The igneous rocks are also known as primary rocks as they form the basis of the formation of other types of bricks. They are formed because of the cooling, solidification, and crystallization of molten material known as magma. These rocks mainly consist of alumino-silicate minerals such as quartz and feldspar.
Characteristics of Igneous Rocks
- Hot lave pours out at the time of volcanic eruptions and cools down, later on, forming rocks.
- The molten materials are known as magma, sometimes cool down beneath the earth’s crust, again forming rocks.
- When the earth’s surface first became solid after it cooled down from its hot liquid state, the original rocks of the earth’s crust were formed. They are the Primary Igneous rocks, which are usually not found today.
- They are generally harder and more granular.
- There are no layers in these rocks.
- Fossils are not found in these rocks.
- The formation of these rocks takes place beneath and above the surface of the earth.
- Rocks formed by the cooling of molten matter beneath the earth’s surface are called intrusive igneous rocks. “Granite’ and ‘Gabbro’ are the main examples of these rocks.
- The intrusive rocks are thus crystalline rocks.
- Sometimes, the molten matter oozes out through cracks in the earth’s crust and spreads on the surface, forming extrusive igneous rocks.
- Gabbro, Obsidian, Basalt, etc are examples of extrusive igneous rocks.
- A very large area of the Deccan Plateau consists of basalt rocks.
- These rocks contain silica from 40 to 80%, others are felspar, magnesium, and iron, etc
- Other examples of Igneous rocks are-Granite, Pumic stone, Basalt, and Gabbro.
Classification of Igneous Rocks
Extrusive Igneous Rocks
- They are formed by cooling molten magma on the surface of the Earth. The molten magma comes on to the Earth’s surface through cracks, fissures and volcanic eruption.
- They are also known as volcanic rocks. Because these rocks solidify at a faster pace, they are smooth, crystalline and fine grained.
- Basalt is a common example of extrusive igneous rock.
Intrusive Igneous Rocks
- They are formed when magma cools and solidifies below the surface of the Earth.
- Because these rocks cool down slowly, they have coarse texture with large crystals. Dolomite and granite are some examples of intrusive igneous rocks.
There are different types of intrusive igneous rocks such as batholiths, laccoliths, dykes, and sills.
Batholiths: Batholiths are formed because of the cooling and solidification of magma below the surface of the Earth. They are generally dome-shaped. They have been exposed to the surface of the Earth because of the action of the agents of erosion.
Laccoliths: When magma is not fully able to reach the crust of the Earth, it solidifies just below the crust. The upper layers of laccoliths are dome-shaped, but the bottom is almost flat.
Sills: When magma is forced to rise, it pushes itself between two layers of rocks and solidifies. The sills are horizontal rocks.
Dykes: When the magma is forced in the upward direction, it fills the cracks and fissures in the existing rocks. It solidifies in these cracks forming dykes. Dykes are often vertical or slanting in position and are often an offshoot of rock.
Necks: When the passage of an extinct volcano is filled with magma, it solidifies. This is known as the volcanic neck or plug.
On the basis of chemical composition
On the basis of chemical composition, igneous rocks are classified into the following:
Acid Igneous Rocks: These rocks have between 65% and 85% of silica but generally lack in iron and magnesium and so are light. Example: Granite
Basic Igneous Rocks: These rocks are generally heavy and dark. Example: Basalt
Distribution of Folded and Faulted Landforms – The fold mountain system arranged in long lines is found almost in every continent. Most of them are interlinked arcs extending through oceans.
Horizontal movements take place because of compressional or tensional forces. They are responsible for the formation of fold mountains. When compressional forces act from two opposite directions, the rocks on the crust of the Earth bend and form arches, troughs, or mountains.
Vertical movements are associated with plate tectonics. They cause the upliftment (rise) or subsidence (sinking) of the land. Because vertical movements are powerful, they are also known as continent building movements