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Igneous Rocks (Volcanic or Plutonic) Volcanic - Extrusive - They cool quickly on the Earth's surface consequently forming small crystals. - These crystals are only visible to a microscope. Plutonic Rocks - Intrusive - They cool slowly over a long period of time beneath Earth's surface forming large interlocking crystals. -These crystals are visible to the naked eye. Sedimentary Rocks (Clastic or Non- Clastic) Clastic - Are composed of grains or fragments of rocks and minerals that have been eroded, transported, deposited and cemented or compacted together. - They are formed from weathering and erosion of igneous, metamorphic or other sedimentary rocks. - These rocks are identified by grain size, grain shape and the composition. Non- Clastic - Identified according to their grain size, include two main types; (i) Inorganic - Rocks from chemicals. - Composed of minerals that have been precipitated from water (sea water, fresh water or ground water). (ii) Organic - Rocks from living things. - Composed of fragments of materials from living organisms that are compacted or cemented together. Metamorphic Rocks (Contact or Regional) - Rocks that have been formed by increased in pressure and/or temperature without melting. - They were once igneous, sedimentary or even metamorphic rocks. Contact - Altered when they come in contact with a heat source. - Rocks are not good conductors of heat so the main zone of metamorphism is relatively narrow and occurs in the rock. Regional - Occur when rocks undergo increased temperatures and pressures and are typically associated with the formation of mountain belts along convergent plate boundaries.