Rocks: Classifications, Rock Cycle and Geological Time Scale

Rocks are naturally occurring solid mass, aggregate of minerals that form the Earth’s crust.


Rocks are classified into three groups:

  1. Igneous Rock: These rocks are made up of cooled and solidified lava.

They are of utmost importance because:

  1. Its chemistry and minerals found give information about the composition, temperature and pressure conditions of the mantle.
  2. On the basis of these rocks, the absolute ages can be determined and can be compared to closest geological strata which helps in a time sequence of events;
  • Helps in understanding characteristic of a specific tectonic environment.
  1. They constitute mineral deposits of industrial importance.

Igneous Rocks are classified into:

  1. Plutonic rocks (Intrusive rocks):
  • They are formed when lava solidifies beneath the surface.
  • They are found at the depth of around 200 km
  • Their crystals are coarse and found.
  • Granite, gabbros, etc. are examples of Plutonic igneous rocks
  1. Extrusive rocks:
  • They are formed on the Earth’s surface.
  • Their crystals are fine.
  • Basalt is example of Extrusive igneous rocks
  1. Hypabyssal rocks (Intrusive rocks):
  • They are formed at a depth of around 2 km
  • They have mixed properties intrusive and extrusive rocks.
  • Porphyries is example of Hypabyssal igneous rocks.
  1. Sedimentary Rock:
  • These rocks are formed from the accumulation of residual materials of other rocks and organic substances like dead shells of marine organisms and plant.
  • Fossils are found in these rocks.
  • Gravel, Peat, Gypsum, limestone, sandstone etc. are examples of sedimentary rocks.
  1. Metamorphic Rock:
  • These rocks were formed when pressure, heat or action of water change the structure, composition or shape of igneous and sedimentary rocks.
  • Gneiss, Marble, Quartzite, etc. are examples of metamorphic rocks.


The rocks found on the Earth undergo through a cycle called rock cycle. This cycle starts when molten magma comes to the surface and solidifies to become igneous rock. Igneous rock is altered when it is exposed to the atmosphere. Agents of weathering and erosion work on it and break it into pieces. These pieces accumulate in a place and thus form sedimentary rocks. Sedimentary and igneous rocks when come into an environment where pressure, heat or action of water change their structure, composition or shape, they transform into metamorphic rocks. New layers of rocks continuously assemble over old rocks. When old rock reaches the depth where they cannot remain in solid state, they melt.  This cycle of solidifying magma to become rock, reaching depth of the Earth to melt is called rock cycle.


It is a chronological dating system. Geologists, palaeontologists, and Earth scientists use this to describe the timing and relationships of events that have occurred during Earth’s history.

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