Trap rock, also known as eithertrapp ortrap, is any dark-colored, fine-grained,non-granitic intrusive or extrusiveigneous rock. Types of trap rock includebasalt,peridotite,diabase, andgabbro.[1] Trap is also used to refer toflood (plateau) basalts, such as theDeccan Traps andSiberian Traps.[2] The erosion of trap rock created by the stacking of successivelava flows often creates a distinct stairstep landscape from which the termtrap was derived from the Swedish wordtrappa, which means "stairs".[1]
The slow cooling ofmagma either as asill or as a thicklava flow sometimes creates systematic vertical fractures within the resulting layer of trap rock. These fractures often formrock columns that are typically hexagonal but could be four- to eight-sided.[3][4]
Trap rock has a variety of uses. A major use forbasalt iscrushed stone for road and housing construction inconcrete,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 asballast for railroadtrack bed and hydraulic engineering rock (riprap) in coast and bank protection for paving embankments. It is also used for the production ofcast rock that is used in corrosion and abrasion protection, such as forsewage pipes.
Other uses include gardening and landscaping,millstones,mineral wool, as aflux in ceramic masses and glazes, for the production of glass ceramics, crushed as a filter aggregate (air filtration of poison gas) inABC bunkers, as filter bed material atwater treatment facilities, and ground as a soil improvement product.[5] Trap rock has been used to construct buildings and churches:Trinity Church on the Green with trap rock quarried fromEli Whitney's quarry is a particularly colorful example of a red-orange-brown-colored, natural-faced trap rock.
Well-known examples ofoutcropping trap rock include bothintrusive sills andextrusive lava flows. They include thePalisades Sill, aTriassic, 200-Madiabase intrusion that formsThe Palisades along 80 kilometers (50 mi) of theHudson River inNew York andNew Jersey. Vast areas of trap rock in the form of thick lava flows and other volcanic rocks comprise theDeccan Traps of India andSiberian Traps of Russia.[6]
Other prominent basalt ridges, mountains, buttes, canyons, and other landscape features include:
