Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Rocks → Material that form crust of earth

Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Igneous Rocks

  • Formed through cooling & solidification of molten material magma ( Erupted from volcanoes, molten material moves towards surface of earth through crack)
  • Normally crystalline in structure, do not occur in strata & do not contains fossils
  • Can be subdivided on the basis of mineral composition
  • Basic rocks contain high proportion of basic oxides mainly of iron, aluminium & magnesium
  • When they contain high proportion of silica they are said to be acidic, which are less dense & lighter in color than basic rocks for ex. Granite
  • Most igneous rocks are extremely hard & resistant hence are quarried for road making & polished as monuments & grave stones
  • Are parent / primary rocks as all other rocks are derived from it.

In terms of origin they can be divided into mainly 2 classes viz.

Plutonic Rocks
  • formed due to solidification of magma (interior of earth)
  • They have cooled & solidified slowly so that large, easily recognized crystals are formed till some depth inside earth crust
  • Examples Granite, Diorite & Gabbro
  • Exposed at the surface by denudation & erosion
Volcanic Rocks
  • formed due to solidification of lava (interior of earth)
  • Molten rocks poured out of volcanoes as lavas
  • They have solidified rapidly on the earth’s surface hence have small crystals
  • For ex. Basalt (which yield Regur or black soil)

Formation of rocks

Sedimentary Rocks

  • Formed due to deposition of layers of sediment usually along the water bodies over a long period of time
  • Sediment is deposited layer by layer in form of strata hence also known as stratified rocks
  • Process of turning sediments into hard rock layers by pressure is known as lithification
  • Rocks may be fine grained or coarse, soft or hard & material forming them may be brought by streams, glaciers, winds or even animals
  • May be derived from Igneous, Metamorphic or Sedimentary rocks
  • Hence, Sedimentary rocks are most varied in formation of all rocks
  • They are non-crystalline & often contains fossils of animals, plants & other microorganisms

May be classified under 3 categories with respect to their origin & composition viz

Mechanically formed
  • Formed from the accumulation of materials derived from other rocks
  • Made from sand grains with tremendously varying texture, composition & colour. For ex : Sandstone
  • Mainly quarried for building purposes or for making grindstones
  • Finer sedimentary materials form clay, widely used for making bricks, shale or mudstone.
  • Sand & gravel may occur in uncemented form
Organically formed
  • Formed from the remains of living organisms such as corals or shellfish, whose fleshy part has been decomposed, leaving behind the hard cells.
  • Most common rocks formed due to this process are Calcareous type for ex. Limestone & Chalk
  • Carbonaceous rocks are also organically formed but from vegetative matter – Swaps and forests
  • Pressure of overlying sediments has compressed the plants remains into compact masses of carbon which eventually becomes Peat, Lignite or Coal
Chemically formed
  • Formed chemically from solutions of one another
  • For ex. Gypsum (Calcium sulphate) is obtained from evaporation of salt lakes having high salinity
  • Same way Potash & Nitrates may be formed

Metamorphic Rocks

  • Formed when original structure of igneous & sedimentary rocks partially or wholly change under the action of heat & pressure
  • Contains no fossils
  • No stratification
  • For ex. Clay Slate
Igneous to Metamorphic

  • Granite Gneiss
  • Mica     Schist
  • Gabro   Serpentite
Sedimentary to Metamorphic

  • Limestone             Marble
  • Sandstone             Quartzite
  • Shale                     Schist
  • Coal                       Graphite
  • Bituminous coal     Anthracite coal
India Yearbook English India Yearbook Hindi Economic Survey 2017


  1. thankyou very much to share the knowledge.

  2. good

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