Types of rocks

Types of rocks

Planet earth is made up of three layers: the crust, the mantle and the core. There is no validated theory to explain the formation of earth 4,500...

Planet earth is made up of three layers: the crust, the mantle and the core. There is no validated theory to explain the formation of earth 4,500 million years ago, but volcanic eruptions, earthquake waves and deep mine operations over the time revealed that the earth possessed three layers.


  • The outer layer in the earth’s crust, called lithosphere comprises of two distinct parts: the upper part consists of granite rock and forms the continents. Granite is made up of aluminium and silica. Therefore it is collectively referred to as SIAL
  • The lower part is a continuous zone of denser ballistic rocks forming the ocean floors that are made up of silica, iron and magnesium, collectively referred to as SIMA
  • The density of crust varies from three to four miles under the oceans to 30 to 40 miles under the continents
  • SAIL is lighter than SIMA. Therefore continents can be said to be floating on a sea of the denser SIMA


  • Mantle is the immediate layer below the crust and is 1,800 miles thick
  • It is mainly composed of dense rocks rich in olivine.


  • Core is the innermost layer of earth; it is 2,160 miles in radius
  • It is mainly made up of Iron and nickel, so called NIFE.
  • The temperature of earth is more than 3,500 degrees F
  • The high temperature and pressure melts the core, but recent studies of earthquakes have established that the core is in a crystalline form
  • The earth’s crust is made up various kinds of rocks. They differ from one another in texture, colour, permeability, chemical composition, mode of occurrence and degree of resistance to denudation. All the rocks are classified into three major groups:
  • Igneous Rocks
  • These rocks are formed by cooling and solidification of magma. They are normally crystalline in structure
  • Igneous rocks are further divided into two types: Plutonic and Volcanic rocks
  • Plutonic rocks are formed deep in the earth’s crust. They are intrusive in nature Examples: Granite, Diorite, and Gabbro
  • Volcanic rocks are molten rocks that pour out of a volcano as magma. They solidify rapidly. These are extrusive in nature. They are very hard and resistant Examples: The Deccan plateau in India and Antrim in Northern Ireland
  • They do not contain fossils

Sedimentary Rocks

  • They are formed from sediments accumulated for a longer period of time
  • They vary from other rocks by layer formation and are termed as stratified rocks
  • The material forming sedimentary rocks may be brought by streams, glaciers, winds or even animals
  • They are crystalline in nature and contain fossils
  • Unlike igneous rocks, sedimentary rocks are classified according to their age
  • Mechanically formed sedimentary rocks: These are formed by the accumulation of sediments from different rock materials. Sandstone is a best example of a mechanically formed sedimentary rock. The rocks differ in colour and are made up of sand grains, quartz and fragments derived from granites
  • Organically formed sedimentary rocks: These rocks are formed from the remains of living organisms such as corals or shell fish. Most of the rocks such formed are calciferous rocks like limestone and chalk. They hold fossil fuels
  • Chemically formed sedimentary rocks: These rocks are precipitated from chemical solutions of one kind or other. Gypsum and Calcium Sulphate are the best examples. Rock salts are derived from strata which once formed the beds of seas or lakes

Metamorphic Rocks

  • All rocks, whether igneous or sedimentary, transform into metamorphic rocks under high temperature and pressure.
  • Dense earth movements change their characteristics.
  • Clay may be metamorphosed into marble, sandstone into quartzite, granite into gneiss, shale into schist and coal into graphite.
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