Trap rock, also known as either trapp or trap, is any dark-colored, fine-grained, non-granitic intrusive or extrusive igneous rock. Types of trap rock include basalt, peridotite, diabase, and gabbro. Trapp (trap) is also used to refer to flood (plateau) basalts, e.g. the Deccan Traps and Siberian Traps. The erosion of trap rock created by the stacking of successive lava flows often created a distinct stairstep landscape from which the term trap was derived from the Swedish word trappa, which means "stairway".
The slow cooling of magma either as a sill or as a thick lava flow sometimes creates systematic vertical fractures within the resulting layer of trap rock. These fractures often form rock columns that are typically hexagonal, but also four- to eight-sided.