Volcanic rock layers on a cliff by a roadside in the Anaga Mountains, Tenerife, Canary Islands. The bands mainly comprise basalts, formed by the cooling of molten rock (magma). Between the bands are layers (strata) of tuff, a rock formed by the compression and consolidation of ash from volcanic eruptions. The basalts were laid down between 6.5 and 3.28 million years ago. The rock layers were horizontal when they were laid down, and were uplifted to this tilted position by geological forces. The Canary Islands are a group of volcanic islands built up by successive undersea eruptions.