Report on Tungurahua (Ecuador) — April 2000
Bulletin of the Global Volcanism Network, vol. 25, no. 4 (April 2000)
Managing Editor: Richard Wunderman.
Tungurahua (Ecuador) Frequent early-2000 explosions with loud reports; ash plumes to 10 km altitude
Please cite this report as:
Global Volcanism Program, 2000. Report on Tungurahua (Ecuador) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 25:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200004-352080.
1.467°S, 78.442°W; summit elev. 5023 m
All times are local (unless otherwise noted)
This report covers the interval from 1 January to 18 February 2000. What follows are excerpts of the Geophysical Institute's daily reports. Basically, they disclose a pattern of noisy ash-bearing eruptions, tremor, and elevated seismicity. The alert status remained at Orange. As noted on table 3, some ash columns rose as high as 5-10 km; the sounds from one explosion were heard 75 km N in Quito. Tephra frequently blew towards the W (the major population center, Baños, lies N of the summit).
|Date||Plume height (km) and Observations|
|01 Jan 2000||2a, W. Banded tremor.|
|02 Jan 2000||0.5 a, W (fines fell on Rio Bamba). Banded tremor.|
|03 Jan 2000||"Significant explosions"a, W. Following the explosions there was high-amplitude tremor for 30 minutes.|
|04 Jan 2000||5a, W.|
|05 Jan 2000||2a, N.|
|06 Jan 2000||3a, NE.|
|07 Jan 2000||Poor visibility, 1-2 a.|
|08 Jan 2000||Poor visibility, 3-4a, SW.|
|10 Jan 2000||6,700-7,000 t/d SO2. High tremor.|
|11 Jan 2000||1.5s, NW.|
|12 Jan 2000||~ 8a, with two plumes seen in satellite imagery, trailing off S and W. 8,400 t/d SO2.|
|13 Jan 2000||Restricted visibility but incandescent ash seen falling on upper slopes. 5,000 t/d SO2.|
|14 Jan 2000||~ 0.2a; the plume was highly concentrated in ash, thus the column collapsed soon after it began discharging; later, incandescent tephra fell over the upper sides.|
|15 Jan 2000||2a, strong detonation heard 10 km away; incandescent blocks seen.|
|16 Jan 2000||Poor visibility and no explosions detected.|
|17 Jan 2000||Small explosion detected.|
|18 Jan 2000||One audible explosion heard during poor visibility.|
|19 Jan 2000||1a, continuous discharge blown W to NW.|
|20 Jan 2000||Small ash columns.|
|21 Jan 2000||Bad weather, falling rocks heard; mudflows noted.|
|23 Jan 2000||Observers saw a short, hazy column with a moderate concentration of ash.|
|24 Jan 2000||1a, blown W and SW.|
|25 Jan 2000||Seismically detected explosions with 5-8 cm2 reduced displacements.|
|26 Jan 2000||Bad weather; small explosions.|
|27 Jan 2000||Seismically detected explosions with 8 cm2 reduced displacements.|
|28 Jan 2000||Small to moderate explosions.|
|29 Jan 2000||Moderate to large explosions; cannon-like sounds reported; two mudflows crossing roads.|
|30 Jan 2000||~ 1a, but American Airlines also reported a plume to ~10 km; moderate-to-large seismically detected explosions, plume blowing NW to W.|
|01 Feb 2000||Slight decrease in the number of explosions with respect to previous days.|
|02 Feb 2000||2a, W; 20 min. of tremor.|
|03 Feb 2000||1a, W and SW; incandescent materials descended 0.5 km down flanks.|
|04 Feb 2000||~2.5a, some dense columns; W, NW, and SW.|
|05 Feb 2000||0.6a, W; rains caused mudflows.|
|06 Feb 2000||Continued outbursts consisting of sporadic low energy explosions. Tremor episode(s) associated with gas emissions.|
|07 Feb 2000||Continual ash emission; S- and W-directed winds.|
|08 Feb 2000||3a, N and NW; explosion at 0438 heard by residents of Quito; ashfall closed airports in Ambato and Riobamba.|
|12 Feb 2000||~8-10a (aviation report); W and SW.|
|13 Feb 2000||Cloudy weather; roaring noises heard; fine ash in NW sector of volcano; at 0743, an Avianca flight en-route to Lima-Bogota reported a 7-km-radius cloud of ash over the volcano at 8 km altitude.|
|14 Feb 2000||2a, SW; small amount of ash in Riobamba.|
|15 Feb 2000||2a, W.|
|16 Feb 2000||Plumes and emissions not discussed.|
|17 Feb 2000||3a.|
|18 Feb 2000||3a.|
Histograms and plots on the Institute's web site tell a story of increased activity during late 1999, but the data for 2000 were generally absent. For example, BGVN 24:11 included their plot of daily explosions during 24 October-3 December 1999. This plot showed 1 event on 24 October 1999 and 1-39 events during the first half of November. A prominent peak in the latter half of November reached 105 events. The number of daily events subsequently declined for the last data shown, ending with 39 events on 3 December 1999.
Based on a reading of the daily reports for early 2000 this decline continued. An approximate tally of daily explosions suggests 300 during the month of January, an average of about 10/day. Similarly, for the first eighteen days of February, explosions again averaged about 10/day. In addition, a regional earthquake of M 3.3 took place on 12 January centered below the volcano's S flanks.
One of the Institute's plots disclosed a positive correlation between near-term rainfall and tremor during 1993-1999. Notwithstanding that longer-term observation, rainfall decreased during the course of 1999 while tremor escalated strongly, ultimately undergoing more than a 15-fold increase over background by the latter part of 1999.
Epicenters and foci mapped for August 1999 clustered near the summit and also occurred with a much lesser density along a radial band trending NW-SE extending out on either side of the summit. Thus, the main cluster of the August 1999 earthquake swarm lay directly below the summit at less than 10 km depth; with close approach to the surface the cluster appears directed toward the volcano's SE side. A plot of measured SO2 fluxes for the interval July-8 December 1999 peaked in September-October at ~9,000-10,000 tons/day (BGVN 24:11).
On 21 and 29 January, and 6 February, rainy weather caused mudflows to descend drainages blanketed by ash deposits. On many days, explosions caused unusually strong acoustical signals; the most powerful signal was generated on 8 February when the sound of an explosion was heard 75 km N in Quito.
Some of the other noteworthy events and ideas discussed in the daily reports follow. At 1544 on 3 January significant explosions occurred, followed by high-amplitude tremor lasting about 30 minutes. The emissions continued and the columns again blew W. On the 4th, undisclosed satellite images showed a column of ash 5 km tall, again blown W. The daily report for 7 January mentioned two possible eruption scenarios involving escalating eruptive vigor and increased hazards.
At 0614 on 12 January ground observers noted a dark gray column rose 3-5 km over the summit. Thereafter, NOAA satellite technicians noted two ash clouds. One cloud extended from the volcano toward the W reaching a length of 16 km and a width of 15 km; it rose to 3 km over the summit (i.e., ~8 km altitude). A larger ash cloud extended toward the S reaching a length of 75 km; it also reached ~8 km altitude (table 1).
Ash clouds remained obscure from the ground on 13 January but in the morning roars were heard associated with the continual emission of gases and ash. A light rain of fine ash fell in the NW sector of the volcano. Peculiar, very dense clouds vented on 14 January, rising only 200 m before rapidly collapsing.
At 0743 on 13 February, an Avianca flight in route to Lima-Bogotá reported an ash cloud over the volcano at 8 km altitude (table 1). In the morning on 14 February some loud explosions foretold of ash columns; they rose 2-3 km above the summit and blew SW. In the morning the next day satellite images showed an ash cloud that had reached a length of 100-150 km and width of 15 km. This cloud was directed from Tungurahua toward the NNW, passing over the towns of Píllaro, Latacunga, and Salcedo. At 10 km distance, falling ash reached a thickness of 1 mm.
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 (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.