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Journal of Volcanologyand Geothermal Research 60 ( 1994) 225-241
The Canary Islands: an example of structural control on the growth of large oceanic-island volcanoes J.C. C a r r a c e d o Volcanological Station of the Canary Islands, Spanish Research Council (CSIC), P.O. Box 195, 38206 La Laguna, Spain
(Received August 16, 1993; revised version accepted December 16, 1993)
Abstract 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.
1. Introduction and geological framework The Canarian Archipelago is a volcanically active alignment of seven islands situated in a band 200 by 500 km off the African continental margin opposite Cape Juby. Fuerteventura, one of the eastern islands, is only 100 km from the African coast (Fig. 1, inset). The magmatic and volcanic evolution of the archipelago has been influenced by the oceanic-continental transitional nature of the lithosphere on which the islands developed (Dash and Bosshard, 1968; Bosshard and Mc-
Farlane, 1970; Banda et al., 1981; Hoernle et at., 1991), the effect of the Atlas tectonics (Schmincke, 1976, 1982) and the near stationary nature of the African plate in this region since the Miocene, about 10 m m / y r for the last 60 Ma according to Duncan (1981) and Morgan ( 1983 ). The African plate was at least stationary during the subaerial growth stage of these islands. The eastern Canary Islands m a y have been active since the Late Cretaceous (Le Bas et at., 1986). Volcanoes were formed through multiple volcanic cycles (Carracedo, 1979; Schmincke,
0377-0273/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10377-0273 ( 94 )00006-3
1 C. Carracedo/Journal of Volcanology and Geothermal Research 60 (1994) 225-241
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Fig. 1. Map of the Canarian Archipelago showing the location and age of historic volcanic eruptions. The index map shows in vertical striped pattern the islands with historic (last 500 yr) volcanism, in horizontal striped pattern, those with Quaternary volcanism, and in empty pattern, those without Quaternary volcanism. The oldest published radiometric ages from subaerial volcanics of each island are indicated in the inset in parentheses. K/Ar ages of La Palma are from Abdel-Monem et al. ( 1972 ), of Gomera and Fuerteventura from Feraud ( 1981 ), of Tenerife from Ancochea et al. (1990), of Lanzarote from Coello et al. (1992) and of Gran Canaria from McDougall and Schmincke (1976). The oldest subaerial volcanism of Hierro is not yet well determined, but it is older than 0.7 Ma since a stratigraphically intermediate volcanic series has reverse geomagnetic polarity (Carracedo et al., in prep. ).
1982; Cantagrel et al., 1984; Ancochea et al., 1990) with different evolutionary histories on each island. The islands can be separated into three categories (Fig. 1, inset) according to their eruptive activity: ( 1 ) the islands ofTenerife, La Palma, Lanzarote and probably E1 Hierro (Hern~indez Pacheco, 1982) which have had eruptions in historic time ( < 500 yr) and are thus, by definition, volcanically active; eruptive activity on La Palma and Lanzarote indicates that volcanism is presently active along the entire alignment of the archipelago; (2) the islands of Fuerteventura and Gran Canaria with Quaternary volcanism; and (3) the island of Gomera, where
no evidence of Quaternary eruptions has been found (Cantagrel et al., 1984), although longer periods without eruptions on Lanzarote, Gran Canaria and Fuerteventura were followed by renewed activity.
2. Main morphological and structural features of the insular edifices
Two relevant volcano-tectonic features (Fig. 2 ) characterize the islands morphologically: ( 1 ) rift-type clusters of aligned eruptive vents or rift
J.C. Carracedo I Journal of Volcanology and Geothermal Research 60 (I 994) 225-241
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