Sometimes, when the crustal rocks are subjected to horizontal compressional pressure, they do not get folded. Instead, they develop fractures or cracks along the line of weakness. These lines of fracture are known as faults. The movement of the part of the Earth’s crust along the line or fault is known as faulting. Block mountains are formed because of faulting. There are three main kinds of faults. They are
Types of Faults
Normal faults: Normal fault is a simple fault where the blocks of the rocks move up or down. In a normal fault, a piece of land may slide down along the fault plane.
Reverse fault: Because of compressional forces acting from opposite directions, a block of rock is forced up and over other rock against the slope of the fault plane.
Tear fault: When the blocks on either side of the fault are displaced horizontally or sideways, tear faults take place. Tear faults generally occur during earthquakes.
Main Features of Faulting
- Usually, faults do not occur singly but run parallel to each other.
- The elevated or subsided rocks are known as fault blocks.
- A piece of block which is elevated is called horst.
- A depressed piece of land between two elevated blocks is called a graben or rift valley.
Landforms Formed by Faulting
Rift valleys and block mountains are two main landforms formed due to faulting.
A rift valley is formed when the middle portion of land between two faults subsides. It is also formed when the middle land remains stable but the two side blocks rise upwards.
The East African Rift Valley is the longest rift valley in the world. Rift valleys have flat bottoms and steep sides.
Many rift valleys also become lakes such as Lake Nyasa in Tanzania. In India, the Damodar Valley and Tapi Valley are examples of faulting.
- The mechanism of the formation of block mountains is similar to a rift valley.
- Because of tensional features, when a piece of land between two parallel folds subsides, a rift valley is formed.
- However, when a piece of land rises, a block mountain is formed. In the above figure, while a graben represents a rift valley, an elevated piece of land known as horst denotes a block mountain.
- Some examples of block mountains are the Vogues Mountains in France and the Black Forest Mountains in Germany.
- According to plate tectonic theory, the outer shell of the Earth is made of several lithospheric plates.
- Each plate moves slowly in the asthenosphere which is a semi-molten sphere existing under the surface of the Earth.
- Each plate moves as a single independent body.
- There are constructive plate margins, destructive plate margins and conservative plate margins.
- Six major tectonic plates are the Pacific plate, Eurasian plate, American plate, African plate, Indian plate and Antarctic plate.
Also, Read Folding and Faulting