Argentina and Chile have declared a mandatory evacuation zone of 25 km around the Copahue Volcano, which straddles the border between the two countries, roughly 500 km to the south of the Chilean capitol Santiago. The volcano started to suffer violent tremors on Monday 27 May 2013, and has released a large volume of gas, though as yet there has been no other eruptive activity.
The Copahue Volcano. Mono Andes/Wikimedia Commons.
Copahue comprises a chain of nine craters along a 2 km east-west line, with the most recent, still active, crater at the eastern end. This chain sits within the 400 000 to 600 000 year old, 6.5 by 8.5 km Trapa-Trapa Caldera, which in turn sits inside the older (more than 2.5 million years old), 15 by 20 km Caviahue Caldera. The active crater contains an acid lake, the Del Agrio, which is fed by acidic hot springs at its east end. The lake is noted for frequent fumerole (gas) emissions, and occasional explosive events.
Copahue is an extremely active volcano, having last erupted on 7 May 2013, when it produced a 350 m ash plume. A series of eruptions in December 2012-January 2013 produced numerous explosions and crater iridescence, as well as ash columns rising as high as 4.6 km.
Copahue, like other volcanoes in the Andes, is fed by the subduction of the Nazca Plate beneath the South American Place along the west coast of the continent. As the Nazca Plate sinks into the Earth it passes under South America, and at the same time is partially melted by the heat and pressure of the planet's interior. More volatile elements in the melted magma to rise up through the overlying South American Plate, fueling the volcanoes of the Andes.
See also Eruptive activity on Mount Sabancaya, Eruption on Mount Copahue, Eruption on Volcán El Reventador, Seismic activity beneath Mount Sotará, Columbia and Seismic activity on Mount Cumbal, Colombia.
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