Dikes, in geology, are sheet-like bodies of rock that form when molten magma or sediment fills a fracture in pre-existing rock. They cut across existing rock layers, usually vertically or steeply inclined, and solidify to form a distinct intrusion. They are typically younger than the surrounding rock and can be composed mainly of igneous rock from cooled magma or sometimes sediment. Their thickness is much smaller compared to their other dimensions, with lengths that can extend for kilometers. Dikes are different from sills, which intrude parallel to rock layers. They play a role in volcanic and tectonic processes and can vary widely in composition, most commonly basaltic.