Report on Cotopaxi (Ecuador) — 19 August-25 August 2015
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 19 August-25 August 2015
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2015. Report on Cotopaxi (Ecuador). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 19 August-25 August 2015. Smithsonian Institution and US Geological Survey.
0.677°S, 78.436°W; summit elev. 5911 m
All times are local (unless otherwise noted)
IG reported that during an overflight of Cotopaxi on 18 August scientists observed continuous but variable amounts of ash and steam rising more than 100 m above the crater before descending the W flank. Significant amounts of ash were deposited on the flanks in an area from the N to the SW flanks. Several new cracks on the top of some glaciers were noted, especially on the E and NE flanks, and possible new tephra deposits on the N flank were observed. Thermal images revealed no hot material on the flanks; emissions prevented measurements of the inside of the crater. During 18-19 August emissions of steam and gas from Cotopaxi were occasionally observed during periods of clear weather. During the morning of 20 August gas plumes rose just above the crater and drifted W. The next day gas-and-steam plumes rose less than 2 km above the crater and drifted NW; cloud cover continued to sometimes prevent visual observations. On 22 August at 0426 the network detected an increase in the seismic amplitude. Steam-and-ash plumes rising 2 km from the crater were more sustained and higher compared to previous days; plumes drifted NW and WSW. Tremor began at 2141, and was accompanied by the onset of continuous ash emissions. Rangers confirmed ashfall at the entrance of Cotopaxi National Park. Throughout 23 August continuous ash emissions occurred with few breaks, rising no more than 1 km above the crater, and drifting SW. IGEPN staff found 2-mm-thick ash deposits that had accumulated during an 18-hour period. On 24 August ash deposits were noted in most of the N parts of Latacunga valley and reached the S moors of Romerillos. On 25 August ash plumes drifted WNW, causing ashfall in Machachi, Chaupi, and Tambillo.
Geologic Background. Symmetrical, glacier-clad Cotopaxi stratovolcano is Ecuador's most well-known volcano and one of its most active. The steep-sided cone is capped by nested summit craters, the largest of which is about 550 x 800 m in diameter. Deep valleys scoured by lahars radiate from the summit of the andesitic volcano, and large andesitic lava flows extend to its base. The modern conical edifice has been constructed since a major collapse sometime prior to about 5000 years ago. Pyroclastic flows (often confused in historical accounts with lava flows) have accompanied many explosive eruptions, and lahars have frequently devastated adjacent valleys. The most violent historical eruptions took place in 1744, 1768, and 1877. Pyroclastic flows descended all sides of the volcano in 1877, and lahars traveled more than 100 km into the Pacific Ocean and western Amazon basin. The last significant eruption took place in 1904.