Trap 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] Trapp (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 East Rock trap rock ridge overlooking New Haven, Connecticut, U.S.
Trap rock forming a characteristic pavement, Giant's Causeway and Northern Ireland
Trap rock cliff overlooking the Hudson River from an overlook on the Hudson Palisades in Bergen County, New Jersey, U.S.
Trap rock forming a characteristic stockade wall, Giant's Causeway, Northern Ireland

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]

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