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
Geological history of the Precordillera terrane range)—literally "pre-mountain range"—and applied usually to the Andes. The Precordillera is in western Argentina and can be traced by the thrust-fold belt. It is about 200 km wide, and stretches 800 km from latitude 29°S to 33°S in western Argentina, with its south end in Mendoza, through San Juan, to La Rioja in the north. The northern and southern boundaries are not precisely defined. In terms of geological regions, the northeastern boundary connects to the western Pampeanas and the Famatina ranges, and the eastern margin connects to the frontal cordillera then the Chilenia Terrace. The Early-Cambrian to Ordovician deposits of