Earth Movements

The face of the earth is constantly being reshaped by internal forces, such as earth movements, volcanoesr earth­quakes and landslides, and external forces, such as river, rain water, glacier, wind and sea waves. In this chapter, we will discuss landforms formed due to activities of earth's internal forces.

EARTH MOVEMENTS
The powerful internal forces operating from within the crust are called earth movements. Such movements may be slow and sudden. Earth movements are classified into tectonic movements, vertical movements and horizontal movements.

Tectonic Movements The earth movements which bring about vast changes on the earth's surface are called tectonic movements (see box). The concentration of great internal forces within the earth raises local areas upwards or cause them sinking downwards. Tectonic movements are divided into sudden movements and slow or secular movements.
Sudden Movements These are commonly noticed during an earthquake.

Slow or Secular Movements These movements continue much longer as compared to our life span. The periodical advance and retreat of continental glaciers and ice caps because of global changes in climate are said to have caused them. These movements are relative to each other, the land advancing against sea is termed a negative movement and the sea advancing on land is known as positive movement.

Vertical Movements Responsible for a rise or a fall of a portion of the earth surface, vertical movements of the earth do not disturb the horizontality of the strata as they were originally laid down. Vertical movements cause uplift and subsidence. When a part of the earth's crust rises in relation to surrounding portions, it is known as uplift. Conversely, when the sinking of a part of the earth's crust, relative to the surrounding portions takes place, it is called subsidence. These earth movements on a large scale build up continents and plateaus.

Horizontal Movements These movements are respon­sible for greatly disturbing the horizontal arrangement of layers of rock. They involve both the forces of compression and tension. Tension is the puIling force. Compression is a force that pushes against a body from directly opposite sides. The tension is responsible for breaking of rock layers with their subsequent sliding or displacement. It is termed as the formation of a 'fault'. The compression leads to the bending of horizontal layers of deep sediments into a shape known as a 'fold'. These two phenomena of folding and faulting lead to the building up of mountains.