The Copahue Volcano, in the Andes on the border of Argentina and Chile, is erupting once again, sending a cloud of ash one mile into the atmosphere. Chile has raised its volcano alert level to red, the highest category, but has not initiated mandatory evacuations. Looking at photos on the internet, one wonders why they are not concerned enough to do so; beyond the volcanic ash fallout and the potential for pyroclastic flows, the mountains are covered with snow and, at the very least, flash flooding and mud flows (lahars) would seem to be significant threats.
Like all volcanoes of the Andes Range, Copahue is the product of tectonic plate subduction. Along the west coast of South America, the Nazca Plate is being forced to dip beneath the South American Plate; this is partly due to the westward motion of the American Plates (as the Atlantic Ocean continues to open) and the eastward motion of the oceanic Nazca Plate (due to sea floor spreading along the East Pacific Rise). As the Nazca plate dips toward the Earth's mantle, its leading edge melts, producing a chain of volcanoes above and along this melting zone. Indeed, the entire Andes Range formed and continues to form from this subduction process.
Copahue, rising to an elevation of 9833 feet, is a stratovolcano that formed during the latter half of the Pleistocene, likely within the last 500,000 years. This "modern" volcano, which has multiple craters, formed within the remnant caldera of a larger volcano that erupted earlier in Andean geologic history. Though Copahue has a long history of mild to moderate eruptions (most recently in 2000), the region is a popular tourist destination due to the numerous geothermal springs that rise along its flanks. Hopefully, the current conservative evacuation policy is devoid of economic considerations.
