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What is igneous rock? How they are formed? What are the 5 major structures of Igneous rocks?

 Before knowing about the structure of igneous rocks, we need to understand the terms “Structure” and “Igneous rocks”. So, the word ‘structure’ includes the size, shape, and orientation of a rock or a group of rocks. The word ‘igneous’ is derived from a Latin word ‘Ignis’, which means ‘fire’. Igneous rocks are a type of rocks, which are formed from magma by cooling and solidification of magma. They are also known as primary rocks because they formed during the formation f Earth's crust. 

The structure of igneous rocks can be classified into two types-

1) Primary structure (Extrusive igneous rock structure): Extrusive igneous rocks structures are formed above the surface of the earth by cooling and solidification of lava. Examples are-

• Pillow structure:- when outer pouring lava occurs in the ocean then the outer surface of it quickly freezes due to a sudden fall of temperature, but the interior parts remain the same hot and molten.  This interior part breaks down the harder part of it and moves forward. This process occurs over and over and some pillow-like structure is formed known as pillow lava.


• Lava flow structure: The volcanic igneous rocks occur as lava flows. The lava flows are tabular in shape and may range in thickness from a few meters to several hundred meters. They are formed when lava erupts on the earth’s surface from the fissure. The Deccan Traps which covers a vast area in Central India is a famous example of lava flows. 


• Vesicular structure: Most lava contains a large amount of gas and volatiles matte. These gases and volatile matters escape into the atmosphere when they solidify on the earth’s surface. As a result, cavities are formed in the igneous rocks, which are known as ‘vesicles’ and the whole structure is called ‘vesicular structure’. 


• Columnar structure: Columnar structure consists of sets of columns of rocks. The cooling of magma is very often accompanied by the development of cracks or joints in the rocks. In the case of some rocks, vertical joints are developed which divide the rocks into polygonal blocks or columns. At the surface, the joints are polygonal-shaped that is they may be square, rectangular, rhombic, or hexagonal. The structure so developed is termed a columnar structure. This structure is usually seen in basalt.  


• Amygdaloidal: The vesicles of volcanic rocks may subsequently be filled up by secondary minerals such as calcite and zeolites. Such filled vesicles are called “amygdale” and the rocks, which contain such amygdale is said to have an “Amygdaloidal structure”.


• Xenoliths structure: Foreign rock fragments are included in the magma when it rises towards the earth’s surface. If they are not separated from the rocks and remain entrapped within the rocks and produce heterogeneous texture on them. Such entrapped foreign rock fragments are called ‘xenoliths’ and the structure is called “xenoliths structure”.


2) Secondary structure (Intrusive igneous rock structure): Intrusive igneous rocks structures are formed below the earth’s surface by cooling and solidification of magma. Examples are-

• Laccoliths: Laccoliths are a lens-shaped intrusion that results in the uplift folding of pre-existing rocks. They have a flat base and a domed top. Laccoliths may be 2 to 3 kilometers in diameter and several hundred meters in thickness. They are concordant types of intrusion. 


• Lopolith: A Lopolith is a saucer-shaped concordant intrusion that results in down lift folding of the pre-existing rocks. They occur between the parallel layers of rock at relatively deeper depths. Its diameter is usually between 10 to 20 times its thicknesses. The composition of Lopolith is commonly basic.


• Sills: Sills are sheet-like intrusion that runs parallel to the bedding plane of the pre-existing rocks layers. They may be horizontal, inclined, or vertical depending upon the direction of bedding planes at which they deposited. The distance from which sills are spread that is depending on the hydrostatic force, their temperature, degree of fluidity, and weight of the block of strata. The thickness of sills varies from a few centimeters to several hundred meters but they are always thin as compared to their length along beds. Sills are commonly made up of basalts and dolerite. 


• Phacoliths: Phacoliths is a crescent-shaped intrusion. They occupy crests and troughs of folded strata. They are formed when igneous materials occur at the folded region and accumulate at the crests and troughs of folded strata due to a zone of minimum stress. 


 • Batholiths: Batholiths are large intrusive igneous bodies that are both concordant and discordant. Their diameter is usually 100 km or more and their outcrop at the surface is roughly circular or oval. They occupy a large area. The composition of Batholith is usually granitic or granodioritic.


•   Stocks: Stocks are just the top of the largest body of Batholith, which has relatively small parts of it exposed at the surface. They are of smaller size and their diameter is usually between 10 km to 20 km.


• Dykes: Dykes is the thin sheet of the igneous body that formed when magma moves vertically through a fracture in pre-existing rocks. It is normally inclined or nearly vertical a tabular sheet of the igneous rock body. Their thickness varies from a few centimeters to a hundred meters or more. A dyke has a circular outcrop and a conical form is called “ring Dyke “.  


• Volcanic neck: A volcanic neck or plug is a vertical cylindrically shaped igneous body that has a roughly oval or circular cross-section. It represents the vent of an extinct volcano. Volcanic plug range in diameter from a few hundred meters to a kilometer or more. 

Intrusive igneous of two types on the basis of depth-

a) Hypabyssal rocks: The igneous intrusive rocks which are formed near the surface of the earth such rocks are called hypabyssal rocks. For example- Basalt.

b) Plutonic rocks: The igneous intrusive rocks which are formed at a great depth below the surface of the earth such rocks are called plutonic rocks. For example-Granite.

          Intrusive igneous rocks structures are classified into two ways, based on the relationship of intrusive igneous structures to country rock-

a) Concordant intrusion: Concordant intrusions are those, which are parallel to the bedding plane. That means they do not intersect the individual layers of country rocks but are deposited above the layers in a parallel manner. Example is-


• Laccoliths

• Lopolith

• Sills

• Phacoliths

• Salt domes


b) Discordant intrusion: Discordant intrusions are those, which are perpendicular to the bedding plane. That means, they cut across the bedding layers of country rocks. The thickness varies widely from an inch up to hundreds of feet. Example is-


• Dykes

• Stocks

• Batholiths

• Volcanic necks

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