What Are Faults?

Block Mountains are formed due to faults in the crust, which are planes where rocks have moved past each other, generating “block mountains“. A mountain can be formed when the rocks on one side of a fault rise relative to the other. Block mountains, or horsts, are raised blocks. Block mountains include the Great African Rift Valley (valley floor is graben), the Rhine Valley (graben) in Germany, the Vosges mountain range in France, the Sierra Nevada in the United States, and the Harz mountains in Germany.

There are two main types of faults that can result in the formation of block mountains: 1) Normal faults , 2) Reverse faults

Normal faults

Normal faults occur when tensional forces pull the Earth’s crust apart, causing one side of the fault to drop down relative to the other side.
This creates a steep slope on one side of the block and a gentler slope on the other side.
Normal faults are the most common type of fault associated with block mountain formation.

Reverse faults

Reverse faults occur when compressional forces push the Earth’s crust together, causing one side of the fault to move up relative to the other side.
This can also result in the formation of block mountains, with a steep slope on one side and a gentler slope on the other side.
Reverse faults are less common than normal faults for block mountain formation, but they can still play a role in their formation.

Types of Faults :

Strike-slip fault

In a strike-slip fault (also known as transcurrent fault), the plane of the fault is usually near vertical, and the blocks move laterally either left or right with very little vertical motion (the displacement of the block is horizontal).

Transform fault

A special class of strike-slip fault is the transform fault or transform boundary when it forms a plate boundary.
A transform fault is the only type of strike-slip fault that is classified as a plate boundary.
Most of these faults are hidden in the deep ocean, where they offset divergent boundaries in short zigzags resulting from seafloor spreading.
They are less common within the continental lithosphere. The best example is the Dead Sea transform fault.
The transformed boundary ends abruptly and is connected to another transform, a spreading ridge, or a subduction zone.

Dip slip faults

Dip slip faults can be either normal or reverse.
In a normal fault, the hanging wall (displaced block of crust) moves downward, relative to the footwall (stationary block). In a reverse fault (thrust fault) the hanging wall moves upwards.
Reverse faults occur due to compressive forces whereas normal faults occur due to tensile forces.
A downthrown block between two normal faults is a graben.
An upthrown block between two normal faults is a horst.
Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary.
Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary.

Rift Valley system

Tension causes the central portion to be let down between two adjacent fault blocks forming a graben or rift valley, which will have steep walls.
The East African Rift Valley system is the best example.
In general, large-scale block mountains and rift valleys are due to tension rather than compression.