. . . During overflights on 4 and 6 August, Maurice Krafft observed frequent ash emission from 1 of 4 WSW-ENE-trending vents in the summit area. The westernmost vent was filled by a blocky lava dome 15-20 m in diameter, partially covered by ash. ENE of the dome, explosions at least every 10 minutes from a 15-m-diameter crater produced thick black cauliflower-shaped ash columns 100-300 m high. Winds blew ash from these explosions to the SW, toward the dome. Each explosion also triggered small ash avalanches from deposits on the upper W and SW flanks. The largest of the four vents, ENE of the active crater, was 80-100 m across and contained two fumaroles that were emitting vapor. The fourth vent, 20-30 m in diameter and slightly N of the trend of the other 3 vents, was not active during the overflights.

Minard Hall reported that activity was generally similar when he visited the volcano in 1976. Although lava was oozing from the westernmost vent at that time, it had not yet built a dome.

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: M. Krafft, Cernay, France; M. Hall, Escuela Politécnica, Quito.