Tuesday, May 29, 2012

Types of Rocks

The earth's solid layer, called lithosphere is about 100 kms in thickness. The word 'lithosphere' literally means 'a sphere of rocks'.  "Rocks are made of individual solid substances called minerals. Each mineral usually contains two or more elements of which the whole earth is made." All rocks are not hard. Clay, for example, is an example of soft rock.

Rock Types : There are three main groups of rocks, Igneous, Sedimentary and Metamorphic rocks.


  • These are formed from lava hurled out of a volcano or from the cooling of hot magma below the crust. Igneous rocks are classified on the basis of chemical composition and texture.
  • There are no layers in igneous rocks.
  • Fossils are not found in igneous rocks.
  • Granite Diorite, Gabbro etc. are example of intrusive igneous rocks.
  • Obsidian, basalt etc. are examples of extrusive igneous rocks.
  • A very large area of the Deccan Plateau consists of basalt rocks.
  • These are normally crystalline in structure.
  • Acidic igneous rocks i.e. granite, are less dense and are lighter in colour than basic rocks.
  • Basic rocks contain a greater proportion of basic oxides e.g. iron, aluminum or Magnesium, and are thus denser and darker in color.
  • Northern Ireland and Columbia Snake plateaus are also basaltic igneous rock.


They're made from the sediments formed by the erosion and weathering of other rock types. Wind, water and snow erode rocks and carry the sediments to low lying areas. When deposited within the sea, they may be compressed and hardened in order to create layers of rocks. As new layers are added, the under-lying layers experience progressively deeper burial. Because of increasing pressure imposed from the overlying load, water is excluded from the sediment. This technique is termed 'lithification'. Sometimes chemical changes also occur. This whole technique of conversion into solid rocks are 'diagenesis'. Sandstone is constructed out of grains of sand, which have been naturally cemented together. Chalk is made up of millions of tiny calcium carbonate skeletons of micro-organisms.

  • They have a layered structure and also softer rocks.
  • Fossils are commonly found in such rocks.
  • These rocks are called stratified or argillaceous rocks also.
  • About 3/4th of the earth's surface is covered by sedimentary rocks.
  • Sedimentary rocks are also called secondary rocks.
  • Mechanically formed sedimentary rocks are sand stones, conglomerate, clay etc.
  • Organically formed sedimentary rocks are limestone (calcareous), peat, lignite, coal (carbonaceous).
  • Chemically formed sedimentary rocks are Rocksalts, Gypsum etc.


These type of rocks are formed whenever a rocks is exposed to heat and/or pressure. The heat and/or pressure customize the characteristics on the original rock by forming new minerals.

Clay gets metamorphosed to slate. Similarly, limestone engenders marble.

Original RockMetamorphic Rock
Sand StoneQuartzite


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