Causes and mobility of large volcanic landslides: application to Tenerife, Canary Islands

Giant volcanic landslides are one of the most hazardous geological processes due to their volume and velocity. Since the 1980 eruption and associated debris avalanche of Mount St. Helens hundreds of similar events have been recognised worldwide both on continental volcanoes and volcanic oceanic islands. However, the causes and mobility of these enormous mass movements remain unresolved. Tenerife exhibits three voluminous subaerial valleys and a wide offshore apron of landslide debris produced by recurrent flank failures with ages ranging from Upper Pliocene to Middle Pleistocene. We have selected the La Orotava landslide for analysis of its causes and mobility using a variety of simple numerical models. First, the causes of the landslide have been evaluated using Limit Equilibrium Method and 2D Finite Difference techniques. Conventional parameters including hydrostatic pore pressure and material strength properties, together with three external processes, dike intrusion, caldera collapse and seismicity, have been incorporated into the stability models. The results indicate that each of the external mechanism studied is capable of initiating slope failures. However, we propose that a combination of these processes may be the most probable cause for giant volcanic landslides. Second, we have analysed the runout distance of the landslide using a simple model treating both the subaerial and submarine parts of the sliding path. The effect of the friction coefficient, drag forces and hydroplaning has been incorporated into the model. The results indicate that hydroplaning particularly can significantly increase the mobility of the landslide, which may reach runout distances greater than 70 km. The models presented are not considered definite and have mainly a conceptual purpose. However, they provide a physical basis from which to better interpret these complex geologic phenomena and should be taken into account in the prediction of future events and the assessment of landslide related hazards.