The Canary Islands: an example of structural control on the growth of large oceanic-island volcanoes

Dike complexes, which are increasingly accepted as a common feature in the growth of most oceanic volcanoes, are well represented in the Canary Islands, where their deep structure can be readily observed through hundreds of infiltration galleries excavated for water mining. These intrusive complexes have their surficial representation as narrow, clearly aligned clusters of emission centers that, cumulatively, form steep topographic ridges. In the subsoil, a narrow band of tightly packed parallel dikes runs through the center of the structure. These volcanotectonic features behave as true active polygenetic volcanoes and show clear rift affinities. The geometry of these rift zones is either single or three-branched. The two-branched stage, probably transitional, has not been observed. The rift zones play a key role in the mass wasting and destruction of mature oceanic volcanoes. Cumulative gravitational stresses related to the growth of the volcanic edifices increase their instability. More ephemeral mechanisms associated with intense eruptive phases, such as dike wedging, increase of slope angles and strong local seismicity associated with magma movement can finally trigger massive landslides. Massive landslides, enhanced by later erosion, may be the explanation for the origin of numerous horseshoe-type valleys and calderas in the Canary Islands. The "least-effort" geometry of complex rift zones seems to fit some mechanism of magma-induced upwelling, such as a hotspot, in the explanation of the genesis of the Canarian Archipelago. The rift zones play a major role in the distribution of historic volcanism in the Canary Islands and, therefore, in their volcanic hazards assessment.