Evolution of the Copahue crater lake (Argentina) during the 2012 phreatic and phreatomagmatic eruption cycle.

AGUSTO M, TASSI F, CASELLI A, ROUWET D, CAPACCIONI B, VASELLI O, CALABRESE S.

Ozu

Workshop; 8th Workshop on Volcanic Lakes; 2013

IAVCEI

 Copahue volcano (37º45’S-71º10’W, 2977 m a.s.l.) is part of Copahue-Caviahue Volcanic Complex (Argentina-Chile), located at the Southern Andean Volcanic Zone. The summit of this stratovolcano, consists of nine NE-oriented craters. Eruptive activity during the last 250 years, occurring from the easternmost crater that hosts a hot acidic lake (30-40°C, pH~1) with a diameter of~200 m, was characterized by at least 13 low-magnitude phreatic and phreato-magmatic events, affecting the nearby villages. Eruptive cycles took place from this crater in August 1992, September 1995 and from July to October 2000; and the last eruptive cycle began in 2012 and is still ongoing. Dimensions and chemistry of the crater lake was significantly affected by such frequent eruptions, Copahue Lake is a typical example of hyperacidic lake whose water composition data served as memory of the eruptive activity of a volcanic system in which geochemical monitoring is very difficult due to the absence of summit fumaroles and the occurrence of a well developed hydrothermal aquifer where magmatic gases are almost completely dissolved, masking any signal of deep magmatic inputs at the surface. In this study the chemical and physical variations of the Copahue crater lake in 2011-2012 are presented. In 2012 phreatic and phreatomagmatic eruptions occurred after 12 years of solfataric state. In November-December 2011, a column of vapour and acidic gases some 200-300 meters above the crater was observed, testifying an increase of the fluid discharge rate from the volcano summit. By March 2012, the waters from hot springs and crater lake showed (1) the highest acidity (pH<0), (2) highest temperature (~65°C), (3) highest contents of magmatic species (SO42-, Cl-, F-) since the 2000 eruption, and (4) a significant decrease on the crater water level due to enhanced evaporation. On July 17 a phreatic eruption occurred, manifested as vertical jets 10 meters above the crater lake level. On July 19, a phreato-(magmatic?) eruption occurred with the expulsion of pyroclastic material. During the following months fumarolic activity and intense bubbling continued, crater lake parameters showed high values of temperatures (~60°), high acidity (pH<0), and extremely high electrical conductivity. The lake water level continued to decrease until becoming a boiling pool only ~20 m in diameter before the December 2012 eruption.  On the 22nd of December a new phreatomagmatic eruption occurred. This eruption caused the disappearance of lake water and the occurrence of several high temperature fumaroles (maximum measured temperature = 420 °C) and liquid sulfur ponds formed at the lake bottom. Two months later the crater lake began to be restored. In March-April 2013, the formation of two incipient water pools with intense gas bubbling was observed. A geochemical monitoring of this newly formed lakes, and that of the fumaroles discharging from their surroundings took place to provide information of on the ongoing evolution of this volcanic system.