Trap_rock
Trap rock
Dark-colored, fine-grained, non-granitic igneous rock
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.[1] Trap is also used to refer to flood (plateau) basalts, e.g. the Deccan Traps and Siberian Traps.[2] 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".[1]
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.[3][4]