Intrusive Rock

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Intrusive Igneous Rocks

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The intrusive rock is also known as plutonic rock. Intrusive rock is also an igneous rock like granite or diorite. When the igneous rock formed from magma is forced down into the older rocks at the depth of the Earth’s surface which then slowly solidifies below the surface of the Earth, it forms intrusive igneous rocks. The intrusive igneous rocks are exposed later on due to processes of erosion. There are many types of intrusive igneous rocks. Examples of intrusive igneous rocks include granites, dikes, sills, etc. 

Formation of Intrusive Rock

The intrusive rock definition clearly states the process of formation of the intrusive rock. The intrusive igneous rock definition tells us that when the magma penetrates some of the existing rock structures, it crystallizes and solidifies below the surface to form intrusions and hence the intrusive rocks. Examples of intrusive igneous rocks include the intrusions such as dikes, sills, volcanic necks, batholith and laccolith. Contrary to this, an extrusive rock is the one that is formed when the magma from the inside of the Earth is thrown out by a volcanic eruption and it solidifies to form pyroclastics or tuff. The major similarity between intrusive and extrusive rocks is that they are both igneous rocks formed by the cooling and solidifying of the magma. 

Thus, there are two ways in which igneous rocks are formed which are the intrusion and extrusion of magma. When the magma passes through the intrusions inside the Earth’s crust it forms the rocks that are intrusive and extrusive rocks are formed when the magma is exploded out of the Earth’s crust into the atmosphere and solidified. Out of the many differences, one particular difference between the formation of intrusive and extrusive rocks is the time taken by them to solidify. Since the intrusive forms are the ones that are solidified inside the Earth’s crust they take a longer time to cool down as compared to the extrusive forms which are solidified above the crust and so they cool down faster due to the open-air or seawater. Thus, these are some of the matters of significance in the formation of intrusive and extrusive rocks.

The intrusive igneous rocks are also referred to as plutonic rocks by some geologists. The ones that do not consider the intrusive rock and the plutonic rock to be the same, subdivide the intrusive rocks by the size of the crystal. The subdivisions are the coarse-grained plutonic rock which is generally formed very deep into the Earth’s crust and the medium-grained subvolcanic or hypabyssal rock which are typically formed higher than the plutonic rock level such as in the case of dikes and sills. In either of the cases, when the surrounding layer of Earth’s crust weathers away due to erosive activities, the intrusive rock lay exposed on the surface. A diagram showing the different types of intrusive volcanic features is given below:

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Intrusive Rock Characteristics

The intrusive igneous rocks are largely characterized by the size, shape and texture of the crystals. Different types of intrusive igneous rocks are also classified as well based on the size of the crystal. For example, when individual crystals are visible then the rock is said to be phaneritic. Although solidified, there are some indications of flow in the intrusive rocks as the texture and structure of such rocks develops during the final stages of crystallization which is when the flow has ended. Sometimes, there are large and well-shaped crystals found in the intrusive rocks lined along the cavities formed by the gases that are trapped and contained within the rocks. Such a texture is described as miarolitic and it is a common occurrence in granites. An interesting fact about such rocks is that the crystals inside such intrusive igneous rocks are roughly of the same size and hence, such igneous rocks are known as equiangular. 

The plutonic rocks are less likely to be as pronounced in showing any porphyritic texture (a texture in which the first generation of large and well-shaped crystals are present inside a fine-grained groundmass) as any other volcanic rocks. Instead, the minerals inside the plutonic rocks have had a definite order with their period of crystallization which could have been distinct or coincided or overlapped with the period of formation of some other ingredients. The crystals that were formed earliest were more or less perfect in shape because most of the surrounding rock was still liquid while at later stage crystals are less regular in shape as they occupy the remaining spaces in-between the already-formed crystals. Because of this, the crystals formed earlier are known as idiomorphic crystals and the ones formed later are known as xenomorphic. 

The hypabyssal rocks usually show structures that lie in-between the structures of extrusive igneous rocks and plutonic rocks or intrusive igneous rocks. They are usually porphyritic or non-crystalline amorphous solids, or sometimes vesicular i.e. present with many cavities. As they are formed near the surface of the Earth, they are indistinguishable in many cases from the lavas that have similar composition.  

Types of Intrusive Igneous Rocks

Since any of the magma that is solidified and cooled within the intrusions of the Earth’s crust forms the intrusive rocks, there is a difference in between the cooling rates of the different intrusive igneous rocks, leading to different textures and hence different types of intrusive igneous rocks. 

It is already known that the cooling of the magma within the intrusions is extremely slow. This is because the solid pre-existing rocks such as the country rock, surrounding the intrusions is an excellent insulator and thus, the temperature of the magma within the intrusions remains almost constant. It later very slowly cools down and solidifies forming the intrusive rocks. But there is a difference between the rates of cooling for the intrusive rocks at various depths. Due to this, the intrusive rocks formed at the very deep intrusions have coarse-grained texture whereas the intrusive rocks formed near the surface or shallow depths have less coarse-grained or fine-grained texture. This also forms the basis of classifying the intrusive rocks into plutonic (coarse-grained and formed at depth) and hypabyssal (less coarse-grained and formed at shallow depth) rocks. 

Another manner in which most of the intrusive igneous rocks are classified is based on their mineral content such as the relative amounts of quartz, feldspar, plagioclase, etc. Both the plutonic and hypabyssal have their classification schemes due to the distinct nature of their granules and the relative mineral composition which can vary from bottom to top of the Earth’s crust. 

FAQ (Frequently Asked Questions)

1. What is Meant by an Intrusive Rock?

Ans: When magma from the volcanoes flows down into the intrusions present in the pre-existing rocks inside the earth’s crust, it gradually and extremely slowly cools down and after a period of crystallisation it solidifies. Any rock formed because of such a process is known as intrusive rock. Thus, intrusive rocks are one of the types of igneous rocks formed by the solidifying of the magma within the intrusions present in the crust leading to the formation of rocks. These rocks are exposed when the nearby rocks are weathered away due to erosive activities. 

2. What are the Common Types of Intrusive Rocks?

Ans: Granite is the most common type of intrusive rocks. It is mostly found on the continents. Gabbro is also a common type of intrusive rock but is mostly found in the oceanic crusts. The igneous intrusive rocks are formed when magma from the volcanoes fills the intrusions present in the pre-existing rocks inside the crust of the Earth and cools down over a long period of time resulting in the formation of rocks. Common forms of such intrusive volcanism are sills, dikes, batholith and laccolith.