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Table of contents
Introduction
The materials of the earth’s crust or lithosphere are called Rocks. The word ‘rock’ included all the materials of the crust, whether it is hard or soft. Rock can be hard like granite and sandstone, soft like clay and sand, porous like chalk and limestone or it can be impervious like slate. Generally, rocks are composed of two or more minerals and form either above or below the earth’s surface. In general (but not always), the rock formed below the earth’s surface is called ‘igneous’, and that formed above the surface is called ‘sedimentary’ rock. The rock that formed due to a change in the composition of either sedimentary or igneous rocks is called ‘metamorphic’ rock.
As far as the origin of rocks is concerned, metamorphic and sedimentary rocks are directly or indirectly formed from igneous rocks. First, igneous rocks formed due to the solidification of molten materials of the earth known as magma and lava. Then, sedimentary rocks formed through lithification and cementation of deposited materials of igneous rock in aquatic areas (eroded by various means of erosion). Later, metamorphic rocks formed as a result of changes in the composition of either sedimentary or igneous rocks.
Classification of Rocks
If rocks on the earth’s crust are analyzed, then it is found that there is a considerable difference in their origin, physical properties, and chemical properties. Rocks are generally classified based on the mode of their formation.
Based on the mode of formation; rocks are divided into three categories –
Igneous rocks
Igneous rocks formed due to the cooling and solidification of molten materials of the earth known as magma and lava e.g., granite, basalt, etc.
Sedimentary rocks
Sedimentary rocks formed through lithification and cementation of the sediments deposited in water bodies or by the accumulation of fossils, e.g., limestone, sandstone, etc.
Metamorphic rocks
Metamorphic rocks formed due to changes in the composition of either sedimentary or igneous rocks, e.g., slate, marble, etc.
Igneous Rock: Meaning
The word ‘igneous’ is derived from the Latin word ‘ignis’ which means fire. As we discussed earlier, igneous rocks are formed by the cooling and solidification of hot and liquid magma and lava below or above the earth’s surface. Because these rocks were formed primarily during the formation of the upper crust, they are also known as primary rocks. Other rocks have been formed directly or indirectly from this rock, which is why igneous rocks are also called parent rocks. During a volcanic eruption, hot and liquid magma and lava solidify inside and outside the earth and take the form of igneous rock.
Characteristics of igneous rock
- Igneous rocks are hard and water percolates through joints with great difficulty. When these rocks get exposed for a long time, they become so soft that they can be easily dug with a spade.
- Igneous rocks are granular, but there is a considerable difference in the texture, size, and form of the grain because these characteristics depend upon the rate and place of cooling of magma and lava. When the lava cooled and solidified on the earth’s surface quickly, then no crystal formed in the resultant rocks because there is no sufficient time for the development of crystal or grain. On the contrary, when magma cooled and solidified below the earth’s surface at a slow rate, the resultant igneous rocks are characterized by coarse grains or crystals.
- These rocks do not have layers or strata like sedimentary rocks.
- Igneous rocks are less affected by chemical weathering because the water does not penetrate easily in these rocks. But more affected by mechanical or physical weathering due to which disintegration and decomposition occur in the rocks.
- Fossils are not found in igneous rocks because the ancient igneous rocks formed by cooling and solidification of hot and liquid materials at the time of the earth’s origin, there was no life on the earth’s surface. Secondly, igneous rocks are formed by the cooling of hot and liquid magma and lava, so the remains of plants and animals, if any, are destroyed due to extreme heat.
- Igneous rocks are often associated with volcanic activity. They formed from the lava that comes out of the volcano. Therefore, these rocks are found in volcanic regions and are also called volcanic rocks.
Classification of igneous rocks
There is considerable variation in the types of igneous rocks. These variations are found in their composition, color, texture, shape of particles, the process of formation, etc. On this basis, igneous rocks can be divided into the following ways-
1. According to the mode of occurrence
Intrusive Igneous Rocks
- Plutonic Igneous Rocks
- Hypabyssal Igneous rocks
Extrusive Igneous Rocks
- Explosive Type
- Quiet Type
2. According to the chemical constitution
- Acid Igneous Rocks
- Basic Igneous Rocks
3. According to the texture of grains
- Pegmatitic (very coarse-grained) Igneous Rocks
- Phaneritic (coarse-grained) Igneous Rocks
- Aphanitic (fine-grained) Igneous Rocks
- Glassy (without grain) Igneous Rocks
- Porphyritic(mixed-grained) Igneous Rocks
1. According to the mode of occurrence
Igneous rocks are formed by the solidification of molten magma and lava. During a volcanic eruption, the lava solidifies in two forms (above and below the surface). On this basis, there are two types of igneous rocks.
Intrusive Igneous Rocks
When magma moves upward during a volcanic eruption, instead of reaching the surface, it cools and solidifies within or below the surface. The resulting rocks are called igneous rocks. These rocks are further divided into two groups.
Plutonic Igneous Rocks
When the cooling and solidification of magma takes place deep inside the earth, the resulting rocks are called plutonic igneous rocks, e.g., granite. These rocks are named after ‘Pluto’ which means ‘God of the Underworld or Netherworld’. Plutonic rocks are coarse-grained because the cooling rate of magma deep inside the earth is extremely slow. They are located at such a depth that they are visible on the surface only after excessive erosion or lifting of the upper part of the earth.
Hypabyssal Igneous Rock
When rising magma (during a volcanic eruption), cools and solidifies in cracks, joints, pores, and hollow places just below the earth’s surface, then hypabyssal rocks are formed. These rocks appear on the surface after excessive erosion. The magma solidifies in different forms or shapes in cracks and joints. These forms or shapes are discussed below.
Batholith
Batholiths are long, uneven, and elevated igneous rocks found deep inside the earth. They are dome-shaped with steep side slopes and a flat base deep in inside the earth. Batholiths are found in volcanically active mountain areas. Granite plays an important role in their formation. Such batholiths are found on the Ranchi Plateau where they are called Ranchi Batholiths.
Laccolith
A laccolith is a form of intrusive hypabyssal igneous rock with a dome-shaped or convex summit and planar base and formed due to the cooling and solidification of magma along horizontally bedded planes of the sedimentary rocks. When lava erupts, the upper part of the stratified sedimentary rocks is transformed into a convex arch or a dome shape structure by the force of ascending gases. Then, the space forms between the upper convex arc and the lower horizontal layers are filled with magma and volcanic ash, gas, lava, etc. As a result of this laccolith form.
Phacoliths
Phacoliths formed when magma cooled and solidified along the anticline and syncline in the areas of folded mountains.
Lopolith
The word Lopolith has been derived from the German word ‘lopas’, which means a shallow basin. When the magma solidified in a concave shape shallow basin inside the earth, then the resultant form of igneous rocks is called lopolith.
Sill
Sills are formed when magma entered and solidified in between layers of sedimentary rocks. The sills are generally laid down parallel to the layers of sedimentary rock. The thick layer of magma is called ‘sill’ and the thin bed is called ‘sheet’. The thickness of the sill varies from a few centimeters to several meters. The sill is much harder than the surrounding rocks and exposed when the adjacent rocks get eroded.
Dyke
It looks like a sill. But it is relatively long and thin. Unlike sills and sheets, dykes are laid down perpendicular to the layers or beds of the sedimentary rocks.
Extrusive Igneous Rocks
Extrusive igneous rocks formed when the hot and molten lava cools and solidifies on the earth’s surface. The lava cools and solidifies on the earth’s surface quickly, therefore no crystal is formed in these rocks because there is no sufficient time for the development of crystal or grain. These rocks are further divided into two groups.
Explosive type
During a violent volcanic eruption, a mixture of volcanic materials like lapilli (pea size rock fragments), bombs (large fragments of rocks), and volcanic dust and gases come out and solidified on the nearby surface, the rock thus formed is called an explosive-type of extrusive igneous rock. When fine volcanic materials are deposited in water bodies, are called tuffs. The mixture of small and large fragments of rocks after deposition is called agglomerate or breccia.
Quiet type
When the magma erupts quietly on the surface through small cracks and vents of volcanoes and solidified in the form of a rock, then the rock formed in this way is called a quiet type of extrusive igneous rock e.g., basalt. Igneous rocks called Gabbro and Obsidian also come in this category.
2. According to the chemical constitution
The chemical composition of different igneous rocks varies from one group to another, but some amount of silica is present there in each type of group of igneous rock. Based on the amount of silica, igneous rocks are divided into four classes-
Acid Igneous Rocks
When the amount of silica in igneous rocks is 65 to 85 percent, they are called ‘acid igneous rocks’. Their average density varies from 2.75 to 2.8. As far as minerals are concerned, they are rich in quartz and white or yellow feldspar and lack in iron and magnesium. That is why these are yellow. These rocks are hard and relatively resistant to erosion. Therefore, these are generally used in building houses. Granite is a prime example of this type of rock.
Basic Igneous Rocks
When the percentage of silica in the igneous rock varies from 45 to 60 percent, it is called ‘basic igneous rock’. Their average density ranges from 2.8 to 3.00. Ferromagnesium predominates in basic rocks and is deficient in feldspar. Due to the dominance of iron, these rocks are dark in color. Basic igneous rocks easily erode when they come in contact with water. Basalt, gabbro, and dolerite are prime examples of these rocks.
Intermediate igneous rock
When the amount of silica in an igneous rock is less than acid rocks and more than basic rocks, then the rock is called an intermediate igneous rock e.g., diorite and andesite. Their density varies between 2.75 to 2.8.
Ultra-basic igneous rock
When the amount of silica in an igneous rock is less than 45 percent and the density vary between 2.8 to 3.4, then that rock is called ultra-basic igneous rock e.g., peridotite.
3. Classification based on the texture of grains or crystal
The texture of grains in igneous rock depends on the following three factors
- Source region of origin of lava and magma and place of their cooling and solidification.
- The rate of cooling and solidification of lava and magma
- The quantity of water and gas with lavas and magmas.
If the lava and magma cool gradually, then the grain developed very well, but when lava and magma cool quickly, at a faster rate, then the grains are not well developed. The rate of cooling of lavas and magma depends on numerous factors viz. when the magma cooled deep inside the earth, the rate of cooling is slow because of the high temperature prevailing there, and hence coarse grains developed there.
On the contrary, when the lava cooled above the earth’s surface, the rate of cooling is very fast because of the low temperature outside and hence no grain formed or if formed they are so small that they cannot be seen without a microscope. Secondly, if the amount of water vapor and gas in magma and lava is present in higher proportion, then the magma and lava cool and solidify slowly, as a result of which the large grains are formed.
Based on the texture of grains, igneous rocks are divided into the following sub-types:
Phaneritic or large-grained igneous rock
When the particles of igneous rock are so large that they can be seen even without the help of a microscope, then that rock is called ‘large-grained phaneritic igneous rock’ e.g., granite and diorite. The size of the grain ranges between a few mm to one inch. granite and diorite are the main ones.
Pegmatitic or igneous rocks with very large particles
The rocks contain very large grains or crystals. Worcester has included those rocks of the ‘Granite Group’ in this category, in which feldspar and quartz are found in abundant.
Aphanitic or fine-grained rock
Although grains are found in this type of igneous rock, but they are so fine and microscopic that they cannot be seen without the help of a microscope e.g., basalt, felsite, and rocks found in dyke, sill, and stock.
Glassy (without grain) igneous rock
In this type of rock, there is a general absence of grains, e.g., obsidian, pitchstone, pumice, and perlite.
Porphyritic or mixed-grained rock
When a rock contains large and small grains in mixed form, then that rock is called porphyritic igneous rock.
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