Metamorphic rocks

Metamorphic rocks

Metamorphic rocks are those which have been changed inside the Earths crust by heat, pressure and chemical activity. Both igneous and sedimentary rocks can be metamorphosed, a word which means changed completely.

Some rocks are metamorphosed over thousands of square kilometres. This happens when rocks that have been buried deep in the Earth return to the surface through earthquakes or the erosion of the rocks above them. This is known as regional metamorphism.

Contact metamorphism occurs in rocks that lie close to where new igneous rocks have been thrust up to the surface. This heats the existing rocks up and changes them. Contact metamorphism is often found in the rock surrounding a dike.

Many metamorphic rocks, such as gneisses, can be readily identified by their striped or banded appearance. They are formed under considerable pressure.

Gneiss is a regional metamorphosed rock. The parent rocks are often sedimentary, but some gneisses have formed from granitelike igneous rocks. The main minerals include feldspars, biotite and muscovite. Gneiss is coarsegrained, with irregular banding. The colour varies from light, in gneisses derived from granite, to dark in rocks derived from sandstones. Gneiss occurs worldwide. In Europe it is common throughout the Alps, while in the USA there are good deposits in New York, New England and Georgia. It is sometimes used as a building stone. The name comes from an old German word meaning to give off sparkles.

Marble is formed by both regional and contact metamorphism. The parent rock is limestone, recrystallized, and the main mineral is calcite. The colours vary from pure white to a mosaic of red, green or brown streaks and patches. Marble is found all over the world, sometimes in huge quantities such as the 130km long bed in Vermont, USA, which contains marbles of several different colours. The most famous marble comes from Carrara, Italy. It has been used by sculptors for hundreds of years. The name marble comes from a Greek word meaning to sparkle.

Schist is the name given to a variety of regional metamorphic rocks, which vary according to the main minerals in them and the parent rocks. Schists can be identified by the parallel arrangement of most of their minerals. The name schist comes from a Greek word meaning to split.

Mica schists come from clayey sedimentary rocks, and contain mica biotite and muscovite and quartz. Chlorite schists come from lava, and the main mineral is chlorite. Talc schist is greasy to the touch, like the mineral talc which is its main ingredient.

Quartzite is a regional metamorphic rock made up entirely or mostly of the mineral quartz, hence its name. The parent rocks are many kinds of quartzrich sedimentary rocks, for example greywacke or flint. Pure quartzite is white, but if there are other minerals present it may be grey even black if dark minerals such as biotite are included. It is found worldwide. It is abundant in the Scottish Highlands, and in the USA there are large deposits in the Carolinas. Quartzite is used in buildings as flooring and as a facing stone. It is also used in glass and ceramics.