Broad Classification of Rocks

Pressure, Temperature and RocksIgenous, sedimentary and metamorphic rocks are all formed under different pressure and temperature conditiion in the rock cycle. The boundaries are zones where rock grades into another. The boundary between the diagenesis of sediments and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational. The boundary between metamorphic and igneous rocks is where partial melting occurs. Metamorphic rocks recrystallise in the solid state but igneous rocks crystallise from magma. Rocks are classified according to the way they are formed.Sedimentary RocksThese rocks form at relatively low pressure and temperature at shallow depths below the Earth's surface. Existing rocks exposed at the Earth's surface are broken down by weathering and erosion. The rock cyce diagram shows that broken fragments of these rocks and ions in solution are transported usually to the oceans by rivers. The energy of the water decreases when it spreads out, where rivers enter the oceans, and the fragments of rock deposited on the ocean floor.Over millions of years more and more fragments are deposited, buried and compacted into layers to make sedimentary rocks for example sandstone. Many sedimentary rocks are made up of clasts held together by cement, rather than interlocking crystals. The grain size of clasts can vary considerably. Other sedimentary rocks are biologically formed from the remains of organisms, or chemically formed by the precipitation of ions from sea water. Sedimentary rocks may contain fossils of plants, animals or traces of animal activity.They are usually built up in layers or beds.Igneous RocksIgneous rocks are formed from magma within the Earth. They may form at high or low pressures depending on depth, but they all form at high temperatures. Magma eventually cools to form igneous rocks formed of interlocking crystals. The mineral crystals do not usually have any preferred alignment. There are two main types of igenous rock: Intrusive - these have medium crystal size if the magma cooled fairly slowly at intermediate depths, for example dolerite, or coarse crystal size if the magma cooled slowly at greater depth inside the Earth's crust, for example granite. The minerals are hard silicates, such as quartz and feldspar, and separate mineral crystals can be seen in the rock. Extrusive - these have fine crystal size as the magma reached the Earth's surface. Erupted as lava or ejected as fragments, it cooled rapidly, producing rocks such as basalt or pumice. Individual crystals cannot usually be seen. Metamorphic RocksWithin the Earth may be subjected to heat or pressure - or both. This causes their texture and mineral composition to change. Minerals in the original or parent rock recrystallise in the solid state to form new minerals. This means that metamorphic rocks are composed of interlocking crystals, but they often show preferred alignment if they have been affected by pressure. There are three main types of metamorphism: burial, contact and regional. If they contain flat crystals, rocks affected by pressure as part of burial or regional metamorphism show foliation. Foliated layers commonly contain mica. Metamorphic minerals tend to be hard silicates but marble is composed of calcite, a carbonate mineral.

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