Igneous, sedimentary and metamorphic rocks
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There are many different types of rocks on the Earth surface. But they fall into one of three groups: igneous, sedimentary and metamorphic. Igneous rocks start out as magma, under the Earth’s crust. Magma is superheated liquid rock. Once magma is erupted onto the Earth’s surface through volcanoes or crusts on the ground it starts to cool and it’s called lava. Some igneous rocks are formed below ground where magma gets into other rocks. Granite is an example of this and can be seen today on the Earth’s surface because other rocks above it have been eroded away over millions of years. Other igneous rocks are formed when magma is ejected onto the Earth’s surface. Basalt is an example of this type of rock. The Giant’s Causeway in Northern Ireland is basalt formed in this way. Sedimentary rocks, as their name suggests are those that have been formed by layers of sediments. Sediments are small particles of rock which have been eroded and transported by rivers, glaciers and wind. Most particles eventually reach the sea, where they are deposited on the seabed. As more and more particles are deposited, the weight of the sediments above compresses the sediments below into sedimentary rock. This is a very slow process taking millions of years. Sedimentary rocks, because of the way they are formed, have distinct layers, know as beds, with lines of weakness or bedding planes between them. The type of rock formed depends on the type of particles that are deposited and compressed. Limestone and chalk are formed from the compressed jaws and skeletons of small marine organisms such as coral. Much of the limestone in the UK was formed during the Carboniferous period, two hundred and eighty to three hundred and forty five million years ago. At that time most UK was covered by warm, shallow sea. Perfect conditions for the organisms which lived and died here, forming carboniferous limestone. Sandstone is formed from the compression of sand and coal is formed from the fossils’ remains from trees and plants which grow here in hot, wet conditions. Clay forms from the accumulation and compression of mud deposits. Metamorphic rocks are created when other types of rocks are erupted by extremes of heat or pressure. For example, marble is formed when limestone is put under extreme heat and pressure. And slate is formed when clay is changed by pressure.
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