Increased sulfur-dioxide degassing and seismicity related to fluid movement, resulting in the overall lowering of pressure in the system, prompted INGEOMINAS to further lower the alert level to Yellow (Level III) on 19 June. By 23 June some rock-fracture seismicity had been detected, though events related to fluid movement had declined significantly. That same day clear weather allowed observation of a small gas column with minor ash content, while scientists on a monitoring flight saw gas emissions near the crater rim and recorded a thermal anomaly within the main crater.

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.

Source: Servicio Geológico Colombiano (SGC)