Report on Tungurahua (Ecuador) — August 2007
Bulletin of the Global Volcanism Network, vol. 32, no. 8 (August 2007)
Managing Editor: Richard Wunderman.
Tungurahua (Ecuador) During March-July 2007, many lahars; variable eruptive behavior
Please cite this report as:
Global Volcanism Program, 2007. Report on Tungurahua (Ecuador) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 32:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200708-352080.
1.467°S, 78.442°W; summit elev. 5023 m
All times are local (unless otherwise noted)
Ecuador's Instituto Geofisico (IG) wrote that significant though variable eruptions and lahars occurred at Tungurahua during mid-2007. Our previous report (BGVN 32:04) focused on early January to 2 March 2007, noting some variations in the pace of eruptive activity then. This report summarizes IG reports for March-July 2007. The substantial eruptions of July and August 2006 left abundant pyroclastic-flow deposits on the mountains slopes, potential source materials for new lahars (mudflows). The abundant seismicity during that interval punctuated a longer-term variable pattern (table 13 and figure 38).
|Time interval||Total earthquakes||Long-period||Volcano-tectonic||Hybrid||Emission signals||Explosion signals|
|Total for Jul 2006||3482||3475||5||2||1185||6442|
|Daily avg Jul 2006||112||112||0.16||0.06||38||208|
|Total for Aug 2006||2546||2518||19||9||467||1643|
|Daily avg Aug 2006||82.1||81.2||0.61||0.29||15.1||53.0|
|Total for Sep 2006||2189||2149||35||5||111||0|
|Daily avg Sep 2006||73.0||71.6||1.16||0.16||3.7||0|
|Total for Oct 2006||3159||3131||20||8||1023||4|
|Daily avg Oct 2006||102||101||0.64||0.25||33.0||0.12|
|Total for Nov 2006||1849||1846||3||0||1049||1|
|Daily avg Nov 2006||61.6||61.5||0.1||0||35.0||0.03|
|Total for Dec 2006||2172||2168||5||0||648||0|
|Daily avg Dec 2006||70.1||69.9||0.16||0||22.8||0|
|Total for Jan 2007||829||817||12||0||10||0|
|Daily avg Jan 2007||26.7||26.4||0.38||0||0.32||0|
|Total for Feb 2007||983||966||15||2||312||54|
|Daily avg Feb 2007||35.1||34.5||0.53||0.07||11.1||1.9|
|Total for Mar 2007||1126||1125||1||0||1215||334|
|Daily avg Mar 2007||36.3||36.3||0.03||0||39.2||10.7|
|26 Feb-04 Mar 2007||427||427||0||0||364||51|
|05 Mar-11 Mar 2007||235||235||0||0||269||87|
|12 Mar-18 Mar 2007||134||133||1||0||203||112|
|19 Mar-25 Mar 2007||241||241||0||0||356||86|
|26 Mar-01 Apr 2007||465||465||0||0||300||47|
|Figure 38. Tungurahua seismicity during September 1999 to March 2007 plotting the number of both explosion (EXP) and long-period (LP) earthquakes. Other kinds of earthquakes also took place but after 2001 were rarely seen. Courtesy of IG.|
The IG report for March stated that a relatively energetic eruptive phase began on 24 February 2007 and continued throughout the month. That phase included abundant, ash emissions, sometimes discharging incandescent material, numerous, sometimes large explosions, and frequent noteworthy ashfall. The ash emissions and ashfalls were sometimes sustained. Blocks ejected in Strombolian outbursts fell up to 1 km below the crater rim.
During March, there were rises in both tremor amplitude and the number of long-period (LP) earthquakes (the later during March averaging 36 per day). SO2 gas fluxes averaged ~ 1,050 metric tons/day (t/d). Flank deformation was minimal. March ash falls came from frequent sustained ash plumes 2-6 km over the summit (figure 39). Seismically detected eruptions took place 29 times per day, including some of large size. Tremor nominally took place around 1 Hz, but its frequency remained irregular, non-harmonic, and pulsating. Intervals of pulsing emissions in mid-March had cycle times of ~ 10 minutes.
|Figure 39. Tungurahua emitting an ash plume on 9 March 2007. Sustained plumes were seen during much of the month. Photo taken from Pondoa, on the N flank by Patty Mothes (IG).|
An explosion on 27 March caused an "overflow" of incandescent material that traveled 800 m down from the head of the Mandur drainage. Other similar eruptions may have occurred but cloudy conditions forestalled clear observations. Hot lahars, however, traveled down the Mandur and Chontapampa drainages. Ash falls were common on the cone's N and NW sectors, and in addition, observers noted a small pyroclastic flow.
During the first weeks of April 2007 the IG noted continuous, strong emissions with a very high ash content. These emissions accompanied conspicuous lava fountains, visible at night, and strong roars that made windows vibrate. Ash columns reached 6 km above the crater (~ 11 km altitude). Activity decreased notably during the last 10 days of April (but were even lower in late May). Seismometers recorded an average of ~ 10 daily low-amplitude LP earthquakes. A differential optical absorption spectroscopy (DOAS) instrument measured SO2 fluxes of 3,600 and 3,700 t/d during the last 10 days of April.
During May, seismicity was low (table 13), with the average number of registered earthquakes each day averaging about 20. The Seismic Activity Index at the beginning of the month indicated a level 5 (moderate-high activity), which later on fell to a level 3 (moderate-low activity). This was the lowest Seismic Activity Index registered since February 2007. Ash emissions were low to moderate with a westerly direction. The SO2 levels were approximately 800 t/d. With the exception of the frequent formation of lahars, the level of volcanic activity was low in May.
The vigor of June 2007 eruptions from Tungurahua remained at moderate to low levels. Seismicity at the start of the month was low, chiefly LP earthquakes. Eruptions columns were modest and charged with moderate to low amounts of ash. June SO2 fluxes were comparatively high, ~ 2,900 t/d; observers heard light roaring noises similar to a turbine engine. Seismicity increased slightly towards the end of the month.
June brought prolonged intervals of low intensity rain, but heavy rains also occurred. The result was lahars (mud flows) that were numerous and in some cases large (table 14). The 21st of June was particularly noteworthy (table 14). Figure 40 shows one such lahar, which was partly eroded resulting in extension of lahars farther downslope. The lahars sometimes closed the route along the N side of the volcano between Baños and Pelileo and also the route from Baños around the volcano's W flank to Penipe (~ 15 km SW of the summit). No fatalities were reported.
|Date||Drainage||Relative size and comments|
|01 Jun 2007||Bilbao||Small|
|06 Jun 2007||Bilbao||Small|
|07 Jun 2007||Vazcun||Small|
|07 Jun 2007||La Pampa||Small; caused road closure|
|07 Jun 2007||Bilbao||Small|
|07 Jun 2007||Motilones||Small|
|07 Jun 2007||Pingullo||Small|
|07 Jun 2007||Rea||Small|
|07 Jun 2007||Viejo Minero||Muddy water|
|11 Jun 2007||Mandur||Muddy water|
|11 Jun 2007||La Pampa||Small|
|12 Jun 2007||La Pampa||Muddy water|
|12 Jun 2007||Viejo Minero||Muddy water|
|13 Jun 2007||La Pampa (2)||Large and medium; a truck remained stuck|
|13 Jun 2007||Viejo Minero||Muddy water|
|14 Jun 2007||Mandur||Small|
|14 Jun 2007||La Pampa||Small|
|14 Jun 2007||Bilbao||Small|
|14 Jun 2007||Pingullo||Small|
|14 Jun 2007||Motilones||Small|
|15 Jun 2007||Mandur||Small|
|15 Jun 2007||Mapayacu||Small|
|15 Jun 2007||Motilones||Small|
|15 Jun 2007||Pingullo||Small|
|15 Jun 2007||La Pampa||Small|
|15 Jun 2007||Rea||Small|
|15 Jun 2007||Choglontus||Small|
|15 Jun 2007||Cusua||Small|
|15 Jun 2007||Vazcun||Small|
|15 Jun 2007||Viejo Minero||Muddy water|
|16 Jun 2007||La Pampa||Muddy water|
|20 Jun 2007||La Pampa||Medium; closing the road|
|20 Jun 2007||Mandur||Small|
|20 Jun 2007||Viejo Minero||Small|
|20 Jun 2007||Achupashal||Small|
|20 Jun 2007||Bilbao||Small|
|20 Jun 2007||Motilones||Small|
|21 Jun 2007||La Pampa||Large, closing the road|
|21 Jun 2007||Viejo Minero||Large|
|21 Jun 2007||Mandur||Large|
|21 Jun 2007||Vazcun||Large|
|21 Jun 2007||Nueva Cusua||Large|
|21 Jun 2007||Achupashal||Large|
|21 Jun 2007||Motilones||Large|
|21 Jun 2007||Pingullo||Large|
|21 Jun 2007||Bilbao||Large|
|21 Jun 2007||Rea||Large|
|21 Jun 2007||Confesionario||Large|
|21 Jun 2007||Ulba||Growing|
|22 Jun 2007||Vazcun||Growing|
|22 Jun 2007||Ulba||Growing|
|22 Jun 2007||La Pampa||Muddy water|
|22 Jun 2007||Viejo Minero||Small|
|22 Jun 2007||Mandur||Small|
|24 Jun 2007||La Pampa||Muddy water|
|Figure 40. A lahar in the La Pampa sector of Tungurahua showing an active, steep sided erosional channel down the axis of the deposit. Photographed 13 June 2007 by P. Ramón (IG). Courtesy IG.|
There was a minor increase in seismicity during the month of July. Distribution of events was variable: 240-330 events per week the first and last week of the month; 50-70 events during each of the other two weeks. They were primarily LPs ~ 2 km below the summit.
The rate of SO2 emission averaged 1,071 t/d with a high of 2,050 t/d. Ashfall was semi-continuous, reaching areas W and SW of the summit, near communities like Bilbao (8 km W of the summit), Chogluntus (SSW of the summit), and El Manzano (7 km WSW). The plume headed toward Manta once the column reached 4 km above the summit.
During July, the road to Baños-Las Juntas was temporarily closed six times due to small-to-moderate lahars.
Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II itself collapsed about 3000 years ago and produced a large debris-avalanche deposit and a horseshoe-shaped caldera open to the west, inside which the modern glacier-capped stratovolcano (Tungurahua III) was constructed. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.
Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).