Extensive Definition
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.