Insights into the internal structure and mechanical behavior of Copahue volcano (Argentina–Chile)

Abstract

Copahue volcano, located at the border between Chile and Argentina, has been among the most active volcanoes in Patagonia for the last 30 years. Copahue hosts a hyperacidic lake in its active crater, and many manifestations of intense hydrothermal activity, motivating studies of the volcano’s internal structure and how alteration may affect edifice stability. In this work, we study the structure, composition, and rock strength of the upper eastern flank of Copahue volcano, which is the most common path used to access the crater by tourists. It is also where the main water springs originating from the crater lake are found, and where most of the ejected material is deposited due to topography and the prevailing wind direction. To study the field-scale edifice structures, we installed a muon detector and computed a muon density image. In addition, we analyzed rock samples from the eastern flank in the laboratory. Using a Scanning Electron Microscope combined with Energy Dispersive X-ray analysis and X-ray Powder Diffraction, we visualized the effects of alteration and identified the minerals present. Finally, we measured the bulk density, dry uniaxial compressive strength (UCS), and static Young’s modulus of the samples. The muon imaging suggests mainly low-density values in the eastern side of the volcano, that we interpret as glacial and/or ice deposits, interleaved with tephras and tuffs, and presence of large edifice fractures. Well-defined high-density anomalies correspond to lava flows that outcrop in some parts of the edifice. Most of the samples analyzed have suffered hydrothermal alteration through rock dissolution, and precipitation of silica and secondary minerals, in accordance with the hyperacidic hydrothermal environment. The laboratory measurements suggest variable rock strength values, which appear to be controlled by rock type and primary porosity. Our multidisciplinary approach indicates that the eastern flank of Copahue volcano is characterized by mainly low-cohesion, low-density and variablestrength materials. The structures mapped, together with the rock properties measured, are key for a future flank stability assessment.