Metamorphic Rock

Metamorphic rock is rock that has been altered by heat or by heat and pressure. ‘Metamorphic' means ‘change of form'; heat and pressure can change the forms of many things, for example, a glassmaker uses heat to change a certain kind of sand into glass. Rocks change when mountain-building forces apply a great deal of pressure and heat to them. Rock is changed by heat produced by nearby molten igneous rock, that is, molten rock, or by both heat and pressure produced mainly by movements in the earth's surface which are associated with the formation of mountains. These rocks formed from other rocks by essentially solid state changes in mineralogy and textures resulting from chemical or physical changes that occur in solid rock buried in the earth's crust are called metamorphic rocks. They have been changed usually by heat and pressure from their original condition into rock with new minerals and structures; some of the minerals in rock are broken down and form new minerals. The grains that make up the rock may become larger. The mineral content of metamorphic rocks depends both on its protolith and the metamorphic conditions the rocks endured. The presence of some specific minerals in a metamorphic rock can indicate the degree of heat and pressure it sustained. Metamorphic rocks are sedimentary or igneous rocks that have been modified or changed in form, that is, the size, shape and arrangement of the minerals in rocks, by heat or pressure. As they are derived from previously existing igneous, sedimentary or even metamorphic rock, their appearance varies from one to the other. Metamorphic rocks are identified by the types of minerals they contain and their texture. Examples of sedimentary rocks changing to metamorphic rock as a result of heat and pressure are limestone changing to marble, and shale changing to slate.