Giant volcanic landslides are one of the most hazardous geological processes. Still, the mechanisms that trigger them remain unresolved. Recent studies suggest that the presence of weak volcanic materials is likely to play an important role. Herein, we present a study of the weakening effect of weathering and hydrothermal alteration of phonolitic lavas, pyroclasts and ignimbrites from Tenerife. A comprehensive geotechnical characterisation of these materials reveals that, from weathering, the weakest units are porous, sandy-silty, non-plastic soils (SM) that are cohesionless, with high peak strengths and significantly lower residual strengths. In the case of hydrothermal alteration, the weakest units are porous, silty, clay-rich, medium plasticity soils (MH) with low cohesion values and varying angles of internal friction (17–45°). Secondary mineralogy produced by alteration, mainly halloysites and the presence of bonding in weathered soils and kaolinites or alunites in hydrothermally altered soils, appears to control the behaviour of the soils.

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