The decrease in the shear strength of volcanic materials with argillic hydrothermal alteration, insights from the summit region of Teide stratovolcano, Tenerife

Abstract

Hydrothermal alteration around the summit region of Teide stratovolcano, in Tenerife, has decomposed lavas to weak, clay-rich soils. This process has been observed at other volcanoes. The purpose of this study is twofold: firstly to identify and geotechnically characterise weak volcanic materials, and secondly to estimate the reduction in shear strength caused by argillic hydrothermal alteration. Hydrothermally altered soil units, the weakest materials found on the flanks of Teide, have been collected and studied using standard geochemical and geotechnical laboratory techniques. The weakest units are exposed only inside the summit crater and are porous, silty, clay-rich, medium plasticity soils. Mineralogical analyses of the summit crater soils reveal that they contain two different alteration assemblages formed by acid-sulphate alteration of potassium-rich feldspars from the parent phonolitic lava flows. Material in the active fumarolic field is now fully altered to alunite, while material at a short distance from the active fumarole field is partially altered to both kaolinite and alunite. Geotechnical measurements performed on the summit crater units show that they have negligible cohesion. The presence of significant amounts of kaolinite in the partially altered soil could explain why this unit appears to have a lower shear strength than the fully altered alunitic one (friction angle φ′ = 16–25° and φ′ = 27–33° respectively). If so, these results would suggest that the type of alteration, which is determined by the temperature and pH, is more important in controlling the shear strength of the altered soil than the amount or degree of alteration. The summit crater soil units have formed in the last ~ 1000 years since the parent phonolitic lava flows were emplaced. Fresh and partially altered phonolitic parent rocks are widely present in the summit region adjacent to the soil units and offer a unique opportunity to quantify the decrease in shear strength caused by hydrothermal alteration. A comparison between the shear strengths of fresh and altered lavas and residual soils suggests relatively small, progressive, decrease in shear strength from fresh to altered rocks, with further alteration causing a large and sharp decrease transforming altered rocks directly to soils. It has been observed in the field that there is a sharp transition from altered rock to soil which takes place only along the top few millimetres on the face of rocks.