Types of Rock: Sedimentary, Igneous, and Metamorphic


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    Rayban AnnaRayban Anna

    Since grade school, we have been studying the different types of rock.

    I know you are all familiar with these rocks so let’s have a refereshing understanding again.

    Rocks are classified into three types based on their geological formation.

    These are the following:

    1. Sedimentary rocks

    2. Igneous rocks

    3. Metamorphic rocks

    Sedimentary rocks

    Sedimentary rocks are formed by the deposition of sediments obtained by the weathering of pre-existing rocks and these sediments are transported by various agents such as water, wind, frost, gravity, etc. These transported sediments form layered structures and give rise to the sedimentary deposits.

    If the sediments remain at the place of origin then the formed deposits are known as residual deposits. Some sediments formed by various chemical reactions such as decomposition, precipitation, evaporation, etc. give rise to the formation of chemical deposits. Similarly, the sediments formed by the action of various organisms such as plants and animals are known as Organic deposits.

    Examples: Sandstone, limestone, lignite, etc.

    Igneous rocks

    Igneous rocks are formed by the solidification of magma below the earth’s surface. When the magma is unable to erupt through the earth surface during its upward journey, it is held up below the earth’s surface and unable to descend. This magma cools down gradually and solidifies into igneous rocks.

    The structure of igneous rocks varies according to the depth at which magma solidified. If the magma hardens at a significant depth from the earth surface, then the rocks possess coarsely grained crystalline structure and these rocks are known as plutonic rocks. Granite is the best example of plutonic rock.

    Similarly, if magma hardens at shallow depth from the earth’s surface, the finely grained crystalline structure of rock will be obtained. These rocks are called as hypabyssal rocks. Dolerite is an example of a hypabyssal rock.

    If the solidification of the rock occurs near to the earth surface, then the rocks obtained are known as volcanic rocks. These rocks possess extremely fine-grained structure. Basalt is an example of volcanic rock.

    Metamorphic rocks

    Metamorphic rocks are formed by the metamorphism process. Metamorphism is the process of changing the characteristics of the pre-existing rocks under the influence of heat and pressure. The pre-existing rocks may be of the sedimentary or igneous type of rocks.

    Examples: Slate, Gneiss, Schist, marble, soapstone etc.

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