What is "trap rock"

Trap \Trap\, n. [Sw. trapp; akin to trappa stairs, Dan. trappe, G. treppe, D. trap; -- so called because the rocks of this class often occur in large, tabular masses, rising above one another, like steps. See Tramp.] (Geol.) An old term rather loosely used to designate various dark-colored, heavy igneous rocks, including especially the feldspathic-augitic rocks, basalt, dolerite, amygdaloid, etc., but including also some kinds of diorite. Called also trap rock.

Trap tufa, Trap tuff, a kind of fragmental rock made up of fragments and earthy materials from trap rocks.

n. A form of igneous rock that tends to form polygonal vertical fractures.

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, i.e. 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.