Galeras is an andesitic stratovolcano that is part of an older volcanic complex. The active cone of the volcano, which has grown in a large horseshoe-shaped caldera created by a large edifice collapse, has been erupting for the past 4,500 years, but the volcanic complex has been active for more than a million years. Recently, eruptions at Galeras have been characterized by Vulcanian explosions, pyroclastic flows, degassing (especially of sulfur dioxide), and ash plumes. All of these types of activity are immediately hazardous to those who live near the volcano; pyroclastic flows are especially a concern, as many people who live in Pasto do not heed evacuation warnings issued by local scientists.
In addition to the hazards posed by volcanic activity, debris avalanches are also a major concern at Galeras. The volcano has areas of extensive hydrothermal alteration, which weakens rock and makes it more prone to collapsing. Such collapses have occurred on at least three occasions, producing large debris avalanches which have swept down the flanks of the volcanic complex. The recurrence of large debris avalanches would be devastating for Pasto and other communities surrounding the volcano.