Faulting – Rift Valley and Block Mountains


Due to the earth movements, movements at plate boundaries place stress and tension on the crustal rocks. These tensional or compressional forces result in parts of the crusts to be fractured. Hence, a fault is a fracture/cracked in the rocks which are being displaced.

We usually associate faults with earthquakes.
But did you know that faults could also form mountains?

Features formed/produced as a result of faulting

  • Block Mountains
  • Rift Valleys

Type of Forces

Tensional Forces occurs when the earth crusts are forced to move apart from each other.  Compressional Forces occurs when the earth crusts are forced to move towards each other.

Block Mountains

Block mountains also known as Horst. It is formed when the tensional forces causes the plates on

the earth crusts to move apart. This movement causes the rocks to be stretched. Temperatures are low and the rocks are brittle. Instead of folding, they break into large blocks.

Due to the compressional forces, the middle section of the rocks are uplifted, resulting it to be higher than the surrounding land.

An example of the block mountain is the Vosges in France and the Black Forest in Germany which are separated by the Rhine Valley. These block mountains are formed from the divergence of the Eurasian Plate and the North American plate.
These mountains have sharp peaks. This is because of the vertical movement and tilting of the rocks. Some of the mountains in this range rise over 12,000 feet. Most have a build up of debris at the base on the steepest side. This is because of erosion. The opposite sides have a more gradual slope.

Rift Valleys

Rift valleys are long, narrow depressions on the earth surface bounded by more or less parallel faults. A rift valley is also known as a graben. Rift valleys are thought to have been developed either from the action of tensional forces in the crust or from the action of compressional forces.

Scientists sometimes argue whether both block mountains (horsts) and rift valleys (grabens) move, or if only one part moves. Many feel that the rift valley (graben) is the part that moves, due to the force of gravity. The block mountain (horst) is left standing higher above the sunken graben. This is difficult to determine.

Many fault-block mountain ranges are a horst and graben system. In these systems, the mountains (horsts) are separated by deep valleys (grabens). Sometimes the valleys are actually wide basins.

2 ways for rift valleys to occur

1. Tensional Forces

a) Tension forces act on the layers of rock.

b) Gradually, two parallel faults appar and the middle block of rocks begin to sink.

c) As a result, it forms the rift valley. The land on both sides of the rift valley remains in place.

2. Compressional Forces

a) Layers of rocks are subjected to compressional forces.

b) Faults developed and the outer blocks move upwards. (Block mountains are formed)

c) The middle block stays in the place and a rift valley form.

Benefits of the Rift Valley to people of East Africa

  • Beautiful scenery to attract tourist arrivals (tourism)
  • Lakes in the rift valley provide fish (Fishing)
  • Provides water for domestic and agricultural use
  • Lakes provide water for transport (water transportation)
  • Forests on the slopes are sources of timber
  • Gentle slopes are used for crop farming and settlement due to the fertile soils
  • Rift valley lakes are used for mining
  • Wildlife conservation

Problems faced by the people living in the Rift valley

  • High temperatures results in shortage of water
  • Earthquakes may destroy property
  • Little rainfall or drought in the rain shadow areas
  • Poor means of transport and communication because of the steep escarpments
  • Salty lakes because of high temperatures and high evaporation rates
  • Soil erosion and land slides especially on the steep slopes


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