Trap rock

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.

Uses

Trap rock, i.e. basalt or diabase, has a variety of uses. A major use for basalt is crushed rock for road and housing construction in concrete, macadam, and paving stones. Because of its insensitivity to chemical influences, resistance to mechanical stress, high dry relative density, frost resistance, and seawater resistance, trap rock is used as ballast for railroad track bed and hydraulic engineering rock (riprap) in coast and bank protection for paving embankments. It is also used for the production of cast rock that is used in corrosion and abrasion protection, as for sewage pipes and acid-resistant rocks.

Other uses include gardening and landscaping, for the production of millstones, for the production of mineral wool, as a flux in ceramic masses and glazes, for the production of glass ceramics, crushed as a filter aggregate (air filtration of poison gas) in ABC bunkers, as filter bed material at water treatment facilities, and ground as a soil improvement product. Trap rock has been used to construct buildings and churches: Trinity Church on the Green with trap rock quarried from Eli Whitney's quarry is a particularly colorful example of a red-orange-brown-colored, natural-faced trap rock.