It is molten rock formed at high temperature deep in the earth’s crust. Rock starts to melt beyond a certain temperature and this temperature is theoretically located in a line called the solidus which below this line, solids begin to melt. Magma which extrudes onto the surface of the earth is called lava. This then is magma and remember that igneous rocks are formed when magma cools and thus forms a solid substance. When magma cools to form rock beneath the earth’s surface, it is called intrusive rock and when above the earth’s surface, it is extrusive or volcanic rock. The cooling of magma is what forms igneous rocks. This cooling is a slow process and the rock is then coarse grained. The classification of igneous rock is important and two important indicators are the composition of the rock as well as the nature of the magma cooling process. The magma cooling process is thus a significant and central process in the formation of igneous rock. There are many minerals which can be specifically identified as important in the composition of igneous rocks. These following minerals have been identified as important: feldspars, quartz, olivines, pyroxenes, amphiboles and micas.
Sedimentary rock is also one of the three main rock types. Common types of rock such as chalk, limestone, dolomite, sandstone, conglomerate and shale belong to the sedimentary rock group. Sediment is an important word and it is defined in a basic way as solids deposited on a bed beneath the water surface or in liquid; thus sediment is solids deposited on a base or floor in a liquid which is usually water. Sediment is thus what is called a particulate matter which is carried through fluid or air or also by wind and glacier to its eventual place where it is deposited. The whole process leading to the deposit of a solid particulate matter is important. Like magma cooling in igneous rocks, it is the transport and eventual deposit of the particulate matter which is important here. The place where the matter is deposited seems to be usually within water e.g. seas and lakes. A note now on the three main classes of sedimentary rock and you can get an idea of the nature of sedimentary rock from this simple classification. There is 1) clastic sedimentary rock which are composed of clasts or fragments of various rocks brought together. Quartz is an important mineral in these types of rocks. Feldspar, amphiboles and clay minerals are also important minerals. Then the second class is 2) biochemical which are rocks formed from materials generated or formed by living organisms. Limestone is an example and Limestone is formed by calcite which is generated from living organisms such as molluscs on the sea floor. 3) Chemical Precipitate is the final class which is formed when mineral solutions evaporate. Note that sea water is classed as a mineral solution. Halite and Gypsum which are known as Evaporite minerals are examples of the Chemical Precipitate class.
Metamorphic is the final example. It is formed by the metamorphism or change in form of an existing rock. The important question is how this change in form happens. Pressure and extreme temperature is what causes the existing rock or Protolith to change in form. Metamorphic rocks can be formed for example within the earth’s crust by extreme temperature and pressure from rock layers above it. It is interesting to note that there are specific minerals called Index Minerals which are formed because of this high temperature and pressure levels. Examples are kyanite, andalusite and garnet and these minerals are thus found specifically in metamorphic rock. An important process in metamorphic rocks is Foliation. This process results in bands of minerals forming and coming together in layers. Gneist, slate and marble are examples of metamorphic rock.