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Types Of Rocks

Updated on October 22, 2011

Rocks are naturally occurring solids which are aggregate of many different minerals and mineraloids. The earth’s crust is made up of rocks and they form the land on which we live.

There are three major types of rocks. They are classified into these three types on the basis their chemical and mineral composition and the texture of their component particles. The three major types of rocks are:

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks

These three classes are then further divided into different subclasses on the basis of their components and particle sizes.

Basalt, an igneous rock
Basalt, an igneous rock

Igneous Rocks

The rocks present at the surface of the earth are termed as the igneous rocks.

The igneous rocks make up the earth’s crust and are the major component of it, constituting more than 50% of the total crust. These rocks are formed as a result of the cooling of magma, which is an exothermic process.

The reason why these rocks are present on the surface of the earth is that it is exposed to cold which is required for the phase conversion of the liquid magma to solid igneous rocks. The igneous rocks can also be called the crystalline form of magma.

Rhyolite, a felsic rock
Rhyolite, a felsic rock
Sedimentary Rock
Sedimentary Rock
Claystone, clastic sedimentary rock
Claystone, clastic sedimentary rock
Quartzite, a metamorphic rock
Quartzite, a metamorphic rock
Gneiss, a foliated metamorphic rock
Gneiss, a foliated metamorphic rock

The igneous rocks are further divided into different types based on two criteria; their composition and their texture.

Based on composition, there are following types of igneous rocks:

  • Felsic Rocks: They are made up of quartz, amphibole, potash feldspar and muscovite.
  • Intermediate Rocks: They are composed of plagioclase, biotite, quartz and amphibole.
  • Mafic Rocks: These rocks are composed of olivine, pyroxene and plagioclase.
  • Ultramafic Rocks: They contain only olivine and pyroxene.

Based on texture, they are:

  • Fine grained or aphanitic
  • Coarse grained or phaneritic
  • Glassy
  • Frothy

Sedimentary Rocks

Apart from the igneous rocks the earth’s surface also consists of sedimentary rocks. The sedimentary rocks make up three quarters of the earth’s surface and are mostly found in areas near water such as beaches.

These rocks are the condensed and solidified form of many small pieces of rocks that were eroded by water.

Many sedimentary rocks are formed under water. Basically when the water slowly erodes a rock, small pieces of that rock are carried along the waves.

These particles either settle at the bottom of the sea or on another land and eventually for the sedimentary rocks. The process of formation of sedimentary rocks is a very slow and a gradual process.

The sedimentary rocks of today are composed of rock particles hundreds and thousands year old. Because of this these rocks are full of fossils and are used to study the land, animals and other factors of the time to which they belong.

Here are further three types of sedimentary rocks:

  • Clastic Sedimentary Rocks:

    The clastic sedimentary are made up of broken bits of other rocks called as sediments. Sediments can also be pieces of sand that cement together to form sedimentary rocks that we have today.
  • Chemical Sedimentary Rocks:

    The chemical sedimentary rocks are composed of crystalline minerals such as: halite and gypsum. These minerals gather up and join to form chemical sedimentary rocks.
  • Organic Sedimentary Rocks:

    The organic sedimentary rocks are made up of the remains of animal and plant bodies of the past. These rocks are very helpful in determining the records of previous plants and animals.

Metamorphic Rocks

The metamorphic rocks are the rocks which are present beneath the surface of the earth.

These rocks are formed when heat and pressure is applied to both igneous and sedimentary rocks.

When these rocks are buried deep in the ground the heat and pressure beneath the surface of the earth causes the rocks to change their forms and convert into metamorphic rocks.

Common examples of metamorphic rocks are: marbles and slate.

There are two types of metamorphic rocks:

  • Foliated Rocks:

    These rocks are formed by direct heat and pressure which brings pieces of similar rocks to join and from rocks. Examples are: slate, phyllite, schist, gnesis etc.
  • Non Foliated Rocks:

    For the non foliated rocks lithostatic pressure and heat directly change the form of the existing rocks present beneath the surface. Examples include: quartzite, hornfels and marble.

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