Simply put, a rock is a solid substance that we see piled up together on the surface of the earth. We see them everywhere either lying around or piled up in big chunks in the form of mountains, cliffs, and all other higher planes.

Rocks are also present in all the man-made structures we see today. Roads, houses, monuments, indoors and outdoors are made using different types of rocks.

What is Rock?

If defined from a more scientific point of view, rocks are formed of a single or more mineral aggregated together naturally. It is created under great pressure and high temperature and then cooled for a prolonged time to take the form of a rock.

Minerals like quartz are pure forms of solidified minerals that show up in the form of rocks. Meanwhile, such rocks are broken down in contact with water and wind to turn to sand or soil. Some of these undergo great pressure and another form of rocks are created. This leads us to the question, “how many types of rock are there?”

Types of Rocks

Rocks usually differ in their formation, color, and composition. While there is an array of naturally colored stones out there, most of them have a prominent criterion which is the formation process of how rocks are made. Based on this, science has divided rocks into three categories-

  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks

Brief description with classifications of these three types of rocks is given below.

Igneous Rocks

Formation Process:

The very primary form of a rock formation is from streams of magma that erupted from volcanoes and as soon as they come in contact with the air in the surface, they start cooling down and ultimately solidify to form Igneous rocks.

N.B.: Magma is a liquid composition of different sorts of molten minerals. They usually form at the core of the earth which is around 700° to about 1300° Celsius. The deeper you go, the hotter it gets.


When magma cools at a slightly lower temperature, it results in the formation of crystals in the rocks. Example: Granite or Pegmatite. On the other hand, some igneous rocks form in a very rapidly changing temperature and thus form air pockets. Example: Pumice. They are also known as porous rocks. Surprisingly, igneous rocks like Obsidian have no crystal or pores but was formed in a gradually changing temperature.


Followings are the types of igneous rocks:

  • Intrusive Igneous Rocks

The rocks that are formed during a slow change of temperature in the atmosphere around the magma is known as Intrusive Igneous rocks. Such rocks are usually non-porous and crystallized.

  • Extrusive Igneous Rocks

The rocks that are formed during a sudden drop in temperature around the magma which usually occurs during a volcanic eruption is known as Extrusive Igneous rocks. Such rocks usually have air pockets, are lighter and less crystallized.

Different Rock Groups

  • Andesites
  • Basalt
  • Pumice
  • Gabbro
  • Granite
  • Diorite
  • Rhyolite
  • Obsidianv
  • Welded Tuff

Sedimentary Rocks

Formation Process

Sedimentary rocks are always formed from the eroding of different pure mineral rocks. Igneous and metamorphic rocks usually erode when in contact with air or water (also known as weathering) and turn into soil or dust. With time, the soil from these rocks settles down at the waterbed of rivers, estuaries, and seas to form layered stones under great pressure.


The simplest way to recognize a sedimentary rock is to find stripes of layer formation on the body of the rock. It is usually found in waterbeds since they are results of eroded igneous rocks. Example: Limestone or Claystone.

Three major identifying features-

  • The presence of mineral chunks covered by sedimented sands around the chunk. Example: Clay.
  • The presence of gravels in different shapes will be seen. Example: Greywacke.
  • The presence of grainy sand-sized minerals. Example: Chalk.


Followings are the classification of sedimentary rocks.

  • Clastic Rocks

This sub-category of sedimentary rocks is mainly formed of different shapes of eroded crystals or silicates. They have no gravel and mainly have clasts.

  • Sandstones

Another sub-category of sedimentary rocks that has a more sand-like texture.

  • Mud Rocks

This sub-category of sedimentary rocks is formed from years of sedimented mud. They have a rather smooth finish.

  • Conglomerates

This sub-category has a distinct presence of gravels or smaller projected crystals of minerals.

  • Chemical Rocks

Rocks that are formed of salt crystals and gypsum.

  • Biochemical Rocks

This sub-category of sedimentary rocks is made from the compression and sedimentation of trees or animals for a prolonged time.

Different Rock Groups

  • Chert
  • Siltstone
  • Diamictite
  • Flint
  • Lignite
  • Sylvinite
  • Oolite
  • Gritstone

Metamorphic Rocks

Formation Process

The term for this category of rocks comes from the word “metamorphosis” which means “change in the form or nature”. These rocks stand true to their naming since they are usually igneous or sedimentary rocks that have changed their properties greatly. The change is so drastic that they miss out their initial characteristics of being an igneous or sedimentary rock. In some cases, some metamorphic rocks change so much that they turn into a new form of metamorphic rocks which display a completely different array of superficial or compositional characteristics.


An easy way to detect a metamorphic rock is the presence of minerals like chlorite or kyanite besides the presence of minerals like quartz in the same rock.


Followings are the classification of metamorphic rocks.

  • Thermal Metamorphic Rocks

As the name implies, when any sort of rock loses its initial properties due to partial heating and gain new characteristics, it is known as a thermal metamorphic rock. The chemical composition is highly affected in this type of rock formation.

  • Dynamic Metamorphic Rocks

A dynamic metamorphic rock is formed when a rock loses its characteristics due to intensive pressure. Usually, the physical features change more than the chemical features.

Different Rock Groups:

  • Granulite
  • Marble
  • Quartzite
  • Slate
  • Tectonite
  • Gneiss
  • Amphibolite
  • Serpentinite

It does not stop here. Questions like “What type of rock is coal?” or “what type of rock is coquina?” could also pop up in your mind. It is given that Coal and Coquina are considered rocks but are not entirely rocks formed from minerals but rather compressed trees and seashells respectively. These special types of rocks are known as biogenic rocks.



Please note that the information in is designed to provide general information on the topics presented. The information provided should not be used as a substitute for professional services.


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