Main Rock Types: Igneous, Sedimentary, Metamorphic

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