dike, in geology, tabular or sheetlike igneous body that is often oriented vertically or steeply inclined to the bedding of preexisting intruded rocks; similar bodies oriented parallel to the bedding of the enclosing rocks are called sills. A dike set is composed of several parallel dikes; when the number of dikes is large, the term dike swarm is used. Dikes have a wide range of rock compositions. They commonly have a porphyritic texture, i.e., larger crystals within a finer-grained groundmass, indicating two periods of crystallization.

Although dikes may range in size from a few centimetres to greater than 10 metres (about 33 feet) in width, they average between 0.3 and 6 metres (between about 1 and 20 feet) wide. The length of a dike usually depends upon how far it can be traced across the surface; dikes can be up to hundreds of miles long. The world’s longest dike is the Great Dike of Zimbabwe, which stretches for more than 550 km (about 342 miles) southwest to northeast across the centre of the country.