Types of Rocks

Alfred Ajibola - Thu, 24th January, 2019 @ 18:28:55 PM

Topics in Geography

The Solar System Layers and Composition of the Atmosphere Types and Branches of Geography Types of Rocks

Academic Questions in Geography

Types of Rock:

We have 3 main types or classes of rock namely, igneous rock, sedimentary rock and metamorphic rock. These classification of rock is based on how they are formed.



Igneous rocks are formed when molten substances below the earth surface cools down and solidifies. These molten substances are called magma or lava. Magma can be derived from melting of existing rocks on the earth crust. The solidification of molten magma into igneous rock may be below the earth surface (intrusive) or on the earth surface (extrusive), giving rise to the two types of igneous rock.

  • Intrusive Igneous Rock: These are igneous rocks that crystallizes below the earth’s surface.  They generally cool slowly during the process of solidification and tend to produce large crystals. They are also referred to as plutonic igneous rock. Examples are gabbro, granite and pegmatite.

  • Extrusive Igneous Rock: They are igneous rocks that form when molten magma erupts on top of the earth's surface. They cool down and solidify quickly, producing small crystals in the process.  Sometimes, the molten magma cools so quickly, allowing no time for crystals to grow. This rapid cooling results in the formation of an amorphous glass. They are also referred to as volcanic igneouse rock. Examples are basalt, rhyolite and scoria.



Sedimentary rocks amount to about 76% of the earyth's surface. They can be formed in different ways;

  • By the deposition of the waethered remains of pre-existing rocks and their subsequent transportation and deposition. Sedimentary rocks formed this way are known as clastic sedimentary rock
  • By the accumulation and lithification of sediment or detrital rock.
  • By the precipitation from solution. Sedimentary rocks formed this way are known as chemical sedimentary rock.
  • As a result of biogenic activity. Sedimentary rocks formed this way are known as organic sedimentary rock.

Clastic sedimentary rock require 4 basic proesseses during their formation:

  1. Weathering
  2. Transportation
  3. Deposition
  4. Compaction

Examples of Sedimentary rocks include chalk, limestone, sandstone and clay.


Weathering refers to the many processes of physical and chemical breakdown of rocks when exposed to the air and water.

Lithification is a process of rock formation whereby pressure squeezes the rock sediments into layered solids while the connate fluids are expelled.

Diagenesis is a term used to describe all the chemical, physical, and biological changes a sediment undergoes after its initial deposition and during and after its lithification. it is not dependent on surface weathering.

We have three basic types of sedimentary rocks are:

  • Clastic sedimentary rocks: formed from mechanical weathering debris. Examples are breccia, sandstone and siltstone
  • Chemical sedimentary rocks: formed when materials precipitate from solution. Examples are rock salt, iron ore, and some dolomites solution.
  • Organic sedimentary rocks: formed from biogenic activity, i.e, the accumulation of plant or animal debris. Examples are coal, dolomites and some limestones.



Metamorphic rocks results from the transformation of an existing rock or original rock called protolithThis protolith may be an igneous rock or a sedimentary rock or another older metamorphic rock. Their transformation may be physical or chemcal, requiring great heat and pressure. Metamorphic rocks are always more dense and also less likely to be broken down by erosion when compared to igneous and sedimentary rocks. They make up about 12% of the earth crust and can also be formed beneath the earth surface.


  • Metamorphism means change in form.
  • Contact metamorphism results when great heat and pressure from inside the earth slightly melts a rock.

Examples of metamorphic rocks are schist (a converted basalt), quartzite (a compressed sandstone), and marble (a compressed limestone or dolomite).

We have two basic types of metamorphic rocks based on their appearances:

  1. Foliated metamorphic rocks: They have a layered or banded appearance. Gneiss and Schist are examples.
  2. Non-foliated metamorphic rocks: They don't have a layered or banded appearance. Marble, and Quartzite are examples.

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Please read on tropical rainforest here.

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