On 26 September a special volcanic activity report issued by OVDAS-SERNAGEOMIN noted a seismic swarm at Copahue; between 1600 and 1900 the seismic network recorded 140 long-period events. Cameras near the volcano recorded increased emissions; mostly white plumes with some ash rose 200-500 m and drifted SE. The report also noted that swarms had been recorded in late July and during 1-16 September. The Alert Level remained at Yellow.

Geological Summary. Volcán Copahue is an elongated composite cone constructed along the Chile-Argentina border within the 6.5 x 8.5 km wide Trapa-Trapa caldera that formed between 0.6 and 0.4 million years ago near the NW margin of the 20 x 15 km Pliocene Caviahue (Del Agrio) caldera. The eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains a briny, acidic 300-m-wide crater lake (also referred to as El Agrio or Del Agrio) and displays intense fumarolic activity. Acidic hot springs occur below the eastern outlet of the crater lake, contributing to the acidity of the Río Agrio, and another geothermal zone is located within Caviahue caldera about 7 km NE of the summit. Infrequent mild-to-moderate explosive eruptions have been recorded since the 18th century. Twentieth-century eruptions from the crater lake have ejected pyroclastic rocks and chilled liquid sulfur fragments.

Source: Servicio Nacional de Geología y Minería (SERNAGEOMIN)