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Types of Rocks: Igneous, Sedimentary and Metamorphic Rocks

  • Last Updated : 14 Apr, 2022

Generally, rock forms by any of the naturally occurring accumulation of mineral particles. The solid part of Earth is made up of various types of rocks differing from one another in texture, structure, permeability, mode of occurrence, and degree of resistance to denudation. On the basis of the origin of rocks, they are classified into three types – Igneous rocks, Sedimentary rocks, and Metamorphic rocks. There are also a few rocks substances of organic origin sometimes they contain fossils, for example, coal, peat, etc.

Igneous Rock:

Due to various reasons for example tectonic movements of plates, crustal loading, density differences, etc. the magma is forced upwards on the surface as lava through the crust along lines of weakness. Thus after cooling of lava two types of igneous rocks formed, the first type of rock which spill out on the surface and consolidate to form extrusive igneous rocks, and another one that does not find an outlet, sometimes it solidifies in cavities and fissures within the crust to form intrusive igneous rocks.

Characteristics of Igneous Rock:

  1. These types of rocks are known as primary or basic rocks as this type was considered to be the first form of rock and the supply materials for other types of rocks.
  2. No fossils contain found in these types of Rock.
  3. These types of rock forms include more than one mineral deposit and these may be either glassy or coarse in nature.
  4. Based on the rate of cooling of lava, the granular size of the rocks was dependent, for very fast cooling the size of granular becomes smaller, whereas for slow cooling the size of granular becomes larger.
  5. Based on the chemical and mineral composition of Igneous rocks, some different types of rocks are formed.
    • Mafic or Basic Rocks: Rocks contain 44-52% of silica in nature e.g., Gabbro basalt, Dolerite, etc. This type of rock is denser and darker in color.
    • Ultrabasic or Ultramafic Rocks: Rocks contain less than 44% of silica in nature e.g., Peridotite, etc.
    • Silicic rocks: Rocks contain more than 66% of silica in nature e.g., Granite, Granodiorite, etc. This type of rock is less dense and lighter in color.
    • Intermediate Rocks: Rocks contain 52-66% of silica in nature e.g., Andesite, Diorite, etc. This type of rock is less dense and lighter in color.

Metamorphic Rocks:

This type of rock is generally a modified form of Igneous, Sedimentary, or previous Metamorphic rock by various external weather conditions like temperature, pressure, volume, etc. and for such reason, the appearance, texture, and composition of rocks were changed from the previous characteristics. 

Characteristics of Metamorphic Rocks:

  • These types of rocks are the hardest among the three types of rocks.
  • These types of rocks do not contain fossils.
  • These types of rocks are formed from all three types of rocks
  • These rock types are generally found at the core of the various mountain ranges.

Examples of Sedimentary Rocks to Metamorphic Rocks:

  • Slate from shale
  • Marble from Limestone
  • Marble from chalk and dolomite

Examples of Igneous Rocks to Metamorphic Rocks:

  • Gneisses from granites
  • Amphibolite from basalt
  • Schist from Basalt

Examples of Sedimentary Rocks to Metamorphic Rocks:

  • Serpentine from Gabbro
  • Schist from Phyllite
  • Phyllite from Slate

Sedimentary Rocks:

These types of rocks are found over about 70% are of the Earth’s upper surface and formed through a series of processes, in the very early stage weathered materials are transported by exogenic agents and in the later time phase, it is deposited at the lower gradient and suitable climate conditions as unconsolidated debris or sediments. In this way due to erosion and deposition of metamorphic, igneous rocks (or some time old sedimentary rocks) are known as sedimentary rocks. 

After the deposit of materials debris or sediments in suitable conditions, it is converted into consolidated sedimental rocks by the process known as Lithification. Then, by the further process of Cementation, the layer of many consolidated sedimental rocks convert into a large bound of sedimentary rock structure.

Characteristics of Sedimentary Rocks:

  • Fossils are generally found in such types of rocks.
  • Maximum sedimentary rocks are found in the form of Shale, Sandstone, and Limestone (More than 97%).

On the basis of the nature of consolidated, sedimentary rocks are classified into three types:

  • Mechanically formed sedimentary rocks: Shale, Clay rocks, etc.
  • Chemically formed sedimentary rocks: Gypsum, Salt rock
  • Organically formed sedimentary rocks: Coal, Peat, Limestones, etc.
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