Seasonal control on phreatic activity of the crater lake of Copahue volcano during the 2018–2022 eruptive cycle

Abstract

The last eruptive cycle of Copahue volcano, located on the border between Argentina and Chile, began in 2012 and continues up to the present. One of the characteristics of this volcano is the hot acidic lake located in its crater, which has undergone various changes in the past 20 years, as a result of different eruptive phases. In particular, between 2018 and 2022, changes in the crater lake level were associated with periods of increased surface activity, characterized by phreatic eruptions and SO₂ emissions into the atmosphere. We carried out a multiparametric analysis, finding correlations between the crater lake hydric balance with annual seasonal changes, gas and ash emissions, and temporal variations in the tremor seismic signal. These correlations allowed us to infer that during the cold months, the ambient temperature prevents the thaw that feeds the shallow hydrothermal system, favoring evaporation and reduction of the crater lake volume. Consequently, less efficient heat dissipation and sulfur scrubbing processes favors vent sealing, likely dominated by liquid sulfur and precipitation of secondary minerals in the shallow hydrothermal conduits. Eventually, the gas pressure in the conduits reaches a critical state, which is manifested through increased tremor activity. When the seal is ruptured, an abrupt drop in the seismic signal is observed. This activity is followed by phreatic eruptions or large SO₂ releases into the atmosphere. This crater lake behavior has occurred annually from 2018 to 2022, evidencing a seasonal control on the shallow hydrothermal system and the phreatic activity of Copahue volcano.