Igneous Rock

Igneous Rock:In geology, rock that has been formed by the cooling and subsequent solidification of a molten mass of rock material, known as magma. Depending upon the conditions under which the magma cooled, the resulting rocks may be coarse-grained or fine-grained. Plutonic rocks, such as granite and syenite, were formed from a magma buried deep within the crust of the earth. The rocks cooled very slowly, thus permitting large crystals of individual minerals to form. Volcanic rocks, typified by basalt and rhyolite, were formed when the molten magma rose from a depth and filled cracks close to the surface, or when the magma was extruded upon the surface of the earth through a volcano. Subsequent cooling and solidification of the magma were very rapid, resulting in the formation of fine-grain minerals or glasslike rocks. Composed almost entirely of silicate minerals, igneous rocks are often classified by their silica content. The major categories are referred to as acid and basic, with granite and rhyolite typical of the former, gabbro and basalt the latter.

Igneous rock :Rock formed from cooling magma or lava, and solidifying from a molten state. Igneous rocks are largely composed of silica (SiO2) and they are classified according to their crystal size, texture, method of formation, or chemical

composition, for example by the proportions of light and dark minerals. Igneous rocks that crystallize from magma below the Earth's surface are called plutonic or intrusive, depending on the depth of formation. They have large crystals produced by slow cooling; examples include dolerite and granite. Those extruded at the surface from lava are called extrusive or volcanic. Rapid cooling results in small crystals; basalt is an example.