Basalt dyke in a tuff cliff by a roadside in the Anaga Mountains, Tenerife, Canary Islands. The majority of theis cliff is red tuff, a rock formed by the compression and consolidation of ash from volcanic eruptions. Down centre is a basalt dyke (grey). This formed as molten rock flowed between cracks in the tuff and then cooled and solidified. Dykes typically form in vertical cracks, giving rise to formations such as this. The Canary Islands are a group of volcanic islands built up by successive undersea eruptions.