Types, Characteristics and Examples of Rocks: Igneous, Sedimentary and Metamorphic Rocks

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

Topics in Geography

Solar System: Classification, Types and Characteristics of Planets Atmosphere: Layers, Functions and Composition of the Atmosphere Geography, Types and Branches of Geography Types, Characteristics and Examples of Rocks: Igneous, Sedimentary and Metamorphic Rocks

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Types of Rock:

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



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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's surface (intrusive) or on the earth's 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.

    Intrusive Igneous Rock 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.

    Extrusive Igneous Rock are also referred to as Volcanic Igneous Rock. Examples are basalt, rhyolite and scoria.



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

  • By the deposition of the weathered remains of pre-existing rocks and their subsequent transportation and deposition. Sedimentary rocks formed in 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 in this way are known as chemical sedimentary rock.
  • As a result of biogenic activity. Sedimentary rocks formed in this way are known as organic sedimentary rock.

    Please read on Separation Techniques here.

Clastic sedimentary rock require 4 basic processes during their formation. These are:

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

Please read on Soil Erosion here.

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

Note the followings

Weathering refers to the processes that involves the 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 lithification.

Diagenesis is not dependent on surface weathering.

From the above explanation, we can conclude that there are three basic types of sedimentary rocks based on their formation process. These are:

  • Clastic sedimentary rocks: They are formed from mechanical weathering of pre-existing rocks. Examples are breccia, sandstone and siltstone
  • Chemical sedimentary rocks: They are formed when materials precipitate from solution. Examples are rock salt, iron ore, and some dolomites solution.
  • Organic sedimentary rocks: They are formed from biogenic activity; that is; the accumulation of plant and animal debris. Examples are coal, dolomites and some limestones.



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Metamorphic rocks are formed from the transformation of an existing rock or original rock called protolith. The protolith may be an igneous rock or a sedimentary rock or another older metamorphic rock. Their transformation may be physical or chemical, 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.


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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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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