Types of Rocks and Rock Cycle

  • Majority of earth’s crust is made of rocks. It constitutes 1% of the total weight of the earth. Rocks are the source of minerals. Rocks are found in several shapes, colours and textures.
  • Geologists classify rocks into three groups on the basis the way they are formed. The three basic types of rocks are igneous rocks, sedimentary rocks and metamorphic rocks.
  • Igneous rocks are formed by cooling and solidification of magma. Igneous rocks are basically divided into two groups on the basis of place where cooling takes place. These are extrusive rocks and intrusive rocks.
  • Extrusive igneous rocks are formed when magma cools on the surface of the earth.
  • Intrusive igneous rocks are formed when magma cools below the surface of the earth.
  • Intrusive rocks can further be classified into hypabyssal rocks and plutonic rocks.
  • Hypabyssal rocks are formed just below the surface of the earth. Plutonic rocks are formed deep beneath the earth’s crust
  • Igneous rocks are mainly composed up of silicates.
  • Sedimentary rocks are formed by the deposition of weathered material on the surface and in water bodies. The important process associated with the formation of sedimentary rock is lithification. Lithification is done by two processes of compaction and cementation
  • Compaction is the slow squeezing of sediments to form hard rock. Cementation is the binding together of compacted sediments.
  • Sedimentary rocks can be broadly classified into three types.
  • Mechanically formed sedimentary rocks contain pieces of other rocks.
  • Organically formed sedimentary rocks consist of remains of animals and plants.
  • Chemically formed sedimentary rocks are formed by direct precipitation of mineral matter from solution
  • Metamorphic rocks are formed by the transformation of igneous and sedimentary rocks under intense heat and pressure.
  • Process of formation of metamorphic rocks is called metamorphism. Metamorphism happens as a result of volcanic activity, movement in the earth’s crust, etc.
  • Process of formation of metamorphic rocks takes a long time.
  • Some important examples of metamorphic rocks are slate, schist, marble, quartzite and gneiss.
  • Cycle of rock transformation is the relationship between three types of rocks.
  • There are various uses of rocks, like making tools and implements, construction of houses, building and roads, etc.
  • Minerals are solid inorganic substances are also found in nature.
  • Minerals can be profitably derived from an ore.
  • For the extraction of minerals from an ore, several refining processes are involved.
  • Minerals can be classified as precious metals, ferrous metals, non-ferrous metals and non-metallic minerals.
  • Some of the precious metals are gold, silver and, platinum.
  • Ferrous metals are iron, and manganese.
  • Non-ferrous metals are copper, lead, zinc, tin and aluminium.
  • Non-metallic minerals do not contain metals, like sulphur, phosphates and nitrates.
  • The hard, sparkling mineral crystals that glitter and reflect light are known as gemstones.

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  • Q1

    What is stratigraphy?


    The study of the composition of a rock, its structure and the fossils it contain is known as stratigraphy.

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  • Q2

    Why sedimentary rocks are also known as stratified rocks?


    Sedimentary rocks are also known as stratified rocks as they are found in layers. ‘Strata’ means layers in Latin.

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  • Q3

    Define ores.


    Some rocks contain metal compounds in large quantities, which can be extracted economically. These rocks are called ores.

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  • Q4

    Into how many types can the rocks be classified? Mention their names.


    Rocks can be mainly classified into three types. They are:

    (i) Igneous Rocks (ii) Sedimentary Rocks
    (iii) Metamorphic Rocks

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  • Q5

    How will you define rocks?


    Rocks can be defined as the solid materials that make up the earth’s crust.

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