Rocks : Kinds of Metamorphic Rock
Metamorphic rocks are made from older rocks, either igneous or sedimentary. These rocks are changed by great heat and/or pressure deep beneath the earth’s surface. The heat and pressure makes crystals in the rock. If the rock already has crystals, the heat makes the crystals larger. Sometimes the pressure flattens the crystals into layers.
Marble is a metamorphic rock, formed by the alteration of limestone under high temperature and/or high pressure. Stonemasons often give the name marble to any rock that takes a good polish, but this is incorrect in geological terminology.
Slate formed from a mudstone or shale. It splits very easily into very thin sheets, a property known as cleavage. This is caused by the alignment of the very fine grains that form the slate, particularly the platy mineral mica, when the rock is metamorphosed under moderate temperatures and quite high pressures.
Slate was commonly used a roofing material on old houses because it splits so readily. Slate was also used for billiard tables and blackboards, and decorative fireplaces made from enamelled slate can sometimes be found in nineteenth Century houses.
Quartzites are formed through the metamorphism of quartz-rich sandstones. It may sometimes look like marble, but can be distinguished because quartzite cannot be scratched with a knife, unlike marble. It is very hard and weather resistant. Quartzite is used in the glass and ceramic industries, and builders sometimes use it for flooring and facing.
Gneiss is a rock formed during regional metamorphism. It is generally a coarse-grained granular textured rock which can develop from a wide variety of igneous and sedimentary material. It is consist of alternating dark and light bands of minerals which can vary in thickness, from milimetres up to a metre and can be highly contorted.
Schist can be formed from basalt, an igneous rock; shale, a sedimentary rock; or slate, a metamorphic rock. Through tremendous heat and pressure, these rocks were transformed into this new kind of rock.
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