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World Geography / Earth

Types of Rocks and their Formation and Properties


    There are 3 types of rocks that are generally found on the earth namely, (i) Igneous Rocks ( Primary Rocks ), first rocks to be formed in the crust and others evolved out of it, (ii) Sedimentary Rocks and (iii) Metamorphic Rocks.

    Igneous Rocks

    • Igneous rocks are formed due to cooling of molten rock liquid ( Magma ) on Earth's surface. Depending on the place of cooling, they can be divided into 3 types, Intrusive igneous rocks, Extrusive igneous rocks and Hypabyssal rocks.
      • Intrusive igneous rocks are also known to be Plutonic rocks. These rocks solidify inside the Earth's crust at greater depth. Very slow cooling in Plutonic rocks results in large sized crystals.
      • Extrusive igneous rocks are also known to be Volcanic rocks. They erupt onto the surface and solidify outside. Absence of crystals in volcanic rocks is due to rapid cooling.
      • If at moderate depth the Magma solidifies, it is called Hypabyssal igneous rocks. Hypabyssal rocks are also called Subvolcanic rocks.
    • All the igneous rocks are hard and rigid. There won't be any fossils inside igneous rocks. Due to high temperatures, organic matter cannot survive.
    • Depending on the Silica content, Igneous rocks can be classified into two categories, (i) Acidic Igneous rocks ( if Silica content is higher, having about 65 – 85% Silica ), e.g. Granite and (ii) Basic Igneous rocks ( if Silica content is lower, having less than 20% Silica ), e.g. Basalt, Dolomite, Gabbro, etc.
    • Groundwater is not rich in igneous rocks as these rocks are non-porous in nature. Most igneous rocks are very good source of minerals particularly metallic minerals. Uranium is present in ultrabasic igneous rocks.

    Sedimentary Rocks

    • Sedimentary rocks are formed out of accumulation or deposition of sediments and their compaction and consolidation. Sediment is nothing but broken rock or rubble material. Rubble material is formed due to weathering or erosion.
    • Weathering happens due to exogenic forces ( happening outside the crust ) like rivers, winds, moving ice ( glaciers ), tidal waves, etc. Due to these forces, the rock is getting converted into heap of rubble material. These materials are transported again by exogenic forces in the form of small sand particles and at convenient places they are deposited.
    • Sediments are deposited over millions of years, layer after layer, leading to piling of layers. When the weight of upper layers falls on lower layers, they are compacted and formed into sedimentary rocks. Sedimentary rock formation does not involve any temperature or pressure. In sedimentary rocks, the layers are visible. This is known as Stratification or Bedding.
    • Sedimentary rocks contain fossils, which are preserved. They are useful in identifying climatic vegetation at different times. Fossils are sometimes converted into fuels ( Coal, Oil, Natural Gas, etc. ) as there won't be any high temperature and pressure.
    • These rocks may be soft or hard and rigid. They are highly porous in nature, their surface runoff is more and they are rich in groundwater aquifers. Sedimentary rocks dominate the surface of earth. Some common types of sedimentary rocks are Sandstone, Loess, Gypsum, Rock salt, Shale, Limestone, etc.

    Metamorphic Rocks

    • Metamorphic rocks are formed of the process known as Metamorphosis. Metamorphosis requires very high temperature, very high pressure or both because of which physical and chemical properties change, which in turn form into new type of rock known as Metamorphic rock. Both Igneous and Sedimentary rocks can undergo Metamorphosis.
    • Metamorphism happens due to Volcanism and Mountain building activities ( Orogenesis ). When crustal subsidence takes place or when the Crust is pushed into Mantle, it undergoes metamorphosis.
    • Igneous rocks like Granite when undergoes metamorphosis, forms into Gneiss. Similarly, when Basalt undergoes metamorphosis, it forms into Schist and when Dolomite undergoes metamorphosis, it forms into Marble.
    • Sedimentary rocks like Limestone when undergoes metamorphosis, forms into Marble. Similarly, when Shale undergoes metamorphosis, it forms into Slate and when Sandstone undergoes metamorphosis, it forms into Quartzite.
    • Sometimes some metamorphic rocks get converted into other metamorphic rocks. Slate undergoes metamorphosis and form into Schist, again when it undergoes metamorphosis, it forms into Phyllite. Quartzite undergoes metamorphosis and forms into Schist.
    • All metamorphic rocks are hard and rigid. These are good source for minerals particularly non-ferrous metals. They are generally non porous in nature.