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The Cañadas Volcano and the Dorsal Ridge Basalts (1.9 – 0.13 Ma)
No surface rocks on the island have been dated in the range of 3.3-1.9 Ma, and this period most likely represents a interval of volcanic quiescence and erosion. This interlude ended with the development of a large composite volcano (Cañadas I), in what is now the centre of the island, at the junction of the three extant basaltic massifs. This large structure is estimated to have been approximately 20km in diameter rising up to 2500m above the submarine basement. Early eruptions (c. 1.9-1.8 Ma) forming the lower stratigraphy of Las Cañadas are mainly basaltic lavas, but are overlain by a complex succession of more highly evolved volcanic rocks including trachytes and phonolites which were erupted between 1.5-0.17 Ma (Marti et al., 1994). The upper part of the Cañadas volcano was partially destroyed by subsidence to form the world famous Cañadas Caldera. Recent studies of the caldera wall have help to reveal the stratigraphy of Las Cañadas (Marti et al., 1994).
Coincident with the later stages of development of the Cañadas volcano, extensive basaltic emissions (Old Basaltic Series II) along the axis between Las Cañadas and the older Anaga massif created the volcanic spine known as the Dorsal ridge (Cordillera Dorsal). On each side of this ridge, two large depressions occurs, the valleys of Guimar (south) and Orotava (north). The formation of these large, steep-sided, valleys has been the subject of much speculation. Palacios (1994) reviews most the commonly proposed theories. These include;
- that the valleys have been created by the lateral accumulation of lava flows creating relief and giving the impression of a large erosional feature;
- that the valleys are asymmetric calderas formed due to volcanic collapse;
- that the valleys are due to large-scale landsliding.
Palacios (ibid.) proposes an alternative theory, in which he suggests that these large features have developed due to rapid valley widening following relief inversion. This occurs when freely flowing basaltic lavas exploit existing valleys and then cool to leave highly resistant material in the old erosion feature. Drainage is therefore displaced to the lateral margins of the lava flow and so widening the valley.
More recently, Watts and Mason (1995) present results of marine surveys in the coastal waters of Tenerife which indicate the presence of an extensive bathymetric high off-shore of Orotava. They interpret this as a significant landslide deposit, appearing to confirm the earlier hypothesis of large-scale mass movement for the formation of the Orotava valley. They speculate that such extensive landsliding may be a particular feature of recent oceanic volcanic systems when, because of incomplete isostatic adjustment (leading to subsidence), oceanic volcanoes attain their highest elevations and thus are most prone of slope failure. K-Ar dating of the back-wall of the Orotava valley suggests that this landslide occurred between 0.78-0.27 Ma.
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