Igneous rock the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. They are formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and fine-grained. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example. The molten rock, with
Big Bone Cave Big Bone Cave (Big) Bone Cave is a cave located in Van Buren County, Tennessee, in the community of Bone Cave that is named after it. It is notable both for its history and current recreational use. It is listed on the National Register of Historic Places and a designated National Natural Landmark. It is a State Natural Area managed by Rock Island State Park. It is named for the discovery of the bones of a giant ground sloth in 1811. The cave was operated as a saltpeter mine, notably during the War of 1812 and the American Civil War.