Investigating subsurface conditions favouring the formation of secondary lahars in the glacier forefields of Cotopaxi volcano, Ecuador

Recently deglaciated terrain is highly active and subject to enhanced geomorphological change. The tropical glaciers on Cotopaxi volcano (5897 masl) in Ecuador are rapidly declining and have lost more than 50% of their surface area within the last five decades, and climate models predict a future rise of the Equilibrium Line Altitude of at least 200 m within the next 50 years (Vuille et al., 2018). The retreat of the presumably polythermal glaciers exposes unconsolidated, previously frozen pyroclastic material and moraine deposits on the steep volcano flanks. In recent years, secondary lahars unrelated to obvious trigger mechanisms occurred at Cotopaxi. As these lahars originated in proglacial areas, we aim to explore a potential connection between glacier retreat and lahar formation.

Here, we provide first insights into scarcely investigated subsurface conditions in periglacial areas of tropical glaciers. In order to gain knowledge on the presence of permafrost and ground ice, which can act as an aquiclude and potential detachment plane, we installed temperature loggers at 5-10 cm depth and performed electrical resistivity and seismic refraction surveys in the glacier forefields between 5000 and 5300 masl. The 1.5-year temperature record shows positive mean annual ground temperatures at all six logger sites. However, the temperature-calibrated electrical resistivity tomogram indicates partly frozen ground at depths of 10-20 m, where high electrical resistivities correspond to calibrated rock temperatures of -1.3 °C. We apply a 1-D thermal model to reproduce temperature changes at the surface with depth due to the retreat of cold-based glaciers. It allows to estimate the effect of the pyroclastic cover with high ice contents, which dampens thermal changes by uptake of latent heat during thawing, and can contribute to maintain ice bodies or relict permafrost lenses for years after deglaciation. In this study, we explore the relevance of degrading permafrost and ice lenses for preconditioning periglacial secondary lahars on rapidly deglaciating tropical volcanoes.

Vuille, M., Carey, M., Huggel, C., Buytaert, W., Rabatel, A., Jacobsen, D., Soruco, A., Villacis, M., Yarleque, C. and Timm, O. E. 2018. Rapid decline of snow and ice in the tropical Andes–Impacts, uncertainties and challenges ahead. Earth-Science Reviews, 176, 195-213.