Vertical and Horizontal Movements Part – 2

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Vertical and Horizontal Movements

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: 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. Besides, these movements are also termed as epeirogenetic movements. Epeiros in Greek language means continent. We have studied that sedimentary rocks are deposited and formed in the oceans and seas. The presence of these sedimentary rocks is widespread in the continents. This clearly shows that these were uplifted or raised to form continents.

On the other hand, there are countless evidences of submerged buildings, river -valleys and cities due to subsidence into the sea. For example, the submerged ancient buildings in Mediterranean in its Crete Island and the ancient city of Dwarka in Saurashtra, India.

Horizontal Movements: There are the forces which act on the earth’s crust from side to side i.e. horizontally or tangentially. They cause a lot of disruption in the horizontal layer of strata as they do involve the forces of compression and tension of the pre-existing rocks since these forces act horizontally or tangentially to the earth’s spherical surface.

We can divide them into two types:

Forces of Compression: It involves pushing of the rock strata against a hard plane from one side or from both sides. The compressional forces lead to the bending of rock layers and thus lead to the formation of fold mountains. 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 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 have been formed by compressional forces resulting in mountain building on a large scale. These are also known as orogenetic movements.

The Earth's Crust Before (A) and After (B) Folding

The Earth's Crust Before (A) and After (B) Folding

The Earth's Crust Before (A) and After (B) Folding

Forces of Tension: They 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 are broken or fractured. As a result, cracks or 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 termed as the fault line. The plane along which displacement of rock strata takes place is known as fault plane.

Fault

Fault

Fault

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 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.

Rift Valley and Block Mountain

Rift Valley and Block Mountain

Rift Valley and Block Mountain

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 examples of horsts are the Vosges and 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.