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Types of Rocks


A rock is a natural substance made up of solid crystals of various minerals that fused together to form a solid mass. The minerals might have formed at the same time or not. What matters is the natural processes glued them all together.

Igneous Rocks

Because igneous rocks are formed from magma and lava from the earth's interior, they are referred to as primary rocks. Igneous rocks (Ignis - Latin for "Fire") occur when magma cools and solidifies.When magma in its upward movement cools and turns into solid form it is called igneous rock. The cooling and solidification process may occur in the earth's crust or on the earth's surface. If it solidifies within the earth's surface it is called intrusive and if it solidifies outside the earth's crust it is called extrusive igneous rock.

Texture is used to classify igneous rocks. Texture is determined by the size and distribution of grains, as well as other physical properties of the materials. Mineral grains may be exceedingly big if molten material is slowly cooled at enormous depths. Rapid cooling (at the surface) produces tiny, smooth grains. Intermediate cooling conditions would result in granules of varying sizes that make up igneous rocks. Igneous rocks include granite, gabbro, pegmatite, basalt, volcanic breccia, and tuff.

Sedimentary Rocks

The term sedimentary comes from the Latin word sedimentum, which means "settling." The earth's surface rocks (igneous, sedimentary, and metamorphic) are exposed to denudational agents and are broken up into different sizes of fragments. These fragments are transported and deposited by several external agents. These deposits become rocks as a result of compaction. This is known as lithification. The layers of deposits in many sedimentary rocks maintain their properties even after lithification. As a result, sedimentary rocks such as sandstone and shale include a number of layers of different thickness.

Sedimentary rocks are categorised into three broad categories based on their mode of formation:

  • Mechanically formed: sandstone, conglomerate, limestone, shale, loess etc.

  • Organically formed: geyserite, chalk, limestone, coal etc.

  • Chemically formed: chert, limestone, halite, potash etc.

Metamorphic Rocks

The term metamorphic refers to a 'change of form.' These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks. Metamorphism is the process through which previously solidified rocks undergo recrystallisation and material reorganisation within the original rocks.

Dynamic metamorphism refers to the mechanical disruption and reorganisation of the original minerals inside rocks caused by breaking and crushing without any discernible chemical changes. Thermal metamorphism causes chemical changes and recrystallization of rock minerals. Thermal metamorphism is classified into two types: contact metamorphism and regional metamorphism. Contact metamorphism occurs when rocks come into contact with hot intrusive magma and lava, causing rock components to recrystallize at high temperatures. New materials formed from magma or lava are often added to the rocks. Regional metamorphism occurs when rocks recrystallize as a result of deformation produced by tectonic shearing combined with high temperature, pressure, or both.

Some rocks undergo metamorphism, which causes grains or minerals to be organised in layers or lines. Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.

Minerals or materials from various groups are sometimes stacked in alternating thin to thick layers that appear in bright and dark colours. Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks.

Metamorphic rock types are determined by the initial rocks that were subjected to metamorphism.

There are two types of metamorphic rocks: foliated rocks and non-foliated rocks. Metamorphic rocks include gneissoid, granite, syenite, slate, schist, marble, quartzite, and others.

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