Trap rock is a form of plutonic igneous rock that tends to form polygonal vertical fractures, most typically hexagonal, but also four to eight sided. The fracture pattern forms when magma of suitable chemical composition (typically basaltic) intrudes as a sill or extrudes as a thick lava flow, and slowly cools.
Because of the regular vertical fracture planes plus frequent horizontal fractures, trap rock tends to appear in orderly structures resembling piles of blocks, sometimes reminiscent of stairs and inspiring the term "trap", which derives from a Scandinavian word meaning "steps" or "stairs".
Trap rock is commercially quarried and crushed to provide hard bulk in composite materials that bond it together, such as concrete and macadam.
Trap rock is the primary constituent of many ridges and other rock outcrops in the Hudson River valley, such as the Palisades, and southern New England.
The Deccan traps and Siberian traps are vast areas of basaltic volcanism in India and Siberia respectively.