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A dike is a vertical or highly inclined sheet of igneous rock, formed when molten rock material or magma has forced its way toward the surface through a cleft, or by melting a passage for itself, and has cooled and solidified there. The magma which is intruding may change direction, branch away into smaller dikes, or give rise to sills, laccoliths, harpoliths, phacoliths, or lopoliths. When exposed at the surface, a dike usually follows a linear path and if it is more resistant than the surrounding rock; it will form a ridge-like structure after long periods of weathering, but if the dike composition is less resistant than the surrounding rock it will form a depression. A dike's thickness may vary from a fraction of an inch to hundreds of feet. Dikes occasionally occur in radial or parallel groups called dike swarms.

Dike in old alluvium
Dike in old alluvium
Earth Science Slides by John S. Shelton
Dike in old alluvium  Salisbury crags sill  Ship Rock and dikes  Large dike north of Spanish Peaks  Broadside view of Spanish Peaks dike 

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