Types of Rocks, Igneous Rock, Sedimentary Rock, Metamorphic Rocks (For CBSE, ICSE, IAS, NET, NRA 2022)

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Types of Rocks

About Rocks

  • Rocks are mineral aggregates with a combination of properties of all the mineral traces. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types.
  • Rock, in geology, naturally occurring and coherent aggregate of one or more minerals.
  • Additionally, different classification systems exist for each major type of rock. There are different types of rocks existing in nature.
  • Such aggregates constitute the basic unit of which the solid Earth is composed and typically form recognizable and mappable volumes.
  • Rocks are commonly divided into three major classes according to the processes that resulted in their formation.
  • Rocks which are found in nature rarely show such simple characteristics and usually exhibit some variation in the set of properties as the measurement scale changes.

Types of Rocks

  • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks

Igneous Rock

Igneous Rock
  • Igneous rocks are those that solidify from magma, a molten mixture of rock-forming minerals and usually volatiles such as gases and steam.
  • Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
  • Since their constituent minerals are crystallized from molten material, igneous rocks are formed at high temperatures.
  • Igneous rock is one of the three main rock types
  • This magma can be derived from partial melts of existing rocks in either a planet՚s mantle or crust.
  • Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.

Types of Igneous Rock

  • Intrusive igneous rock: These rocks crystallize below the earth՚s surface resulting in large crystals as the cooling takes place slowly. Diorite, granite, pegmatite are examples of intrusive igneous rocks.
  • Extrusive igneous rock: These rocks erupt onto the surface resulting in small crystals as the cooling takes place quickly. The cooling rate is for a few rocks is so quick that they form an amorphous glass. Basalt, tuff, pumice are examples of extrusive igneous rock.

Sedimentary Rock

Sedimentary Rock
  • The sedimentary rocks are formed by the deposition and subsequent cementation of that material within bodies of water and at the surface of the earth. The process that causes various organic materials and minerals to settle in a place is termed as sedimentation.
  • They are those that are deposited and lithified (compacted and cemented together) at the Earth՚s surface, with the assistance of running water, wind, ice, or living organisms.
  • The particles that form a sedimentary rock by accumulating are called sediment.
  • Most are deposited from the land surface to the bottoms of lakes, rivers, and oceans.
  • Before being deposited, the sediment was formed by weathering and erosion from the source area and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.
  • Sedimentary rocks are generally stratified — i.e.. , they have layering. Layers may be distinguished by differences in colour, particle size, type of cement, or internal arrangement.

Types of Sedimentary Rock

  • Clastic sedimentary rocks: These rocks are formed from the mechanical weathering debris. Sandstone, siltstone are examples of clastic sedimentary rocks.
  • Chemical sedimentary rocks: These rocks are formed from the dissolved materials that precipitate from the solution. Iron ore, limestones are examples of chemical sedimentary rocks.
  • Organic sedimentary rocks: These rocks are formed from the accumulation of plant and animal debris. Coal, some dolomites are examples of organic sedimentary rocks.

Metamorphic Rocks

Metamorphic Rocks
  • The metamorphic rocks make up a large part of the Earth՚s crust and are classified by texture and by chemical and mineral assemblage.
  • They are those formed by changes in preexisting rocks under the influence of high temperature, pressure, and chemically active solutions. The changes can be chemical (compositional) and physical (textural) in character.
  • They may be formed simply by being deep beneath the Earth՚s surface, subjected to high temperatures and the great pressure of the rock layers above it.
  • Metamorphic rocks are often formed by processes deep within the Earth that produce new minerals, textures, and crystal structures.
  • Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means “change in form.”
  • The recrystallization that takes place does so essentially in the solid state, rather than by complete remelting, and can be aided by ductile deformation and the presence of interstitial fluids such as water.
  • The original rock is subjected to heat with temperatures greater than 150 to and pressure around 1500 bars, causing profound physical and/or chemical change.

Types of Metamorphic Rock

  • Foliated metamorphic rocks: These rocks are produced by the exposure to heat and pressure which makes them appear layered. Phyllite, gneiss are examples of foliated metamorphic rocks.
  • Non-foliated metamorphic rocks: These rocks don՚t have layers. Marble, quartzite are examples of non-foliated metamorphic rocks.

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