FORCES EFFECTING THE EARTH MOVEMENTS, Endogenetic forces, Geometry of Folds, Types of Folds, Exogenetic forces, Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS
FORCES EFFECTING THE EARTH MOVEMENTS
The forces which affect the earth’s movement are involved in the creation, destruction, recreation and maintenance of various types of relief features of varying magnitudes. On the basis of origin these forces are divided into (i) endogenetic forces and (ii) exogenetic forces. While endogenetic forces create relief features on the earth’s surface, the exogenetic forces through their erosional and depositional activities destroy them and help in the planation process.
Term endogenic refers to internal processes and phenomena that occur beneath the Earth’s surface. These forces are divided into sudden forces and diastrophic forces.
(a) Sudden forces: events like earthquake and volcanic eruption occur suddenly and the resultant forces work very quickly. They are constructive forces as they create cones, lakes, plateaus, lava plains etc.
(b) Diastrophic forces: they include both vertical and horizontal movements.
(i) Vertical movement: they include emergence and subsidence of land masses. Emergence may occur due to upliftment of the whole continent or part there of or upliftment of coastal land of the continents. Submergence may occur when the land near the sea coast subsides below sea level.
(ii) Horizontal movement: these forces work into two ways a) In opposite direction – this includes tensional or divergent forces which create faults, rupture, fracture, cracks etc.
- b) Towards each other – This includes compressional or convergent forces which create folding, warping etc.
It is the process whereby the rock strata are bent into a series of arches (anticlines) and toughs (syncline) as a result of horizontal earth movements which cause compression within the crust. The anticlines of the folds generally form the mountains and the adjacent synclines form the valleys. Most of the mountain ranges of the world consist of Fold Mountains e.g. the Alps, the Andes, the Rockies and the Himalayas.
Geometry of Folds – Folds are described by their form and orientation. The sides of a fold are called limbs. The limbs intersect at the tightest part of the fold, called the hinge. A line connecting all points on the hinge is called the fold axis. In the diagrams above, the fold axes are horizontal, but if the fold axis is not horizontal the fold is called a plunging fold and the angle that the fold axis makes with a horizontal line is called the plunge of the fold. An imaginary plane, that includes the fold axis and divides the fold as symmetrically as possible, is called the axial plane of the fold.
Types of Folds
Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length are termed symmetrical, and those with highly unequal limbs are asymmetrical. Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on. Other kinds of folds are:
- Anticlines – Up folds. When the upper part of the fold is eroded away, the oldest rocks are in the center of the fold, and the youngest rocks are on each side. Also, the rocks dip (or slope) away from the central axis of the fold.
- Synclines – Down folds. When the upper part of the fold is eroded away, the youngest rocks are in the center of the fold, and the oldest rocks are on each side. Also, the rocks dip toward the central axis of the fold.
- Monoclines – a bend in otherwise horizontal strata.
- Isoclinal folds have undergone greater stress that has compressed the limbs of the folds tightly together.
- The limbs of overturned folds dip in the same direction, indicating that the upper part of the fold has overridden the lower part. Depending on where the exposure is in an overturned fold, the oldest strata might actually be on top of the sequence and be misinterpreted as the youngest rock unit.
- Recumbent folds, found in areas of the greatest tectonic stress, are folds that are so overturned that the limbs are essentially horizontal and parallel.
- Chevron: angular fold with straight limbs and small hinges.
Faulting: It is the process by which the tensional earth movements under the effect of considerable pressure create a fracture in the earth’s crust. Faulting gives rise to relief features like block mountains (horsts), rift valleys, etc. A rift valley is a long, relatively narrow depression formed by the sinking of a block of land between two more or less parallel faults. Examples: East African rift valley, Narmada and Tapti valleys.
Columns of faulting bring four distinguishable landforms as:
- Rift valley: it is the result of the subsidence of the central column. When the central column of the two fault-lines subsides, the rift valley is made. “Damodar valley” is such an example.
- Ramp valley: when both side columns are raised and the central column is standstill, then the made landform is ramp valley. “Brahmaputra” river passes through a ramp valley.
- Block Mountain: this is the result of the subsidence of side column. The central column gets steep rim along the fault scarps and the raised landform is Block Mountain. “Satpura hills” of India is such an example.
- Horst: Horst is a similar landform but is supposed to be due to upward force from beneath the central column. Side-columns are standstill. “Harz Mountain” of Germany is an example.
Exogenic forces refer to external processes and phenomena that occur on or above the Earth’s surface. Comet and meteoroid impacts, the tidal force of the moon and sun’s radiations are all exogenic. Weathering effects and erosion are also exogenic processes. They also affect the planation processes. These are also called denudational or destructive forces. The erosional process is affected by running water, ground water, glaciers, sea waves etc. These processes form erosional and depositional land forms.