Low-intensity seismic activity continued during November and December 1999, similar to previous months. During this period, 66 volcano-tectonic (VT) earthquakes were registered, releasing a total energy of 5.03 x 10¹⁴ ergs. The VT events occurred at depths between 0.5 and 25 km below the summit within 20 km of the active crater. The largest magnitude event (coda magnitude 2.4) occurred NE of the summit on 28 December at a depth of 19 km. Fifty-six (56) long-period events (LP) occurred during this period (compared to 19 in the previous two months) releasing 6.53 x 10¹⁴ ergs. The most remarkable aspect of this seismicity was the occurrence of 15 tornillo events with dominant frequencies between 1.6 and 1.9 Hz although some had peaks between 9 and 18 Hz.

Radon-222 emissions at soil stations located around the volcano measured values between 40 and 5,468 pCi/l, levels similar to those of previous month, with the peak value occurring at Barranco station 6 km NW of the summit.

Low-pressure, gray and white, gas emissions came from active vents. Temperature measurements taken in the Deformes fumarole at the SSW edge of the main crater registered values from 128 to 132°C.

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: Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).