The slow movements can further be divided into vertical and horizontal movements on the basis of the uplift or subsidence of a part of the Earth’s surface.
Vertical movements originate from the centre of the earth and affect its surface. Consequently large scale uplift or subsidence of a part of the earth’s surface takes place. These movements are slow and wide spread and do not bring changes in the horizontal rock strata.
These movements are mainly associated with the formations of continents and plateaus, hence these are also known as continent building or plateau building movements.
These movements are also called epeirogenetic movements. ‘Epeiros’ in Greek language means ‘continent’.
The sedimentary rocks are deposited and formed in the oceans and seas. The presence of these sedimentary rocks is wide-spread in continents. This clearly shows that these were uplifted or raised to form continents.
Contrary to the above, there are countless evidences of submerged buildings, river -valleys and cities due to subsidence into the sea. Some of such examples include the submerged ancient buildings in Mediterranean in its Crete Island and the ancient city of Dwaraka in Saurashtra, India. These changes clearly point out the downward movement of the Earth’s surface.
There are forces which act on the earth’s crust from side to side i.e. horizontally or tangentially. Naturally, they cause a lot of disruption in the horizontal layer of strata as they do involve a good deal of compression and tension of the pre-existing rocks since these forces act horizontally or tangentially to the earth’s spherical surface. These are known as horizontal or tangential movements.
We can divide them into two types –
- (i) Forces of compression
- (ii) Forces of tension
Forces of compression : involve pushing of the rock strata against a hard plane from one side or from both sides.
To understand their working, let us take a piece of cloth and spread it on the table. Push the cloth with your both hands towards its centre, it will form wrinkles rising into up and down folds. Likewise rock strata also bend in the same fashion when forces of compression act on them from opposite directions. In this way, the compressional forces lead to the bending of rock layers and thus lead to the formation of fold mountains.
In them the rock strata primarily of sedimentary rocks get folded, into wave like structure. This process of bending, sometimes warping and twisting of rock strata is referred to as their folding. The upfolds are called anticlines and downfold are called synclines.
When folding takes place on a gigantic scale, it represents the mountain building process. Most of the great mountain chains of the world viz, the Himalaya, the Rockies, the Andes, the Alps and others of this sort have been formed by compressional forces resulting in mountain building on a large scale. These are also called Orogenetic Movements.
Forces of tension : are produced when these forces are working horizontally in opposite directions i.e, away from a given plane or point.
Under the operation of intense tensional forces, the rock strata is broken or fractured. As a result cracks and fractures develop.
The displacement of rocks upward or downward from their original position along such a fracture is termed as faulting. The line along which displacement of the fractured rock strata takes place is called the fault line. Like wise the plane along which displacement of rock strata takes place is known as fault plane.
Forces of compression give rise to the operation of the forces of tension. Thus faults are closely related to the formation and occurrence of folds. It implies that folding generally leads to or is accompanied by fracturing and faulting in rock strata.
Faulting results in the formation of well known relief features such as rift valleys and the block mountains. A rift valley is formed by sinking of rock strata lying between two almost parallel faults.
The classical examples of rift valleys in the world include the Midland Valley of Scotland, the Rhine Valley, the Valley of Nile, the Dead Sea basin and the Great Rift Valley of East Africa comprising few lakes of this region.
Some geographers are of the opinion that the Narmada and Tapti valleys are also rift valleys. The coal deposits of the Damodar valley are said to be originally laid in a synclinal trough resembling a rift valley.
A rift valley is a trough with steep parallel walls along the fault lines. Such a valley is also called a graben. A rift valley may also be formed by upliftment of two blocks along the fault line. These uplifted blocks are called horsts or block mountains. The well known examples of horsts are the Vosgesand the Black forest mountains on both sides of Rhine rift valley and the Plateaus of Palestine and Trans Jordan.
The escarpments are the characteristic features of rift valleys and horsts. They are very steep or have highly precipitous slopes in a continuous line facing one direction. The escarpments of Western Ghats ones looking the Arabian Sea are thought to be the result of faulting. The escarpments of Vindhyachal Mountain are also ascribed to the faulting and formation of narrow Narmada Valley.
Large scale uplift or subsidence create continents, plateaus and oceans.
Bibliography : NIOS Geography Book