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Galeras, a 4270-m high andesitic stratovolcano in southwestern Colombia near the Ecuadorian border (Figure 1), gradually reawoke in 1988 after more than 40 years of dormancy. In 1991, after a request from the Geological Survey of Colombia (INGEOMIAS) and the National Disaster Prevention Office (ONAD), Galeras was named a “Decade Volcano” by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) as part of the United Nations' International Decade for Natural Disaster Reduction (IDNDR) program. A workshop was held in 1993 to research, monitor, and mitigate the hazards of the volcano.

Galeras lies at latitude 1°14′N, longitude 77°22′W (see Figure 2), and its active cone rises about 150 m above the floor of a small summit caldera that is open to the west [Calvache and Williams, 1992]. The active crater is located about 6 km west of Pasto, a city of about 300,000 inhabitants. At least six major Galeras eruptions have been identified during the past 4500 years. These eruptions were mainly vulcanian, with inferred low-altitude eruption columns (<10 km) that produced small-volume pyroclastic flow deposits containing a high proportion of nonjuvenile material and lava flow fragments [Calvache and Williams, 1992]. During the last 500 years, eruptions have been characterized by gas and ash emissions, small lava flows, and explosive eruptions producing pyroclastic flows that have traveled up to 15 km from the crater [Calvache, 1990; Cepeda, 1993].