Seismicity, gas emission, and deformation were generally at low levels during November, although a swarm of 22 long-period events was recorded on the 10th. Most of the month's seismicity was thought to be associated with movement of fluids. Rock-breakage events were less frequent than in October, and were centered E of the active crater. Tremor episodes remained at low energy levels, with maximum durations and amplitudes of 287 seconds and 3.2 mm, respectively. SO₂ flux measured by COSPEC ranged from 46 to 547 t/d, similar to October values.

Tilt data from [Crater Station] showed 6 µrad of deflation in November. A second station, 1.6 km E of the crater, measured 6 µrad of inflation during the first half of the month. A 40-µrad change registered by its tangential component during the second half of November may have been caused by local movements at the site.

Geological Summary. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: F. Muñoz, INGEOMINAS—Observatorio Vulcanológico del Sur.