Geologists from GEVA visited the volcano in June and August. Throughout June, only summit fumarolic activity was noted, but in early August, explosions every 5 minutes ejected small ash clouds. A warning was issued to aircraft flying in the vicinity of Sangay on 4 August at 2249. On 11 August, a new lava flow extended from the summit ~ 200 m below the crater rim. Blocks from the crumbling flow front rolled down the W and SW flanks to 4,400 m altitude. Larger avalanches from the flow front were observed on 12 August between 1400 and 1500. At 0005 on 13 August, explosions from a S-flank vent covered the volcano's S slope with red-hot lava blocks up to 1 m in diameter. Explosions ejecting blocks and ash continued until 0900. By 1600, the flow had moved to ~ 400 m below the crater rim and numerous blocks fell from the two lobes of the flow front.

Geological Summary. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within the open calderas of two previous edifices which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been eroded by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of an eruption was in 1628. Almost continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: J. Durieux, GEVA, Lyon, France; G. Heiken, LANL, Los Alamos.