A dike is a sheet like structure of intrusive rock. Magma exploits a weakness in rock such as a bedding plain in sedimentary rock or structural weakness and injects magma. Rings of dykes often surround volcanoes and dike swarms can accompany larger intrusions. Dikes are vertical or near vertical and although narrow may extend for several kilometres. They are by defintion younger than the rocks through which they pass. Compositions range from dolerite a, finer grained version of gabbro,to granitic dikes called pegmatites which have very large crystal sizes and mineraliation. Ophiolite complexes ,such as the Lizard Peninsular in Cornwall, show a high concentration of dolerite dikes.
An Ophiolite is a section of Oceanic crust which has been thrust onto the the land. Basalt,dolerite and Gabbro can all be present.