Rocks and types of rock

Rocks are the most common material on the Earth; they ordinarily lie everywhere on the ground. They constitute most of the landforms, and there is a close relation between rocks and landforms. The earth s crust is composed of rocks. A rock is an aggregate of one or more minerals that have been fused together into a solid lump. For example, granite, a common rock, is a combination of the minerals quartz, feldspar and biotite.

Properties of rocks

  • The rocks may be made up entirely of one mineral or various minerals.
  • Rocks do not have a definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.
  • Rock may be hard or soft. For example, granite is hard, soapstone is soft.
  • Rocks may have varied colours. Some rocks are dark and some are light coloured. For example, Gabbro is black and quartzite can be milky white.

Types of rocks

Rocks are classified according to characteristics such as mineral and chemical composition, permeability, the texture of the constituent particles, and particle size. In the long run, all types of rocks can transform from one type into another, as described by the rock cycle model. These transformations usually take thousands or millions of years.

There are many different kinds of rocks which are grouped into three families by their mode of formation. They are:

  1. Igneous Rocks
  1. Sedimentary Rocks
  1. Metamorphic Rocks

Igneous Rocks

  • Igneous rocks are called as primary rocks as they are formed out of magma and lava from the interior of the earth.
  • The igneous rocks (Ignis in Latin means Fire ) are formed when magma cools and solidifies.
  • Igneous rocks are classified based on texture. The texture of these rocks depends upon arrangement and size of grains and other physical properties or condition of the materials.
  • Most igneous rocks are extremely hard and resistant. For this reason, they are quarried for road construction and polished as monuments and gravestones.
  • Igneous rocks do not contain fossils as their forming material are super hot magmatic materials.
  • Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

Formation of Igneous rocks

  • During the volcanic eruptions, the molten rock materials - Magma and Lava, find their way to the surface.
  • When magma in its upward movement cools and turns into a solid form, it is called igneous rock.
  • This process of solidification can happen on the surface of the earth or inside the earth s crust.
  • The rate of cooling of the magma determines the size of its grain structure. Magma cooled on the surface are fined grained whereas magma cooled slowly inside the earth surface have coarse grain structure.


Formation of Igneous rocks

Classification of Igneous rocks

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified into two types:

  1. Volcanic rocks (Extrusive rocks) - cooling at the surface - extrusive rocks are formed when molten magma, erupted from volcano, cools on the surface. The magma on the surface (lava) cools faster on the surface to form igneous rocks that are fine grained. Pumice and basalt are examples of extrusive igneous rocks.
  1. Plutonic rocks (Intrusive rocks) - cooling in the crust - Intrusive igneous rocks are formed when the magma cools off slowly under the earth s crust and hardens into rocks. Intrusive rocks are very hard in nature and are often coarse-grained. Gabbro and granite are examples of intrusive igneous rocks.

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Sedimentary Rocks

  • The word sedimentary is derived from the Latin word sedimentum, which means settling.
  • Sedimentary rocks are those that are formed through deposition and lithification (compaction and cementation) of sediment particles at the Earth s surface, with the assistance of running water, wind, ice, or living organisms.
  • They may be coarse or fine-grained, soft or hard.
  • Sediments are naturally occurring particles derived from weathering and erosion of pre-existing rocks.
  • The particles that form sedimentary rocks may be brought by winds, streams, glaciers and even animals.
  • They are non-crystalline and often contain fossils of animals, plants and other micro-organisms.
  • Sedimentary rocks are distinguished from other rock types in their characteristic layer formation and are termed stratified rocks. The strata may vary in thickness.
  • Layers may be distinguished by differences in colour, particle size, type of cement, or internal arrangement.
  • Classification of sedimentary rocks is done according to their age and different kinds of rocks formed during the same period are grouped together.
  • Most of the solid surface of our planet (roughly 70%) is represented by sedimentary rocks.
  • Sandstone, Limestone and Shale are some examples of sedimentary rocks.



Weathering, Erosion and Deposition (W.E.D.)

Formation of sedimentary rocks

  • Sedimentary rocks are formed from sediments accumulated over long periods, usually under water.
  • All types of rocks (igneous, sedimentary and metamorphic) of the earth s surface are exposed to denudational agents (water, winds, ice and glacier etc.) and are broken up into various sizes of fragments.
  • Such fragments are transported by different exogenous agents and deposited or settled down in low lying areas or underwater, through the process of sedimentation.
  • These deposits finally turn into rocks through compaction and compression by weight of overlying material. This process is called lithification.
  • In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. That is why we can see a number of layers of different thickness in sedimentary rocks like shale, sandstone etc.

Formation of sedimentary rocks

Classification of sedimentary rocks

Depending upon the mode of formation, sedimentary rocks are classified into three major groups:

  1. Mechanically formed they are known as Clastic sedimentary rocks because of their formation by accumulations of clasts: little fragments of broken up rock material which have been piled up and "lithified" by compaction and cementation. Conglomerate, sandstone, shale, limestone, loess etc. are some of the examples of mechanically formed sedimentary rocks.
  1. Organically formed these rocks are formed by an accumulation of living organisms such as corals or shellfish. These rocks have significant amounts of organic material. The most famous rocks formed in this way are of the calcareous type such as Chalk. However, carbonaceous rocks are also organically formed but from vegetative matter. Geyserite, chalk, limestone, coal etc. are some examples of organically formed sedimentary rocks.
  1. Chemically formed many of these forms when standing water evaporates, leaving dissolved minerals behind. These sedimentary rocks form when mineral constituents in solution become supersaturated and inorganically precipitate. Chert, limestone, halite, potash etc. are some examples of chemically formed sedimentary rocks.

Metamorphic Rocks

  • The word metamorphic means change of form whereby meta means change and morph means form.
  • Metamorphic rocks are formed due to the transformation of a pre-existing rock.
  • They are predominantly sedimentary or igneous rocks that have undergone physical and chemical changes under the action of extreme heat and pressure.
  • Formation of metamorphic rocks can take place under different physical conditions i.e. in different temperatures (up to 200 C) and pressures (up to 1500 bars)
  • Their natural characteristics undergo a massive transformation in such extreme conditions of temperature and pressure.
  • Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface
  • Gneissoid, granite, syenite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.

Formation of Metamorphic rocks

  • Metamorphic rocks can simply be formed when the sedimentary or igneous rocks moves deep inside the earth and come under the influence of high temperature and pressure of the overlying material.
  • Formation of metamorphic rocks can take place through tectonic processes including continental collisions, which may cause changes in pressure and temperature.
  • The intrusion of magma on the earth's surface is also an important cause of metamorphic transformation.



Formation of metamorphic rocksMetamorphism is a process by which already consolidated rocks undergo re-crystallisation and re-organisation of materials within original rocks.

Dynamic Metamorphism Dynamic metamorphism refers to mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any significant chemical changes.

Thermal Metamorphism - Due to thermal stress the material of rocks undergo crystal transformation which alters the chemical properties of the rock. There are two types of thermal metamorphism.


  1. Contact Metamorphism - In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures. Quite often new materials form out of magma or lava are added to the rocks.
  1. RegionalMetamorphism - In regional metamorphism, recrystallisation of rocks takes place as a result of deformations caused by tectonic shearing along with high pressure and temperature.


Foliation or Lineation - In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. This arrangement of minerals or grains in metamorphic rocks is referred to as lineation or foliation.

Banding - Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades. Such arrangement in metamorphic rocks is commonly referred to as banding. Rocks that display banding are called banded rocks.

Classification of Metamorphic rocks

Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism. Metamorphic rocks are classified into two major groups

  1. Foliated rocks foliated rocks are formed where pressure squeezes or elongates the crystals due to differential stress, they have a clear preferential alignment. Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated. For example, the slate is a foliated metamorphic rock, originating from shale.
  1. Non-foliated rocks they are formed where 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

Rock cycle


Rock Cycle

Like most Earth materials, rocks are created and destroyed in natural cycles. Rocks do not remain in their original form for long but may undergo a transformation. The rock cycle is a model that describes the formation, breakdown, and reformation of a rock as a result of sedimentary, igneous, and metamorphic processes. Rock cycle is a continuous process transforming old rocks into new ones.

Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) are formed from these primary rocks. Igneous rocks can be changed into metamorphic rocks. Sedimentary rocks are formed from the fragments derived out of igneous and metamorphic rocks. Sedimentary rocks themselves can turn into fragments and the fragments can be a source for the formation of sedimentary rocks.

In the subduction zones in the earth's crust. The crustal rocks can get inside the mantel where they melt and become a source of the igneous rock, thereby completing the rock cycle.

Related terms

Mineral - a mineral is an organic or inorganic substance that occurs naturally on the earth surface. They have an orderly atomic structure and a definite chemical composition and physical properties. A mineral consists of two or more elements. However, sometimes single element minerals like copper, sulphur, gold, silver, graphite etc. can be found.

Magma and Lava - The mantle contains a weaker zone called asthenosphere. It is from this that the molten rock materials find their way to the surface. The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava

Petrology - Petrology refers to the science of rocks. Under petrology, rocks are studied with respect to all their aspects viz., mineral composition, structure, texture, occurrence, origin, alteration and relationship with other rocks.

Weathering - Weathering is an action of elements of weather and climate over earth materials. Weathering includes mechanical disintegration and chemical decomposition of rocks through the actions of various aspects of weather and climate.

Erosion Erosion involves acquisition and transportation of rock debris. When massive rocks are broken into smaller pieces through the process of weathering or any other process, erosional geomorphic agents like groundwater, running water, wind, waves and glaciers remove and transport it to other places depending upon the dynamics of each of these agents.

Diagenesis - Diagenesis is a process which includes the formation of sedimentary rocks by compaction and cementation of grains, or by crystallization from water or solutions and recrystallization.

Cementation Cementation is the process by which clastic sediment is lithified by precipitation of mineral cement, such as calcite cement, among the grains of the sediment.

Compaction compaction is a process through which the porosity of a given sedimentary material is reduced as a result of its mineral grains being squeezed together by the weight of overlying sediment.

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