Snow-contact volcanic facies and their use in determining past eruptive environments at Nevados de Chillan volcano, Chile

Studies of the eruptive products from volcanoes with variable ice and snow cover and a long history of activity enable reconstruction of erupted palaeoenvironments, as well as highlighting the hazards associated with meltwater production, such as j¨okulhlaups and magmawater interaction. Existing difficulties include estimation of ice/snow thicknesses and discrimination between iceand snow-contact lithofacies. We present field evidence from the Cerro Blanco subcomplex of Nevados de Chill´an stratovolcano, central Chile, which has erupted numerous times in glacial and non-glacial periods and most recently produced andesitic lava flows in the 1861–1865 eruption from the Santa Gertrudis cone on the northwest flank of the volcano. The main period of lava effusion occurred during the winter of 1861 when the upper flanks of the volcano were reportedly covered in snow and ice. The bases and margins of the first lava flows produced are cut by arcuate fractures, which are interpreted as snow-contact features formed when steam generated from the melting of snow entered tensional fractures at the flow base. In contrast, the interiors and upper parts of these flows, as well as the overlying flow units, have autobrecciated and blocky textures typical of subaerial conditions, due to insulation by the underlying lava. Similar textures found in a lava flow dated at 90.0±0.6 ka that was emplaced on the northwest flank of Cerro Blanco, are also inferred to be ice and snowcontact features. These textures have been used to infer that a small valley glacier, overlain by snow, existed in the Santa Gertrudis Valley at the time of the eruption. Such reconstructions are important for determining the long-term evolution of the volcano as well as assessing future hazards at seasonally snow-covered volcanoes.