A simple model for the genesis of large gravitational landslide hazards in the Canary Islands

Natural hazards in the Canarian Archipelago are mainly in relation to volcanism (at least five islands with Holocene eruptive activity, 17 eruptions in the last 500 years) and massive gravitational landslides related to edifice overgrowth and dyke intrusion. In the volcanically active islands, the emission vents tend to group in clearly aligned clusters, evolving to form steep ridges that behave as true polygenetic active volcanoes with clear rift affinities, and constitute by far the most probable location of any future volcanic eruption in the archipelago. Tunnels excavated for water mining show a narrow band of tightly packed parallel dykes running through the centre of the rifts. The rift zones play a key role in the growth and shape of the island edifices and in the generation of massive landslides. Cumulative gravitational stresses related to the growth of the volcanic edifices and more ephemeral mechanisms associated with intense eruptive phases, such as dyke wedging, increase of slope angles and strong local seismicity associated with magma movement can finally exceed the trigger-threshold of gravitational slides. This mechanism may be the explanation for the numerous horseshoe-type valleys and calderas developed in the Canary Islands. The geometry of these rift zones is frequently three-branched at 120°C (Mercedes-Benz star configuration), suggesting a least-effort fracturing by magma-induced upwelling. The rift zones play a major role in the distribution of geological hazards: eruptive vents and the failure planes of gravitational landslides are located preferentially along these volcanic lineaments.