Intrusive Igneous Rocks

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More About Intrusive Igneous Rocks

Intrusive Igneous rocks are those rocks that are formed from the magma beneath the surface of the Earth. Igneous rocks are those types of rocks, which are formed with the cooling of molten rock to a solid state. These rocks are formed from magma and thus these are the rocks that begin the rock cycle, they are also known as primary rocks. These can be easily identified with their features such as texture, colour, density and mineral composition. These are generally further divided into two types: Intrusive or Extrusive. Here, in this article, we will learn about the intrusive volcano, intrusive forms, Intrusive rocks, its diagram, features, types, etc.

Introduction

A volcano is an opening on the surface of a planet that leads to the escape of various warmer materials such as lava, gases, dust, ashes, etc. from its interior. This escape occurs because of the eruption of the volcano and this eruption can be explosive or on the other hand, it can be calmer which includes a gentle flow of the lava out of the volcano. Basically, these can be of two types i.e. Intrusive or Extrusive.

Intrusive Volcano

Intrusive volcanism is a type of activity where magma is forced into the rocks that make up the crust of the Earth and when with time it cools and later becomes solid but still remains underground, this leads to the formation of different features such as plutons. Due to intrusive volcano activity, intrusive igneous rock formation occurs here.

Intrusive Forms

During volcanic eruptions, the lava which is discharged out of it leads to the formation of igneous rocks after cooling down. This cooling of the lava may take place in two ways; either after arriving on the surface or while the lava is still in the crustal portion. Igneous rocks are generally classified into two types i.e. plutonic rocks as well as volcanic rocks depending upon the location of the cooling of the lava. The lava which cools inside the portions of the crust of the Earth takes various diverse forms and these forms are known as intrusive forms.

Intrusive Igneous Rocks

Intrusive igneous rocks are types of rocks that cool underground and deep in the crust below the surface magma cools slowly and this slow cooling gives crystals enough time and a chance to grow. Therefore, these types of rocks have relatively large crystals which are easy to see. These are also known as plutonic rocks where pluton means an igneous rock body that forms within the crust.

Intrusive Igneous Features

The various features of these type of rocks are mentioned below:

  • These are one of the important types of igneous rocks along with the Extrusive ones.

  • These types of rocks formed beneath the surface of the Earth with the cooling as well as solidification of the magma.

  • It leads to the formation of various features such as stocks, sills, dikes, laccoliths, etc.

  • If we talk about plutons, then the mass of cooling magma is known as a pluton whereas the rock around refers to country rock. The formation of large plutons can occur along the convergent tectonic plate boundaries. 

  • Intrusive rocks are those which include slow cooling and slow cooling leads to the formation of large crystals which are visible.

  • Common examples of these types of rocks are granite, gabbro, and diorite, etc.

  • Plutonic rocks are hard as well as large and slow process of erosion leads to exposing of these rocks at the surface due to geological processes.

Intrusive Igneous Rocks Diagram

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Here, in the aforementioned diagram, we can clearly see how Intrusive rocks are formed beneath the surface of the Earth due to the trapping of magma inside the Earth's surface and their magma cools and solidifies with time and leads to the formation of various types of rocks. Long and slow timing of cooling leads to the formation of the coarse-grained texture of these rocks.

Types of Intrusive Igneous Rocks

The Intrusions which actually intrude the rocks at shallow levels of the crust are known as hypabyssal intrusions. Here, shallow refers to the depths of generally less than about 1 km. These kinds of intrusions always show sharp contact relations with the rocks they intrude.  Several types are mentioned below:

  • Dikes are one of the important types which are small, generally <20 m wide shallow intrusions and which usually show a discordant relationship to the rocks in which they actually intrude. Here, discordant refers to that they cut across pre-existing structures and they may also occur as isolated bodies or as swarms of dikes emerging from a large intrusive body.

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  • Sills are also small shallow intrusions that are generally <50 m thick which show a concordant relationship with the rocks that they actually intrude. These usually are fed by dikes and on the other hand, these may not be exposed in the field. [Image will be Uploaded Soon] 

  • Laccoliths are those intrusions, which are somewhat large that lead to the upliftment as well as folding of the preexisting rocks above the intrusion. These intrusions are also concordant.

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Besides these, if we talk about plutons, these are generally much larger intrusive bodies that have intruded much deeper.   They may show sharp contacts with the surrounding rocks but on the other hand at deeper levels, the contacts are often gradational. Several types are mentioned below:

  • Lopoliths are those small plutons that usually show the upper surface as concave downward.  This shape may have formed because of the reduction in volume due to crystallization of magma, with the weight of the overlying rocks that leads to collapse into space which was once occupied by the magma at the time when it had a larger volume (as a liquid).

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  • If we talk about Batholiths, these are very large intrusive bodies whose bottoms are rarely exposed and sometimes they also consist of several smaller intrusions as well.

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  • If we talk about what Stocks are, then these are those smaller bodies that are likely to feed on batholiths of deeper levels. Stocks may have been feeders for the volcanic eruptions, but associated volcanic rocks are rarely exposed because of the large amounts of erosion that are required to expose a stock or batholith.

Fun Fact

Intrusive rocks formed below the surface but due to geological processes, some intrusive igneous rocks have been brought to the surface. For example The following landscape in the Sierra Nevada Mountains of California which are made up of granite that has been raised to create mountains.

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Conclusion

Thus, in this article, we have covered the Intrusive rocks which are an important type of Igneous rocks that are formed from the magma, especially the magma which cools into a solid form under the surface of the Earth. The volcanic eruptions of the intrusive volcano lead to the formation of such rocks and various other forms such as dikes, sills, laccoliths, etc. These occur below the surface but sometimes due to the geological processes, these can be exposed on the surface as well. This topic will be helpful to increase your understanding of igneous rocks in Geology, Geography, Earth Sciences, Physical Geography, Geomorphology, etc.

FAQs (Frequently Asked Questions)

Q1. What Do You Mean by Intrusive Igneous Rocks?

Answer. These are those types of igneous rocks that are formed from the magma that has trapped inside the crust of the Earth and these are also known as Plutonic rocks. In simple words, these are those Igneous rocks that form under the surface and their magma cools and becomes a solid mass under the influence of longer time where crystals get enough time to grow and develop into visible and coarse-grained.

Q2. What are the Various Features of Intrusive Igneous Rocks?

Answer. The various features include that these are one of the important types of igneous rocks which formed beneath the surface of the Earth. They form because of the cooling as well as solidification of the magma and leads to the formation of several features such as stocks, sills, dikes, laccoliths, etc. Plutons are also associated with these types of rocks whose formation can occur along the convergent tectonic plate boundaries. 


These are the rocks that include slow cooling which leads to the formation of large crystals which are visible. Some of the examples of these types of rocks are granite, gabbro, and diorite, etc.

Q3. What is the Difference Between Intrusive Igneous Rocks and Extrusive Igneous Rocks?

Answer. Igneous rocks are formed because of the solidification of the molten magma and can be classified into two types. Intrusive rocks are formed when lava is cooled into a solid form beneath the surface of the Earth whereas when lava comes above the surface, then the rocks formed are known as extrusive. The former one has a coarse-grained texture whereas the latter has a very fine-grained as well as glassy texture.

Q4. What are the Common Types of Intrusive Igneous Rocks?

Answer.  The common types of Intrusive forms include Batholiths, Laccoliths, Sills, Dykes, etc. Batholiths are those large bodies of magma that cool in the crust deep down and mould into the form of large domes whereas laccoliths are also large & dome-shaped bodies that have a level base and are also linked by a pipe-like channel from below. Sills are horizontal bodies found in these rocks whereas dykes are considered as feeders for eruptions.

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