29 Structure Of Igneous Rocks

The composition of igneous rock is largely characterized by many factors.(28 Igneous Rocks: Introduction, Classification, & related terminology)

Igneous Rocks

Fig.1:- Structure of Igneous Rocks

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Fig. 2:- Structure of different rocks

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Igneous structures are usually classified into three main groups, as follows:

1. Mega- Structure:

These are usually formed at the extrinsic magma (Extrusive Rock) stage-

a. Vesicular and Amygdaloidal structure-

When lavas heavily charged with gases and other volatiles have erupted on the surface, the gaseous constituents escape from the magma as there is a decrease in the pressure. Thus, near the top of flows, empty cavities of variable dimensions are formed. The individual openings are known as vesicles and the structure as a whole is known as vesicular structure. If, however, the vesicles thus formed are subsequently filled in with some low-temperature secondary minerals, such as calcite, zeolite, chalcedony, etc., these infillings are called ‘Amygdales’ lavas containing amygdales are said to have amygdaloidal structure. These are the typical character of a basic lava flow.

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b. Cellular or Scoriaceous Structure

Awakening from the gases, from lava that is heavily involved in volatile and gaseous movements, a number of caves are formed by the consolidation of the lava. When the caves are plentiful, the term ‘pumice’ or ‘rock-froth’ is applied. Such structures are termed cellular or scoriaceous struts and are common in siliceous lava.

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c. Lava-drain Tunnels-

At times as long as the upper surface of the lava solidifies, the interior can remain mobile. When the closed lava drains out through some weak spots lying at the edge of the stream, the resulting structure is called lava drainage tunnels.

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d. Block-Lava-

Because lava of acidic composition, due to their high viscosity, does not flow to greater distances, they are found after solidification to offer a very rough surface. Such lava flow is called black lava. It is also called the “aa” structure.

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e. Ropy Lava-

The lava of basic writing is highly mobile due to its low volume and can flow to greater distances and after solidification provides a very smooth surface. Such lava flows are called lava and are also called ‘pahoehoe’ structure.

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f. Pillow structure-

It is made up of the shape of a remote pillow. Mass was piled on top of each other. These are made by taking lava into wet air, under ice sheets, under water-filled sediments, or in seawater. Spilite, albite-rich lava (sodium-rich) usually exhibits a pillow structure.

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g. Sheets Structure-

The development of a single set of joints is well defined, sometimes having a chipping effect on the body of igneous rocks. If all these slices are flat, the structure is said to be a leaf structure.

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h. Platy structure-

This is also due to the development of different sets of joints, which cause only blades of rock mass, to hit the rock. Such a feature is called a platy structure.

I. Columnar structure-

As a result of shortening due to cooling, a few sets of vertical joints develop. Such joints result in the formation of columns, which may be square, rectangular, rohombic, or hexagonal in outline.

j. Flow structure-

After lava debris has erupted on the surface the species are slow, flowing from one place to another with great difficulty and in an attempt to do so, the uneven patches within the lava are drawn. out in the form of long lenticels. Sometimes the crystalline particles already inside the magma are arranged parallel to the direction of the lava flow. They naturally indicate the direction of flow of the mass, before it is fixed. These are more commonly known as steering structures or flow structures.

k. Rift and Grain-

These are the result of a connection. In granite, three bonds are called together, which are surrounded by each other, and which are advantageous while making cubic blocks. But for processing the blocks down to smaller dimensions, the ties that are tightly wrapped together are brought to benefits. These joints are called rift and gravel.

1. Minor structure:

These structures are formed in the fluid stage of the magma and include the following-

a. Primary foliation-

Many plutonic rocks are sometimes catalyzed by foliation due to the parallel accumulation of platy and ellipsoidal mineral grains.

b. Banding in Rock-

These are also referred to as layered rocks made up of other bands of different compositions. It can lead to lamellar edema, from the removal of minerals from crystalline magma, or continuous injection.

c. Schlieren-

These are wavy, irregular, irregular leaves, usually without a sharp connection to the surrounding igneous rocks. They can alter inclusion, separation, or represent a density of residential moisture in layers in the rock that has otherwise crystallized.

2. Micro-structure-

these are formed due to a reaction between already solidified crystals and the rest of the magma and include the following-

a. Reaction Rims-

When the reaction between one crystal mineral is already present and the rest are surrounded by the results of infinite magma, scattered crystal refraction. Such zones are called reverse rims. When reproductive currents are produced by primary mesmeric regression. When they are in high school, they are called ‘corona structure’ and ‘calcific border’.

b. Graphic structure-

It results from an intergrowth of quartz and orthoclase feldspar.

c. Myrmekite structure-

It is produced by an intergrowth of quartz and plagioclase feldspar where quartz occurs as bulbs or drops in plagioclase.

d. Xenolithic structure-

The return of foreign rock fragments inside the Ignatius rock gives rise to the Xenolithic structure. Xenoliths are called “knowledgeable” when they are genetically linked to rocks and “accidental” when they are fragments of native rocks with no genetic connection to the cycle.

e. Orbicular structure-

These spheres differ in that they are concentrated shells of different mineral compositions and textures, which are occasionally found in granite rocks.

f. Spherulite structure-

Its essential feature is the simultaneous crystallization of fibrous diseases with radiating arrangements about a common center. The large sphere is known as ‘lithophytes’. In lava and infiltrates, they are called ‘ovarioles’ and contain variolates in the rocks.

g. Perlitic cracks-

These are the focus lines of cracked, fractured, often seen in volcanic glass. These are only due to the shrinkage of the glass mass on cooling.

Texture of Igneous Rocks

The texture of Volcanic rock textures describe the actual relationship between crystals, or between crystals and the vitreous material found in igneous rocks. The rocks were formed in a diverse physicochemical environment, and textural studies indicate a history of magma cooling. The texture of igneous rocks depends on three important factors: Degree of crystallization

  • Size of the grains
  • Fabric, which consists-
    • Mutual relations of grains or of crystals and glassy matter
    • Shape of crystals
  1. Crystallinity:
    • Holocrystalline- When an igneous rock is made up of mineral grains only.
    • Hemi-Crystalline- When a rock contains both crystallines as well as glassy matter in variable proportions.
    • Holohyaline- When the igneous rock consists wholly of glass.

The degree of crystallization depends on the following factor:

  • Rate of cooling
  • Depth of cooling
  • The volume of the magma
  • Viscosity of magma
  1. Granularity: It refers to the grain size of the crystals present in the igneous rocks. These are-
    1. Phaneritic- When individual crystals are visible to the naked eyes and are:
      1. Coarse grain- When the grain size is 5 mm or above.
      2. Medium grain- Grain size is 1 mm to 5 mm.
      3. Fine grain- Grains are smaller than 1 mm in diameter.
    2. Aphanitic- When individual grains can not be distinguishable unaided vision and are:
      1. Micro-crystalline- When individual crystals are distinguishable only under microscope.
      2. Meso-Crystalline- its found as the form of intermediate in range.
      3. Crypo-crystalline- When individual crystals are too small to be separately distinguished, even under the microscope.
      4. Glassy- When there is no crystallization at all.


It is the process of conversion of glassy material to crystallized state. Perlitic cracks in crystals are evidences of their original glassy condition.

  1. Fabric:
    • Shape of the grains- It refers to the degree of development of crystal faces and are:
      1. Euhedral- When the mineral grains are found to have developed a perfect crystal outline. These are known as idiomorphic ar automorphic crystal.
      2. Sub-hedral- When the crystal outlines have partially developed. These are also known as hypidiomorphic or hypautomorphic crystal.
      3. Anhedral- When the crystal faces are absent.
    • Mutual Relationship- It refers to the relative size, shape and dimensions of crystals and their relation to one another. This may be:
      1. Allotriomorphic-  In this case all the crystals are Anhedral synonymous terms are xenomorphic, aplitic, mosaic, surgary, sacchraoidal.
      2. Hypidiomorphic- Here all the crystals are subhedral
      3. Pan idiomorphic- Here all the crystal are euhedral. This is the characteristics texture of lamprophyres.

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