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After reading this article you will learn about:- 1. Formation of Igneous Rocks 2. Classification of Igneous Rocks 3. Forms 4. Textures 5. Engineering Uses.
Formation of Igneous Rocks:
Igneous rocks are formed by the solidification of magma or lava. At about a depth of 40 kilometres in the earth, the rocks are believed to be in a molten state. Due to enormous temperature and pressure, this molten material always has a tendency to penetrate into the cracks and lines of weakness of the thin solid crust of the earth, and thus tries to make its way on the surface of the earth.
Sometimes, the magma, during its endeavour to come out on the earth’s surface, is successful in coming out. But sometimes the magma is held up by strong and massive rock masses, below the earth’s surface, during its upward journey.
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If the magma is successful in coming out on the earth’s surface, it is erupted out from a weak point with a great force and spreads out on the surface of the earth. This spread out lava solidifies due to cool temperature of the atmosphere, but if the magma is held up below the earth’s surface, during upward journey, it is then unable to descend.
This magma then slowly cools down, and ultimately solidifies. The process of solidification of the magma or lava gives birth to igneous rocks.
Classification of Igneous Rocks:
1. Igneous rocks are classified on the basis of Silica into four types.
They are:
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(a) Acidic igneous rocks:
These are those rocks in which the Silica presence is more than 66%.
Example:
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Granite, Pegmatite, and Rhyolite.
(b) Intermediate igneous rocks:
These are those rocks in which the Silica presence is in between 55 and 66%.
Example:
Syenite, Trachyte.
(c) Basic igneous rocks:
These are those rocks in which the Silica presence is in between 44 and 55%.
Example:
Basalt, Dolerite, and Gabbro.
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(d) Ultrabasic igneous rocks:
These are those rocks in which the Silica presence is less than 44%.
Example:
Olivine basalt.
2. Igneous rocks are classified on the basis of mode of origin into three types.
They are:
(a) Plutonic:
These are those rocks that are formed beneath the earth’s surface at a greater depth.
Example:
Granite, Syenite, Diorite, and Gabbro.
(b) Hypobyssal:
These are those rocks that are formed beneath the earth’s surface at a lesser depth.
Example:
Pegmatite, Porphyries, and Dolerite.
(c) Volcanic:
These are those rocks that are formed in the surface of the earth.
Example:
Rhyolite, Trachyte, Andesite, and Basalt.
Forms of Igneous Rocks:
Magma i.e., the liquid rocks, has a tendency to penetrate into the cracks and lines of weakness of the thin solid crust of the earth. It tries to make its way out on the surface of the earth and consolidates in different shapes, known as forms of igneous rocks.
The forms of igneous rocks in general may be studied under the following headings:
1. Concordant bodies:
Sometimes the magma during its upward journey does not possess enough energy to push, drag or cut through the existing rock.
Example:
Sill, Phacolith, Laccolith, and Lopolith.
2. Discordant bodies:
Sometimes the magma, during its upward journey, possesses enough energy to push, drag or cut through the existing rock.
Example:
Dyke, Batholiths.
The various forms in which igneous bodies occur are as follows:
3. Extrusive bodies:
Lava flows:
The volcanic igneous rocks occur as lava flows. They are tabular in shape; in thickness they range from few centimetres to many hundreds of metres. Lava flows are formed when lava breaks through the Earth’s crust along fissures. Central India is an example of great lava flows.
4. Intrusive bodies:
(a) Batholiths:
These are large intrusive igneous rocks, which are general granite in components. In plain view their outline is irregular and the area of outcrop exceeds 100 sq. kms most Batholiths increase in size with depth and they are thought to be bottomless as shown in Fig. 3.2.
(b) Stocks and Boss:
A stock is a small Batholith, in which the area of outcrop is less than 100 sq. kms. A stock having a circular outcrop is called a Boss.
(c) Lopolith:
It is a lenticular igneous body, which is bent or sagged downwards into a basin like shape as shown in Fig. 3.3.
(d) Laccolith:
Laccoliths are piano convex intrusive igneous bodies, which cause the overlying beds to arch in the form of a dome as shown in Fig. 3.4.
A laccolith may be 2 to 3 kms in diameter and several 100 metres in thickness. It differs from batholiths in being much smaller and having a known floor.
(e) Phacolith:
Phacoliths are intrusive igneous rocks, which occupy crests and troughs of folded strata as shown in the Fig. 3.5.
(f) Sill:
A sill is a sheet-like igneous body, which runs parallel to the bedding planes of the enclosing rocks. A sill may be horizontal, inclined or vertical depending upon the attitude of the strata in which they are intruded. A sill varies in thickness from a few centimetres to several kilometres to their length along the beds.
(g) Dyke:
Vertical wall-like structure cutting the bedding planes. Dykes probably represent a crustal fracture into which the magma was injected.
(h) Volcanic neck:
A volcanic neck or a plug is a vertical intrusion of igneous mass, which has a roughly oval or circular section as shown in Fig. 3.8.
Textures in Igneous Rocks:
The term ‘texture’ is defined as the mutual relationship among the minerals and the glassy materials in a rock. Textures are best observed under a microscope, because many textures are microscopic.
The following textures of igneous rocks are important from subject point of view:
1. Equigranular:
In this texture the minerals of the rock are more or less equal in size.
2. Inequigranular:
The mineral of the rock will be of different sizes.
Three important textures of inequigranular texture are:
(a) Porphyritic textures,
(b) Poikitic texture, and
(c) Ophitic texture.
(a) Porphyritic texture:
Bigger grains surrounded by smaller grains.
Examples:
Granite porphyry, Syenite porphyry and Diorite porphyry.
3. Intergrowth textures or Pegmatitic textures or Graphic textures:
Quartz occurs within the feldspar.
Example:
Pegmatite
4. Trachytic texture or Directive texture
Textures formed by flow of magma during crystallization process. In magma, flowing small crystals like microlites and crystallites and well-developed crystals may arrange themselves in parallel line.
Examples:
Trachytes and Andesite.
5. Vesicular texture:
The lava contains hot gas. This gas begins to escape when the pressure is reduced. The reduction of pressure takes place when the lava reaches the surface of the earth. If the lava contains numerous cavities with irregular shapes then it is known as vesicular textures.
Example:
Vesicular basalt.
6. Amygdaloidal texture:
If the cavities are filled by secondary minerals they are called Amygdales because they resemble the shape of almonds. Lava containing Amygdales (calcite, zeolite, silica etc.) are said to show Amygdaloidal structure.
Example:
Amygdaloidal basalt.
Engineering Uses of Igneous Rocks:
Many igneous rocks, especially plutonic, have high crushing and shearing strengths and are thus considered to be the most satisfactory rocks for all types of engineering purposes. Basalt and dark coloured rocks are largely used as road metals and concrete aggregates.
Almost all igneous rocks being crystalline compact and impervious are always considered to be safe for use as foundation rocks, abutment of dams, walls or roofs of tunnels.