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In nature, there are different types of rocks available, i.e., rocks are classified into three categories, Geologically, chemically, and structurally.

This classification is based on the mode or process of formation of a rock. Thus, some rocks may be formed from natural hot molten materials.

Others may be formed at ordinary temperatures from compaction of particles or sediments, and still.

Therefore, in the geological classification of rocks following three types of rocks are recognized.

Igneous Rock

They form from the cooling of magma deep inside the earth. They often have large crystals (you can see them with the naked eye).

Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state.
The earth is made of igneous rock – at least at the surface where our planet is exposed to the coldness of space.

Igneous rocks are given names based upon two things:
1)composition (what they are made of) and
2)texture (how big the crystals are)

The most common types of igneous rocks are:

  • andesite
  • basalt
  • dacite
  • dolerite (also called diabase)
  • gabbro
  • diorite
  • peridotite
  • nepheline
  • obsidian
  • scoria
  • tuff
  • volcanic bomb

Metamorphic Rock

They are formed through the change (metamorphosis) of igneous and sedimentary rocks. They can form both underground and at the surface.

They can be divided into many categories, but they are typically split into:

  • Foliated metamorphic rocks — pressure squeezes or elongates the crystals, resulting in a clear preferential alignment.
  • Non-foliated metamorphic rocks — the crystals have no preferential alignment. Some rocks, such as limestone, are made of minerals that simply don’t elongate, no matter how much stress you apply.

Metamorphic rocks can form in different conditions, in different temperatures (up to 200 °C) and pressures (up to 1500 bars). By being buried deep enough for a long enough time, a rock will become metamorphic. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion; they can also form when the rock is heated up by the intrusion of magma from the Earth’s interior.

The most common metamorphic rocks are:

  • amphibolite
  • schist (blueschist, greenschist, micaschist, etc)

Sedimentary Rock

They are formed through the solidification of sediment. They can be formed from organic remains (such as limestone), or from the cementing of other rocks.

Sedimentary rocks are quite difficult to classify, as they have several different defining qualities (the chemical make-up, the sedimentation process, organic/inorganic material), but the most common classification is the following:

  • clastic sedimentary rocks — small rock fragments (many silicates) that were transported and deposited by fluids (water, bed flows). These rocks are further classified by the size and composition of the clastic crystals included in the sedimentary rocks (most often quartz, feldspar, mica and clay).
  • conglomerates (and breccias)  — conglomerates are predominantly composed of rounded gravel, while breccias are composed of angular (sharper) gravel.
  • sandstones — as the name says, it’s a rock made from many-sand-sized minerals and rock grains. The most dominant mineral in sandstone is quartz because it is the most common mineral in the Earth’s surface crust.
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