EVOLUTION OF COPAHUE VOLCANO (37º45’S), SOUTHERN ANDES

Copahue (37º45’S-71º10.2’W, 3001 m als) is an andesitic and basaltic andesitic active stratovolcano, located in the west border of the Pliocene Late caldera in the Southern Volcanic Zone and situated in the border between Chile and Argentina. In the summit volcano present glaciars that cover an area of 9.3 106 m2 and nine alined craters in N60º. The most oriental one, which is located in Argentinian territory, is the active crater. It has a acid lake inside of it that had variable volumen of 3-4.5 106 m3 before the eruption of 2000. Copahue had 12 phreatic and pheatomagmatic eruptions in the past 252 years, three of them ocurring in the last decade. The last one was the eruption of the biggest magnitude reported in this volcano (VEI<3). It began in July 2000 with phreatomagmatic explosions, ejection of incandescent bombs and ash. These pyroclasts covered an area of up to 50 km from the source and the plume was detected 250 km NE from the crater. During medium Pleistocene (0.6-0.4 My) the activity was essentially efusive composed of an andesite and basaltic andesite lava flows secuence (53-60% SiO2) and volcanic breccias probably originated by lahars. In the 0.4 My, the activity changed to acid and explosive volcanism. This volcanism was composed of a riolite dome (74-75% SiO2) in the south flank of the volcano, depositation of pumice and ash flows (71% SiO2) more than 30 km west of the source and welded pyroclastic flow in the east. Later, from Late Pleistocene to Postglacial, the activity was efusive once again. It was composed of basaltic andesite lava flows (55-57% SiO2) together with laharics events and thin pyroclastic levels. Finally, during Holocene, andesite lava flows (56-57% SiO2) were deposited in NE and SW volcano flanks and between 8.770 and 2.280 BP at least six explosive events ocurred, which were composed of pyroclastic ash flows distributed in a radial form.