Seismicity was marked by frequent intervals of intense harmonic tremor. Digital seismic recordings (figure 34) using a 3-component Mark L-4 3D geophone revealed the dominance of E-W horizontal motions, typically at 1-2 Hz, accompanied by very little vertical motion, and sometimes completely without vertical motion. The motion may reflect sub-horizontal flow of the viscous andesitic magma in an E-W subvolcanic conduit."

Figure 34. Three-component seismogram of harmonic tremor at Arenal, November 1990. Courtesy of W. Melson.

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: G. Soto and R. Barquero, ICE.

The strong seismicity . . . has declined since late October. Only 27 earthquakes and one tremor episode were recorded in November, compared to 105 and 31 respectively in October. Similar activity was noted in early December.

Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the horseshoe-shaped remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asama-yama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has an historical record dating back at least to the 11th century CE. Maekake has had several major plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

Information Contacts: JMA.

Crater 1, active July 1989-June 1990 (table 6), weakly emitted ash on 12, 18-19, and 25-29 November; white steam was emitted steadily on other days. The highest plume observed in November reached 1,000 m above the crater. Ash had last been emitted on 17 September. The area within 1 km of the crater, which had reopened to tourists on 15 October, was closed on 12 November and remained closed in early December.

Table 6. Brief chronology of activity at Aso, January-14 December, 1990.

Glow from many points on the crater floor was observed during a night visit on 13 November, the first crater glow seen since June. Glow remained visible through early December. During 17 November fieldwork, incandescent scoria was being ejected to 30 m height from a small vent on the crater floor. Scoria ejection had last been observed in June. By the 24 November crater visit, a vent 10 m across had developed on the crater floor and was ejecting blocks to 5 m height. The vent was named 902, the second new vent of 1990 . . . . An infrared thermometer detected a maximum temperature of 811°C in the vent.

Ash emission became frequent in early December. An eruption on 4 December at 1410 ejected a 1200-m ash cloud, December's highest (as of the 14th), and similar activity occurred on 6, 7, 8, and 13 December. Vigorous ash emissions had last occurred in June. Ejections of blocks and scoria were also more frequent and higher (to 150 m) in early December. A visit on 6 December revealed that a new vent . . . had opened near 902.

The amplitude and number of volcanic tremor episodes has gradually increased since October and remained high through November (figure 18).

Figure 18. Daily number of tremor events at Aso, January-8 December 1990. Longer arrows at top of figure mark eruptions, shorter arrows indicate weaker ash emissions. 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 cu km 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 AD. 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.

"Recent reports of increased activity at El Chichón motivated a visit to the region. Analysis of records from nearby seismic stations (the nearest at Ostoacán, 115 km NW) showed no increased seismicity. A helicopter flight over the volcano on 10 November allowed observation of hydrothermal activity that had increased from last year's levels. The crater lake, which in the last few years had shrunk to a small pond, had recovered to the level of November 1982, and hydrothermal activity was similar to that observed in January 1983 (as described in Casadevall and others, 1984). The greenish-yellow lake now covers most of the crater floor, and numerous small fumaroles release steam through and around it. This activity is probably a result of the increased rainfall that reached 3,829 mm in the area January-October, compared to 3,219 mm measured during the same period last year. This increase was particularly high in October 1990, when 1,069 mm of rain was reported, compared to 701 mm measured in October 1989. As expected, no evidence of the start of a dome growth episode has been detected."

Reference. Casadevall, T., de la Cruz-Reyna, S., Rose, W.I., Bagley, S., Finnegan, D.L., and Zoller, W.H., 1984, Crater lake and post-eruption hydrothermal activity, El Chichón Volcano, México: JVGR, v. 23, p. 169-191.

Geologic Background. El Chichón is a small, but powerful trachyandesitic tuff cone and lava dome complex that occupies an isolated part of the Chiapas region in SE México far from other Holocene volcanoes. Prior to 1982, this relatively unknown volcano was heavily forested and of no greater height than adjacent nonvolcanic peaks. The largest dome, the former summit of the volcano, was constructed within a 1.6 x 2 km summit crater created about 220,000 years ago. Two other large craters are located on the SW and SE flanks; a lava dome fills the SW crater, and an older dome is located on the NW flank. More than ten large explosive eruptions have occurred since the mid-Holocene. The powerful 1982 explosive eruptions of high-sulfur, anhydrite-bearing magma destroyed the summit lava dome and were accompanied by pyroclastic flows and surges that devastated an area extending about 8 km around the volcano. The eruptions created a new 1-km-wide, 300-m-deep crater that now contains an acidic crater lake.

Information Contacts: S. de la Cruz-Reyna, UNAM.; Romeo León Vidal, CFE, Tuxtla Gutiérrez, Chiapas, México.

Seismicity declined rapidly after . . . 4 October (figure 22). No additional eruptions had occurred as of early December. Steam emission continued steadily through November, with the plume reaching 1,300 m above the crater. A series of 10 microearthquakes, centered in the E part of Oshima Island 3 km E of the summit (Mihara-yama) cone occurred 7-10 November, the first seismicity there since 21 November 1987. Seismicity at the summit continued unchanged through November at relatively low levels. A seismometer near the summit recorded 160 earthquakes during November, down from 633 in October. Seismicity and steam emission remained similar in early December. No tremor has been recorded since late April.

Figure 22. Daily number of earthquakes at Oshima, April-November 1990. The small 4 October eruption (arrow) was accompanied by high seismicity. Most of the earthquakes were centered on the summit (Mihara-yama) cone. Courtesy of JMA.

Geologic Background. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island and was constructed over the remnants of three dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous historical eruptions. More than 40 parasitic cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. Historical activity dates back to the 7th century CE. A major eruption in 1986 produced spectacular lava fountains up to 1600 m height and a 16-km-high subplinian eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: JMA.

Lava . . . continued to flow into the ocean through November. Coastal activity was generally most vigorous on the W side of the flow field (near Wahaula) . . . . Lava feeding the W ocean entry was enclosed in tubes at the beginning of the month, but by 5 November, lava had broken out at ~50 m elevation and the flow volumes at the coast had declined. Lava re-entered the sea near Wahaula on 8 November and entries were dispersed along a 700-m front by the 11th. During the following weeks, several breakouts were active behind the flow front, but lava continued to enter the ocean, and at least three of the entries were explosive late in the month.

On the E side of the flow field (figure 74), small amounts of lava entered the ocean (at Hakuma Point), but breakouts from the tube system were frequent at low elevations (in the Kalapana area). On 10 November, lava destroyed a home (at the base of the Hakuma Horst). A second, slow-moving flow advanced to within 30 m of the four remaining houses in upper Kalapana Gardens, but did not reach them. Lava outbreaks overran new land in the Kalapana area in mid and late November, covering parts of the old coast highway and cutting off a temporary access road built over the lava in September, but did not destroy any additional houses.

Figure 74. The extent of flows on the E side of the lava field as of 22 November 1990.

Harmonic tremor near Kupaianaha and Pu`u `O`o vents continued through November, but at low levels. An earthquake swarm centered ~5 km W of the caldera occurred between 11 and 13 November. Activity spread over a 5-km-long region at the E end of the Kaoiki fault zone. Over 200 swarm events with magnitudes of up to 3.3 were located during the 3-day period. Hypocentral depths ranged from very close to the surface to 12 km. HVO's preliminary locations suggest that the deeper earthquakes clustered at the E end of the active zone, with shallower events more concentrated to the W. Activity returned to normal regional levels of ~20-30 events/day by 15 November.

Geologic Background. Kilauea volcano, which overlaps the east 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 sq km, destroying nearly 200 houses and adding new coastline to the island.

Information Contacts: T. Moulds, P. Okubo, and C. Heliker, HVO.

Seismicity has remained at high levels since February (figure 4). During November, 117 earthquakes and 27 volcanic tremor episodes were recorded . . . . Seismicity remained similar in early December. No changes in surface activity were observed.

Geologic Background. The summit of Kusatsu-Shiranesan volcano, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the east, south and SW flanks. The latest eruptive stage began about 14,000 years ago. All historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

Information Contacts: JMA.

"Activity remained at a low level in November . . . . Crater 3 emitted mainly weak to moderate white-grey ash and vapour clouds. However, stronger emissions on 12 and 25 November produced eruption columns ~200 m high and ashfalls ~10 km downwind. Weak deep explosions from Crater 3 were heard on 28 and 29 November. Emissions from Crater 2 consisted mainly of weak to moderate white and grey vapour and ash, and rarely, blue vapour. Steady weak glow was observed throughout the month. The only sound from this crater during November was a deep loud explosion on 28 November and rumbling noises on the 29th and 30th. Seismic activity was at a moderate level."

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: C. McKee and I. Itikarai, RVO.

Oblique air photos taken by Steve Cunningham . . . on 7 September (figure 19), showed minor changes in crater activity since a 7-8 August crater visit. There was no sign of very fresh lava on T4/T7, . . . although the W end of the ridge was surrounded by fresh lava. A large dark-gray patch of lava extended from the top to near the base of the largest cone in the crater, T5/T9, suggesting recent spatter or minor overflow of lava from its summit vent. T15 . . . continued to erupt, as indicated by the presence of dark gray lava and a possible small fresh flow just N of it. Most of the crater floor was covered by older pale gray to white lava, including a large flow active 7-8 August (F18). Two new flows (F19 and F20 in figure 19) were visible, the longer extending 200 m across the crater floor and into the S depression. It was estimated that this flow was 1-2 days old at the time of the photograph.

Figure 19. Active crater at Ol Doinyo Lengai, 7 September 1990, looking NE. Shaded areas show fresh lava flows. Tracing of photo courtesy of C. Nyamweru.

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.

"Activity remained at a low level in November. Both craters continued to release very weak to moderate emissions of thin white vapour. Additionally, thin blue vapour emissions were observed on two occasions from Main Crater, and brown ash emissions were reported once from Southern Crater. No noises or glow were reported from either crater. The daily total of volcanic earthquakes declined further from an average of ~150 at the end of October to <30 from mid-November to the end of the month. Amplitude of these events was very low."

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: C. McKee and I. Itikarai, RVO.

Research continues at Lake Nyos and on the broader problem of gas-charged lakes. Quoted material is from William Evans.

"The International Working Group on Crater Lakes (IWGCL) met 11-12 September in Nancy, France to discuss the hazards associated with Lake Nyos. Scientific teams from Cameroon, France, Germany, Japan, Nigeria, Switzerland, the UK, and USA attended the meeting, held at the 15th Colloquium on African Geology. The goal of the meeting was to prepare a document containing conclusions and recommendations regarding Lake Nyos that could be presented to the government of Cameroon. Each IWGCL member present could prevent the use of any statement in the document. At the conclusion of the meeting, there was unanimous agreement on the wording to be used. The following is a complete list of the conclusions and recommendations drawn up at the meeting."

Conclusions. 1) The 1986 gas disaster in Cameroon was caused by a massive release of CO₂ from Lake Nyos. 2) Lake Nyos now contains ~300 x 10⁶ m³ of CO₂ and therefore remains dangerous. 3) This danger is increasing because CO₂ is currently being added to Lake Nyos at a rate of at least 5 x 10⁶ m³/year.

Recommendations. 1) Another gas disaster could occur at any time. Therefore the amount of CO₂ in lake Nyos should be reduced as a matter of urgency. 2) Prior to controlled degassing, equipment should be installed for continuous monitoring of the stability of the lake. 3) Pipes should be installed to remove gas-rich water from the bottom of the lake. 4) Degassed water should be discharged outside the lake basin to avoid disturbing the natural stratification of the lake. 5) The rate of water extraction should not exceed the natural water recharge rate. Lowering the lake level would increase the risk of a gas release from beneath the lake. 6) Any system for gas extraction should be tested first at Lake Monoun. 7) The stability of the natural dam at the exit to Lake Nyos and the possibility of lowering the lake level should be further investigated. [A modified version of this document was published as Freeth and others, 1990].

"There is continued interest in the problems associated with Lake Nyos and its smaller analog, Lake Monoun. Cameroon, Nigeria, and several other countries maintain long-term research efforts. Scientists from the U.S. have made three field trips to Lake Nyos in . . . December 1989 and September and December 1990, to study the stability of the natural dam and the recharge rates of water and gas. The dam, which forms a spillway for the rainy season outflow, is 43 m wide at the top but is undercut several meters at its base. It maintains lake level at ~36 m above bedrock on the downstream side. The current rate of erosion due to water flow over and through the dam is unknown, but large-scale flooding and loss of life could occur when the structure ultimately fails. Dissolved salts and gases are transported into the lake by slightly thermal soda springs on the lake bottom at 210 m depth. Because of strong stratification of the water column, the incoming fluids are trapped in the deepest water layers. Studying the recharge process thus involves measuring temporal changes in bottom waters after the August 1986 gas burst when a partial mixing of the water column occurred. Near 200 m depth, water temperature has increased to 24.5°C, 1.0°C higher than in September 1986. Ionic strength has now risen from 0.015 to 0.024 at this depth. Total dissolved gas pressures, due mainly to CO₂, have also increased. The highest pressure measured, 10.6 bar at 206 m depth, is almost 50% of the saturation pressure. When completely analyzed, these data will refine existing estimates of the various recharge rates."

Further References. Freeth, S., Kling, G., Kusakabe, M., Maley, J., Tchoua, F., and Tietze, K., 1990, Conclusions from Lake Nyos disaster: Nature, v. 348, no. 6298, p. 201.

LeGuern, F. and Sigvaldason, G., eds., 1989, The Lake Nyos event and natural CO₂ degassing, I: JVGR, v. 39, nos. 2-3, p. 97-275 (15 papers); II: JVGR, v. 42, no. 4, p. 307-400 (10 papers).

Lockwood, J., Costa, J., Tuttle, M., Nni, J., and Tebor, S., 1988, The potential for catastrophic dam failure at Lake Nyos Maar, Cameroon: BV, v. 50, p. 340-349.

Nojiri, Y., Kusakabe, M., Hirabayashi, J., Sato, H., Sano, Y., Shinohara, H., Njine, T., and Tanyileke, G., 1990, Gas discharge at Lake Nyos: Nature, v. 346, no. 6282, p. 322-323.

Sano, Y., Kusakabe, M., Hirabayashi, J., Nojiri, Y., Shinohara, H., Njine, T., and Tanyileke, G., 1990, Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst: Earth & Planetary Science Letters, v. 99, p. 303-314.

Geologic Background. Numerous maars and basaltic cinder cones lie on or near the deeply dissected rhyolitic and trachytic Mount Oku massif along the Cameroon volcanic line. The Mount Oku stratovolcano is cut by a large caldera. The Oku volcanic field is noted for two crater lakes, Lake Nyos to the N and Lake Monoun to the S, that have produced catastrophic carbon-dioxide gas release events. The 15 August 1984, gas release at Lake Monoun was attributed to overturn of stratified lake water, triggered by an earthquake and landslide. The Lake Nyos event on 21 August 1986, caused at least 1,700 fatalities. The emission of ~1 km3 of magmatic carbon dioxide has been attributed either to overturn of stratified lake waters as a result of a non-volcanic process, or to phreatic explosions or injection of hot gas into the lake.

Information Contacts: (Conclusions and Recommendations document) M. Kusakabe, IWGCL, Japan; (Fieldwork) W. Evans, USGS; G. Kling, The Ecosystems Center, Woods Hole, MA; J. Lockwood, R. Schuster, and M. Tuttle, USGS.

Fieldwork was conducted at the volcano by a group from the Univ de Genève (mid October-4 November), and scientists from Michigan Technological Univ and Guatemala (25 November).

The following is modified from a Univ de Genève report. Strombolian activity increased during October, to >400 recorded explosions/day. Scientists visting the volcano observed explosions every 30 seconds to 5 minutes (17 and 21 October), and counted up to 17 explosions during a 15-minute period (28 October). The explosions ejected incandescent material to 10-50 m above the 25-m-tall cone in MacKenney crater, and were visible on clear nights from Guatemala City (25 km NNE). The number of seismic events (recorded by a joint INSIVUMEH-Univ de Genève digital seismic station) increased steadily, from 147 on 13 October to 457 on 20 October, and averaged over 450 daily during the following several days. Tremor was recorded during periods of closely spaced explosions (including the 28th), with 5 hours of continuous tremor recorded on the 20th. As of 4 November, two lava flows were moving down the N flank of the volcano, and explosive activity was unchanged.

The following report is by Michael Conway. "Vertical Strombolian eruptions (lasting from seconds to <1 minute) from the crater of MacKenney cone occurred every 3-5 minutes, hurled incandescent bombs (to 2 m in diameter) to 150 m above the vent, and were accompanied by a jet-like sound. Black eruption clouds, with the coxcomb geometry characteristic of phreatomagmatic blasts, were rarely observed, suggesting that the conduit is moderately well sealed from infiltrating meteoric waters. Low-temperature fumaroles (up to 100°C) were active on the E summit of MacKenney cone, and patches (areas of one to tens of m²) of yellow sublimates were common.

"An aa lava flow was being extruded from a pit crater (50 m long, 10-15 m wide, and 2-5 m deep) on the N flank of MacKenney cone, 40-65 m below the summit; opening of the pit crater may be related to an eruptive episode that occurred on 16 September. A lava channel, 5 m wide and 2-4 m deep, delivered lava to the low-lying area between MacKenney cone and Cerro Chino. The lava flux was variable and flow velocities ranged, roughly, from 1 to 6 km/hour. Collapse of the lava flow front was common (slopes of the cone are about 30°), exposing fresh lava and sending hot block avalanches down the channel. The latest stage of lava flow activity began on 3 November and has erupted two flow units, one of which was still active. Lava flows erupted since March have effectively armored the N-central flank of the cone. A result of continued construction of MacKenney cone is that, for the first time, the cone is visible from San Francisco de Salas (2.5 km downslope, to the N)."

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: Jean-Jacques Wagner, Thierry Basset, and Jean-Charles Gentile, Univ de Genève, Switzerland; Michael Conway, Michigan Technological Univ; Ricardo Mata, Guatemala City, Guatemala; E. Sánchez and Otoniel Matías, INSIVUMEH.

Fumarolic activity continued during November, with the greatest activity concentrated in the SE and N parts of the yellow-green crater lake. Fumaroles created mud pots, and small sprays of mud and sulfur constructed cones within the lake. The temperature of the lake oscillated between 61 and 85°C (maximum 80°C in October), and lake level was relatively high due to heavy rainfall. Hot springs at the periphery of the lake had temperatures of 40°C, cold springs at the edge of the crater were 16°C, and fumaroles on the 1953-55 dome were 90°C.

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: G. Soto and R. Barquero, ICE.

"Seismicity remained at a low level in November. The total number of caldera earthquakes increased slightly to 160, from 101 in October. All events were of small magnitude (M_L <1). Of the three events that could be located, two were on the NW side and one was on the NE side of the caldera seismic zone. No significant changes were observed in ground deformation measurements."

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: C. McKee and I. Itikarai, RVO.

The following report, from the AVO staff, covers the period 12 November-14 December. "No significant change in activity has been observed at Redoubt since the last report. Steam and ash emissions from the lava dome, detected seismically, continue at a rate of several/day. No reports of ash deposition on the flanks of the volcano have been received, suggesting that these phreatic events continue to be relatively minor. However, decreasing daylight hours and infrequent overflights have limited observations.

"Overall seismicity at Redoubt is at a low level, and continues to decline slowly. The automatic event detection system triggered on 246 events between 12 November and 14 December (about 8 events/day). These are mostly shallow B-type (low-frequency) events in the vicinity of the lava dome, although a few A-type (high-frequency) events daily continue at 4-10 km depth beneath the volcano. Shallow tremor associated with steam and ash emissions continues to be recorded several times/day, with typical durations ranging from 5 to 20 minutes; occasionally, periods of nearly continuous tremor may last several hours. These longer periods appear, in some cases, to be 'triggered' by one of the steam and ash events.

"AVO field crews observed the partly snow-covered lava dome on 14 December, and reported that it continued to steam vigorously, as did some of the pyroclastic flow deposits at the N base of the volcano. A minor amount of ash mantled the dome's surface. No significant ash deposits were seen elsewhere in the crater or on the volcano's flanks.

"COSPEC measurements have not been made at Redoubt since 8 November. Insufficient ultraviolet light precludes operation of the COSPEC until sometime in February."

Geologic Background. Redoubt is a 3108-m-high glacier-covered stratovolcano with a breached summit crater in Lake Clark National Park about 170 km SW of Anchorage. Next to Mount Spurr, Redoubt has been the most active Holocene volcano in the upper Cook Inlet. The volcano was constructed beginning about 890,000 years ago over Mesozoic granitic rocks of the Alaska-Aleutian Range batholith. Collapse of the summit of Redoubt 10,500-13,000 years ago produced a major debris avalanche that reached Cook Inlet. Holocene activity has included the emplacement of a large debris avalanche and clay-rich lahars that dammed Lake Crescent on the south side and reached Cook Inlet about 3500 years ago. Eruptions during the past few centuries have affected only the Drift River drainage on the north. Historical eruptions have originated from a vent at the north end of the 1.8-km-wide breached summit crater. The 1989-90 eruption of Redoubt had severe economic impact on the Cook Inlet region and affected air traffic far beyond the volcano.

Information Contacts: AVO Staff.

Many small ash emissions occurred and high-frequency seismicity was at high levels during November. Hypocenters were located around the crater at shallow depths. Pulses of tremor occurred frequently, often associated with the ash emissions. Low-frequency seismicity was at low levels and there was no measured ground deformation. The SO₂ flux continued to decrease, averaging 860 t/d.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers >200 sq km. 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.

"Interpretation of telemetered seismic data by volcanologists at INSIVUMEH indicates a general increase in volcanic activity (pyroclastic eruptions, rock avalanches, and lava flows) at Caliente vent from June 1988 through August 1990 (figure 15). Five periods of increased lava flow activity have been documented, the most recent beginning in July 1990 (BGVN 15:06) and continuing as of early December. The number of explosions ranges from about 5 to 90 daily, while rock avalanches are more abundant, with 100 to as many as 600/day. Explosions, rock avalanches, and lava flow flux at the dome were greatest from June through September 1988, 1989, and 1990, corresponding to the rainy season. Small decreases in explosions and avalanches were noted during mid-October through March 1988-89, 1989-90, and from October through November 1990, and are roughly correlative with the dry season in Guatemala, suggesting a link between eruptive and climatic patterns at Santiaguito.

Figure 15. Appoximate number of daily explosions (solid line) and rock avalanches (dashed line) recorded by seismic stations near Santiaguito, June 1988-23 November 1990. Five periods of relatively high lava flow flux are marked by horizontal arrows. Vertical arrows mark dates of major pyroclastic eruptions. [Courtesy of Otoniel Matías.]

"Beginning in April 1990, more than 20 powerful pyroclastic eruptions, similar in magnitude to the 19 July 1989 eruption, have occurred at Caliente vent (table 1). Direct observation of pyroclastic eruptions is often impossible because of weather conditions, but reports from four events indicate that they are characterized by large eruption columns rising 4-5.5 km above the vent, durations on the order of 7-15 minutes, and are heard as far away as Retalhuleu [25 km SSW]. Simultaneous collapse of a small plug dome atop Caliente generates pyroclastic flows and lateral blasts. Block and ash flows accompanied by ash cloud surges typically sweep 4-7 km down the Río Nimá II (figure 16); phreatic blasts in nearby drainages are common during violent mixing of hot pyroclastic flows with stream water. Repeated lateral blasts have devastated an area of 4 km² E of Caliente, stripped away or blown down all vegetation, and buried it in ash and lapilli-sized debris. On 19 July 1990, exactly 1 year after the onset of major pyroclastic eruptions at the dome, 4 hikers climbing along the E rim of Santa María's 1902 explosion crater, roughly 1 km E of the dome, were killed by a lateral blast. Tephra fallout (to 4 cm thick) blanketed the dome and surrounding area, and measurable airfall deposits (<1 cm thick) occurred as far away as San Martín, 20 km SW of the dome. Numerous smaller explosions accompanied major explosions at Caliente, and continuous explosive activity of up to 3 hours has been reported.

Table 1. Dates and intervals between major pyroclastic eruptions at Santiaguito Dome, July 1989-November 1990. Courtesy of Michael Conway.

      Date             Days since last eruption

    18 July (1989)
    20 April (1990)
    22 April                      1
    5 May                         12
    8 May                         2
    9 May                         0
    3 June                        24
    19 June                       5
    13 July                       23
    19 July                       5
    29 July                       9
    5 August                      6
    18 August                     12
    22 August                     3
    26 August                     3
    22 September                  26
    4 October                     11
    26 October                    21
    14 November                   18
    20 November                   5
    23 November                   2

Figure 16. Map of Santiaguito and environs showing zones affected by the 1929, 1973, and 1989-90 pyroclastic flows. The zones affected in 1989-90 are marked by vertical lines (devastation by lateral blasts), and diagonal lines (area affected by pyroclastic flows and ash cloud surges). The Santiaguito Observatory is marked by a star. Courtesy of Michael Conway.

"Periods between major explosions have been characterized by passive fuming of Caliente and by minor phreatomagmatic and possibly phreatic vertical explosions. On the morning of 28 November, from the 'Hotel de Magermann', NW of the dome, we observed a series of 15-20 small explosions; each was accompanied by a gray to white steam and ash column, rising 1.5-3 km above Caliente. Individual explosions were accompanied by a jet-like sound and lasted anywhere from a few seconds to 2-3 minutes. Passive fuming preceded and followed each blast.

"Since July, a viscous block lava flow, fed by a plug dome on Caliente, has advanced down the E side of the dome, and recently entered the headwater extension of the Río Nimá II system (figure 17). The flow is roughly 2 km long, 30-50 m wide, and 15-20 m high; a rough estimate of the average extrusion rate is 7,500 m³/day. Collapse of the lava flow front occurs frequently, and small-volume block-and-ash avalanches are common events. Merapi-type block and ash flows are less common and travel between 2 and 3 km down the Río Nimá II.

Figure 17. Simplified geologic map of Santiaguito Dome, 1922-November 1990. Streams near Santiaguito are approximately located. Unit dates, such as Rc (1922-90), represent periods of discontinuous activity at each vent. Patterned areas represent very recent activity: Rl - area of active laharic and stream deposition, and very high aggradation rates; Rd - area of recently initiated extensive mass wasting possibly indicating inflation of the El Monje vent and potential reactivation; Rc (v pattern) - active block lava flow on Caliente's summit, with very common (hourly) collapse of the broad toe resulting in hot rock avalanches; Rc (dotted pattern) - extent of the 1986-88 block lava flow from Caliente. Lava flows erupted since July 1990 are shown by diagonal and horizontal line patterns; the S-most unit, extending into the Río Nimá II drainage, was active as of 28 November. Courtesy of Michael Conway.

"Lahars originating at Santiaguito, common during the past rainy season, extended S down the Río Nimá II to its confluence with the Río Samala, and continued for up to 50 km from the dome (figure 18). Diversion of lahars from the Río Samala into the Río Ixpatz occurred as it has in every year since 1983. Hot lahars (temperatures to 45°C were measured 25 km S of the dome) were observed and occurred hours to days after a major pyroclastic eruption. A particularly large lahar on 16 September destroyed the pedestrian bridge at El Palmar, forcing people of the surrounding area to ford the river on foot - a particularly hazardous endeavor during the rainy season. Rapid aggradation from lahars and hyperconcentrated floods continues in the Río Nimá and Río Samala systems.

Figure 18. Sketch map of rivers and towns S of Santiaguito. Locations of drainages are approximate. Areas affected by pyroclastic flows, lahars, and hyperconcentrated floods are marked. Field studies during the 1990 rainy season indicate four zones with distinct hydraulic characteristics. [Courtesy of O. Matías.]

"In order to monitor activity better at Santiaguito, INSIVUMEH and Centro de Prevención de Desastres Naturales en América Central (CEPREDENAC) have constructed a permanent observatory at Finca El Faro, 7 km S of the dome (figure 16). The observatory opened in the second week of November and will be manned around-the-clock, by trained observers. Equipment at the observatory includes: a paired seismometer-seismograph, seismographs for two outlying seismometers; deformation and survey equipment; and hand-held radios and radio-telephone equipment. A key function of the observatory is to act as a training post for geoscientists, and at present 25 geoscientists from throughout Central America are receiving training in seismology, deformation, and volcanic hazards at Santiaguito." [The following originally appeared in BGVN 16:02] The building site was donated by the owners of Finca El Faro and construction costs were paid by the government of Sweden through CEPREDENAC. The facility has laboratory space and a small dormitory, and is intended as a base of operations for volcanologists to work with local scientists at Santiaguito (through INSIVUMEH and other agencies).

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, INSIVUMEH; Michael Conway, Michigan Tech.

A small explosion occurred from the lava dome on 20 December at 1259. The explosion was marked by a small seismic signal that decreased to low levels after several minutes, but continued for several hours. Airplane pilots reported a light gray plume to as much as 7.5 km altitude that was carried SSW by strong winds. A diffuse plume was first evident on satellite images at 1320, moving SSW at ~30 km/hour. By 1600, the plume could no longer be detected on satellite imagery. Light ashfall was reported to 15 km SW of the volcano. No mudflow or water flow event was detected.

Geologic Background. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.

Information Contacts: CVO; SAB.

Although the eruption was apparently somewhat less vigorous in late October, activity observed by Boris Behncke on 7-8 November was stronger than during his previous visits in September 1989 and March-April 1990. Vigorous gas emission fed a dense plume that obscured the vent area during the day, but visibility improved after sunset and a clear view of the craters was possible after 2000 on 7 November. Although vent morphology had changed somewhat, vent configuration was much the same as in April.

The main focus of activity was a cluster of at least four vents in C1 (at the NE end of the summit crater group) that were almost continuously erupting. Intense bomb and spatter ejection started at 1717 on 7 November and continued for at least 2 hours, with loud roaring like a jet aircraft. The strongest eruptions occurred from C1's easternmost vent (1), site of nearly continuous bomb ejections in early October. On 7 November, vent 1 ejected lava fountains to 100 m height, often followed within seconds by eruptions from vent 3, the southernmost vent in C1. Individual bursts of spatter, mostly from vent 3, were accompanied by loud explosions; gas had been emitted from this vent nearly once a second in early October. Between stronger bursts, very small lava fountains were continuously active within vents 2 and 3. Spatter was ejected to ~20 m height every 10-20 seconds from vent 3 and another vent to the NE. At 0100 on 8 November, fountains rose 40-50 m from the latter vent, and loud roaring was continuing. None of the vents produced ash plumes after 1800 on 7 November. The former vent at the NE end of C1 had apparently ceased erupting and may have been buried by the growing cone at vent 1, a prominent feature that had been too small to be visible from the summit in April.

Eruptive episodes from C3 (at the SW end of the summit crater group) occurred about twice an hour, producing lava fountains that rose as much as 100 m, and sometimes diffuse brown ash plumes and light tephra falls onto the summit platform. Most episodes consisted of several pulses of fountaining over a period of ~30 seconds. A strong eruptive episode at 1710 on 7 November was followed by bursts of spatter at intervals of 10-20 seconds until 1717. Another particularly violent burst at 2000 covered most of the crater area with glowing bombs and spatter. An area of 3 pits in C3 that had contained actively degassing lava in April was occupied by two small (<1 m diameter) vents that emitted low fountains of spatter. Much of C3 had been filled with recent pyroclastics.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small, 924-m-high island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period from about 13,000 to 5000 years ago was followed by formation of the modern Stromboli edifice. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5000 years ago as a result of the most recent of a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: B. Behncke, Ruhr Univ, Germany.

The following supplements the preliminary report of 17 November activity in 15:10.

Ground and aerial surveys [after the eruption began] by JMA and Kyushu Univ revealed that steam plumes were being continuously erupted from two new craters ~650 m E of the summit (Fugen-dake). One was at the E end of Jigoku-ato crater, the other was 100 m S of Jigoku-ato at the E end of Tsukumo-jima crater; the older craters were larger than the new active vents. The steam plumes were about 300-400 m high, occasionally containing ash on 17-18 November; weak ashfall was noted downwind. No explosion sounds were heard and no clear shocks were recorded. The amplitude of continuous tremor gradually declined, and tremor faded away at around 1900. Steam emission from the vent at Jigoku-ato crater stopped on 18 November, but a steam plume continued to rise from the vent at Tsukumo-jima crater through early December, gradually declining to a few tens of meters in height (figure 7). No damage was reported but a ropeway and an area within 2 km of the active vents were closed to tourists on the morning of 17 November.

Figure 7. Estimated plume heights from Unzen, 17 November-10 December 1990. Courtesy of JMA.

Earthquakes and tremor episodes had begun in July, with the strongest burst of activity on 25-26 July, and continued at relatively high levels September-November (tables 2 and 3). Seismic activity was almost unchanged before and after the eruption, although seismicity was somewhat stronger in November than during the three previous months. Earthquake swarms occurred at the volcano on 5, 13, 20, and 23 November, contributing to the month's total of 843 recorded events, up from 549 in October and 248 in September. Eighteen shocks were felt in November [at UWS], compared to 15 in October and two in September. The distribution of earthquakes (figure 8) was unchanged from October; most were about 5 km W of the summit at shallow depth. Normal seismicity continued weakly in the Tachibana Bay area about 15 km W of the summit. Few tremor episodes were recorded in the first half of November, but tremor increased in the second half of the month to levels similar to active periods in previous months. Tremor episodes totaled 46 in November, down from 81 in October, while amplitudes were similar to those of previous months. Earthquakes declined in early December, and as of 14 December, no tremor episodes had been recorded since 27 November.

Table 2. Earthquake swarms at Unzen, 1990. Courtesy of JMA.

    Date (1990)      Recorded Earthquakes    Felt Earthquakes

    07 Jul-08 Jul            35                      2
    24 Jul-25 Jul           432                     26
    09 Aug-11 Aug           108                      3
    17 Oct                   56                      0
    23 Oct                  112                      8
    31 Oct                  115                      2
    05 Nov-06 Nov            87                      0
    13 Nov                   76                      4
    20 Nov                  134                     11
    23 Nov                  136                      1

Table 3. Monthly number of earthquakes and tremor episodes at Unzen, 1990. December data are through the 14th. Courtesy of JMA.

    Month      Recorded       Felt      Tremor
    (1990)    Earthquakes    Events    Episodes

    Jan           74            0          0
    Feb           53            1          0
    Mar           66            0          0
    Apr          162            1          0
    May          149            2          0
    Jun          208            1          0
    Jul          922           34         11
    Aug          345            6         56
    Sep          248            2         45
    Oct          549           15         81
    Nov          843           18         46
    (Dec)       (140)          (2)        (0)

Figure 8. Earthquake epicenters in the vicinity of Unzen, July-November 1990. Seismicity typically occurs in Tachibana Bay, but began W of the summit (Fugen-dake) in July. Dots near Fugen-dake mark the 17 November eruption vents. Courtesy of JMA.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. A 30-40-km-long, E-W-trending graben extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: JMA.

The Vanuatu arc was visited by an ORSTOM mission 5-18 September. The following is modified from their report in the LAVE Bulletin.

Volcanic activity, consisting of block and ash emissions, and bubbling lava lakes, seemed slightly decreased since visits during 1988. The configuration of the main crater and its three principal sub-craters (A,B,C; figure 1) remained relatively unchanged. The depth from the summit to the base of the crater was estimated at >350 m, placing activity at or below sea level.

Figure 1. Sketch map of Yasur showing the locations of the principal craters and sub-craters (from Nairn and others, 1988).

In sub-crater A, a new lava lake (~20-25 m in length) was visible; strong turbulence in the lake due to rising gas bubbles caused lava to move N-S. Explosions at other vents (notably one in the S part of sub-crater B) corresponded with increased intensity of lava lake activity. Two other vents in sub-crater A had explosions that ejected ash and incandescent blocks. The blocks had loud detonations on impact.

Projectiles and night glow were visible from a lava lake in sub-crater B, hidden from view by a ridge. Explosions were identified from at least three vents, with frequencies of 1 explosion/5 minutes to 1/hour. Sub-crater C was less active, occasionally emitting puffs of ash or gas following explosions in sub-crater B. There were no visible shock waves or ejecta being deposited outside of the crater, as there were in 1988, suggesting a decrease in the intensity of activity.

Concentrations of 5-10 ppm SO₂ were measured in the plume, 1 ppm from the ash plain below the plume, and 0.5 ppm, 3 km from the volcano. The SO₂ flux was estimated to be 1,200 ± 600 t/d, based on the measured concentrations and a visual estimate of the plume volume. This is greater than the flux usually registered at other volcanoes in the Vanuatu arc (100-600 t/d). During 1987-88, vegetation in areas downwind from the volcano was affected by gas, ash, and acid rain, causing damage to gardens and coffee plantings.

Reference. Nairn, I.A., Scott, B.J., and Giggenbach, W.F., 1988, Yasur volcano investigations, Vanuatu, Sept. 1988: New Zealand Geological Survey Report, no. G134, 74 p.

Further Reference. Eissen, J.P., Monzier, M., Robin, C., Picard, C., and Douglas, C., 1990, Report on the volcanological field work on Ambrym and Tanna Islands (Vanuatu) from 2 to 25 September 1990: Rapport Missions Sci Terre Geologie-Geophysique - ORSTOM (Noumea), no. 22, p. 1-22.

Geologic Background. Yasur, the best-known and most frequently visited of the Vanuatu volcanoes, has been in more-or-less continuous Strombolian and Vulcanian activity since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island, this mostly unvegetated pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide, horseshoe-shaped caldera associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

Information Contacts: M. Lardy, ORSTOM, New Caledonia; B. Marty, CNRS, France; LAVE.