28 Igneous Rocks: Introduction, Classification, & related terminology


Igneous Rocks are formed by the solidification of magma below or above the surface. This is a branch of geology that deals with the study of rocks and include:

a). Petrogenesis:  

It denotes the origin and modes of occurrence, as well as the natural history of rocks.

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b). Petrography:

its deals with classification and description of rocks.

• The branch of petrology that deals solely with the study of rocks is called “lithology”.

• Stones must be strong, durable and dense.

• As we know, rocks are necessarily complementary parts of the crust.

Constituents of Elements the Earth’s Crust

ElementsWeight %Atoms %Ions Redius
  • Rocks are made up of minerals. Some rocks are monomineral, consisting of a single mineral, while most rocks are multi-mineral minerals, which are essential components of more than one mineral species.
  • Igneous and meta-igneous rocks make up 95% of all crustal rocks.
  • Sedimentary and metasedimentary rocks make up 5% of the crustal rocks.

Classification Of Rocks

According to the original mode, all rocks are classified into three major groups:

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1. Igneous Rocks or Primary Rocks:

These are rocks formed by the solidification of magma below or above the surface; accordingly, they are divided into two groups:

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Credit: https://cdn.pixabay.com/photo/2014/12/05/18/16/ladybird-558330_1280.jpg

Igneous Rock

Credit: https://www.researchgate.net/profile/Juan_Gil-Berrozpe/publication/325313516/figure/fig5/AS:[email protected]/Detail-of-the-IGNEOUS-ROCK-hierarchy.png.

(a) Intrusive bodies:

Which are made underneath the surface of the earth. On the basis of the depth of formation, intrusive rocks are of two types:

  1. Plutonic Rocks– which are raised at very great depths.
  1. Hypabyssal Rocks– which are raised at the shallow depths.
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Credit: https://www.researchgate.net/profile/Rais_Latypov2/publication/283736523/figure/fig9/AS:[email protected]/A-section-through-an-intrusion-illustrating-the-hypothesis-that-magma-chambers-start.png

(b) Extrusive bodies:

These occur as a result of the consolidation of magma on the earth’s surface. These are also known as volcanic rocks.

Important Characteristics Of Igneous Rocks

  1. Its Hard, Massive, compact with interlocking grains
  2. The entire absence of fossils
  3. No bedding planes
  4. Enclosing rocks are baked
  5. Contains many feldspars

2. Sedimentary Rocks:

These rocks are derived from existing rocks through the processes of erosion, transport, and deposition, such as wind, water, glaciers, and so on. the products that are formed are known as sedimentary rocks.

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Credit: https://cdn.pixabay.com/photo/2015/02/13/12/16/fossil-635079_1280.jpg

On the basis of place of formation, sedimentary rocks are of two types:

(a) Sedentary rocks:

That is, residual deposits generated at the site of the pre-derived stones. These are not made up of transportation processes.

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Credit: https://www.researchgate.net/profile/Kenneth_Edgett/publication/344518154/figure/fig2/AS:[email protected]/Sedimentary-rock-fragments-and-fragments-from-veins-formed-in-fractures-in-sedimentary.png

(b) Transported rocks:

In this case, the crushed and decomposed rock materials are transported from the original place and stored in a suitable site. Depending on the mode of transport of the deposits, these rocks are divided into three parts:

  • Mechanically Deposited– Clastic Rocks
  • Chemical Precipitation– Chemical Deposits
  • Organically Deposited– Organic Deposits.

Important Features of Sedimentary Rocks

  • Its occur soft rocks, stratified, bedding available
  • Fossils occur
  • Lamination, Cross-bedding, Ripple marks mud- cracks, etc. are the usual structure.
  • No effect on the enclosing or the top and bottom rocks.
  • Quartz, clay mineral, calcite, dolomite, hematite, are the common minerals.

3. Metamorphic Rocks:

These are caused by changes in existing rocks under the influence of temperature, accompanied by pressures (non-magmatic) assisted by groundwater fluids.

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Credit: https://cdn.pixabay.com/photo/2017/07/03/11/16/rock-2467294_1280.jpg

Important features of Metamorphic Rocks

  • Its hard, interlocking grains and bedded (if derived from stratified rocks).
  • Fossils are rarely preserved in the rock of sedimentary origin except for slates.
  • Foliated, gneissose, schistose, granulose, slaty, etc. are the common structure.
  • Common minerals are andalusite, sillimanite, kyanite, cordierite, wollastonite, granite, graphite, etc.

Important Rock Types Of India

Igneous Rocks TypeSedimentary RocksMetamorphic Rocks
Granite- with its volcanic equivalent, ex-rhyoliteSandstoneGneiss  
Syenite- with its volcanic, ex- trachyteLimestone and DolomiteQuartzite  
Nepheline-Syenite (and phonolite)ShaleSchists  
AnorthositeSaline rocksCharnockite  
Granodiorite and MonzoniteLateriteKhondalite  
Gabbro, diorite, and norite, and their volcanic equivalents basalts (Deccan traps), andesite, etc. Marble and Phyllites.  
Peridotite Gondite
Carbonatite Kodurite and Slate

Forms Of Igneous Rocks

The shape, shape, and shape of igneous bodies are mainly dependent on the following factors:

  • Mode of formation
  • The viscosity of the magma, which in turn depends on the
    • Temperature
    • Composition of Magma
  • Relation with the surrounding country-rocks
    • Physical characters of the invaded rocks
    • Weight of the overlying rock mass in case intrusive bodies
    • Structure of rocks.

The shapes that intrusive bodies take are dependent on major geological structures, such as faults, folds, bed planes, and so on. Thus, there are two main categories of intrusive body shapes.

  • Discordant-Bodies:
    In this case, an intrusive mass that crosses the existing rock structure of the country occurs. There are different types of discordant shapes in both extended and highly folded regions.
    • Unfolded Region:
      • Dykes-These discordant igneous bodies have an indirect relationship with the rocks of the country. Dikes usually appear in groups and these groups can be irradiated, arched, or any other type. Since the magma must be mobile enough to form dikes, the most basic composition of dikes is Ex-doleritic. The dikes are evidence of tension in the crustal regions within the hardening effect on both sides.
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Credit: https://www.researchgate.net/profile/Ramasamy_Prabhakaran/publication/299626862/figure/fig3/AS:[email protected]/Doleritic-dyke-rock-exhibiting-spheroidal-Weathering.png

  • Ring Dyke – The dike of the contracted crop appears to be approximately a full or nearly complete circle.
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  • Cone-Sheets- These operate inwards (in the form of reverse coaxial cones) with dike-shaped masses with circular surfaces.

Credit: https://www.researchgate.net/profile/Sadhbh_Baxter/publication/242578849/figure/fig9/AS:[email protected]/Outcrop-of-metasedimentary-rocks-and-cone-sheet-at-picnic-area-north-of-Carlingford.png

  1. High Folded Regions:
    • Batholiths- These are the largest invaders. Most of the foundation rock exists in the inner deformation zone of the earth’s crust and is composed of granite. These are spreading downward to unknown depths. The rock foundation with a smaller scale is called a stock, and the stock with a round surface is called a boss. Rural rock residues on or near the top of these intrusive materials are called roof overhangs.

Credit: https://www.researchgate.net/profile/Michael_Petterson2/publication/327023739/figure/fig4/AS:[email protected]/Field-photographs-from-the-Kohistan-batholith-Gilgit-area-a-The-c-102-Ma-Matum-Das_Q640.jpg

  • Ethmolith- These are funnel-shaped basic bodies with circular outcrop.
  • Harpolith- A sickle-shaped matrix formed by stretching the stratum after or during the injection.
  • Chonolith- Any irregular intrusive body.
  1. Concordant-bodies: These are infiltrating entities that operate in the same way as the rock formations in which they occur.
    1. Unfolded Region:
      • Sills- It is a thin parallel table sheet of magma inserted with bed planes, schistosity planes, non-adaptations, etc. They may find a tendency in the space in which they are based on the attitude of the bed of rocks.
rocks types

Credit: https://www.researchgate.net/profile/Olivier_Galland/publication/322203693/figure/fig5/AS:[email protected]/A-Schematic-drawing-of-a-dyke-feeding-sills-B-Schematic-drawing-of-sills-feeding-sills.png

  • Laccolith- The bottom surface of these infiltrating bodies is flat and has an optimal head. This is due to the accumulation of adhesive magma which is usually acidic in the composition, which pushes the upper rocks upwards to make room for the mass of the top.
rocks types1

Credit: https://www.researchgate.net/profile/Tobias_Mattsson/publication/322935547/figure/fig7/AS:[email protected]/1-The-formation-of-the-Sandfell-laccolith-initiated-as-a-sill-intrusion-cf-Johnson.png

  • Lopoliths- These are saucer-shaped bodies that have wide directions and are ultra-basic in composition.
  • Bysmalith- Sometimes the magma breaks from the overlying rocks and after uniting the Ignes mass is known as vyhsmaliths.
  1. Highly Folded Regions:
    • Phytoliths- These are lunar-shaped intricate bodies that are encircled in and around the country’s rocks. They are made of basalt.
rocks types2

Credit: https://www.researchgate.net/profile/Irfan_Temizel/publication/265421031/figure/fig6/AS:[email protected]/figure-fig6_Q320.jpg

Forms of Extrusive Bodies:

(A) Concordant –

Compressed-lava flows and pyroclastic which are products of volcanic activity are common forms of extruded igneous rocks.

(B) Discordant –

This is a group of hot rocks formed by the confluence of lava and pyroclastic materials in the eruption channel of an extinct volcano.

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