Endogenic ForcesUPSC Past papers
The forces acting on the earth’s surface leads to the creation, destruction, recreation and maintenance of geo-materials on the earth’s crust. The forces are divided as Endo-genetic and Exo-genetic forces.
– Various agents of denudation like running water, rain, frost, sun, wind, glaciers and waves have constantly reshaped the face of the earth making it very varied and diverse. However, the plateau and plains have only modified these structures which have been modeled by the movements of the earth’s crust.
– Since the down of the geological time there have been about nine orogenic or mountain building movements, folding and fracturing the earth’s crust. Some of them occurred in Pre-Cambrian times between 600 – 3500 Million years ago.
– The three most recent orogenies are the:
• Caledonian (about 320 million years ago) raised the mountains of Scandinavia and Scotland in North America. These mountain ranges have been worn down by the agents of denudation and are no longer exhibit the striking forms as existed earlier.
• Hercynian (about 240 million years ago) raised the Ural Mountains, the Pennines and Welsh Highlands in Britain, the Harz Mountains in Germany, the Appalachians in America and the high plateaux of Siberia and China. These mountains have also been reduced in size by the various sculpturing forces.
• Alpine (about 30 million years ago) the recent and major orogenic movements of the earth, raised mountain ranges such as Alps, Himalayas, Andes and Rockies which are the young loftiest mountains and are most imposing.
– The above developments of the landforms are the results of Endo-genetic andExo-genetic forces.
• The forces which originate inside the earth due to volcanism, diastrophism and massive crustal rearrangement are known as Endo-genetic Forces.
• The external forces which bring changes in the landforms through the process of erosion, mass wasting and weathering are known as Exo-genetic Forces.
The forces coming from within the earth are called as endo-genetic forces which cause two types of movements in the earth, viz:
a) Horizontal Movements: Side to side movements of the earth’s crust (Horizontal and Tangential), causes lot of disruptions in the horizontal layer of strata as they involve compression and tension of the pre-existing rocks.
b) Vertical Movements: Slow and widespread bringing changes in the horizontal rock strata. Originate from the centre of the earth causing large scale upliftment or subsidence of a part of the earth’s crust forming continents and plateaus. These movements motored by the endo-genetic forces introduce various types of vertical irregularities which give birth to numerous varieties of relief features on the earth’s surface, eg., mountains, plateaus, plains, lakes, faults, folds, etc.
The energies in the endo-genetic forces are mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth. This energy is due to geothermal gradients and heat flow from within induces diastrophism and volcanism in the lithosphere.
The endo-genetic forces and movements are divided, on the basis of intensity, into two major categories:
a) Diastrophic Forces: It is a general term applied to slow bending, folding, warping and fracturing of the earth’s crust which becomes discernible after thousands of years. From the point of view of areal distribution, Diastrophic forces can further be divided into two sub-groups:
I. Epeirogenetic Movements: A slow process causing movements of the earth’s crust resulting into the displacements of continents and ocean basins. It acts along the radius of the earth and hence also known as radial movements.
• Their direction may be towards (subsidence) or away (uplift) from the center. The formation of mid-oceanic ridges, ocean trenches, the Great Rift Valley of Africa can be attributed to these movements.
• Some of the beaches which have been elevated (uplifted) as much as 15 m to 30 m above the present sea level such as Kathiawar, Nellore and Thirunelveli coasts. Several places which were the flourishing sea ports about 1000 to 2000 years ago are now a few miles inland such as Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Thirunelveli.
• An earthquake in 1819 submerged a part of Rann of Kachchh. Presence of peat and lignite beds below the sea level in Thirunelveli and the Sundarbans is an example of subsidence. The Andamans and Nicobars have been isolated from the Arakan coast by submergence of the intervening land.
• Trees have been found embedded in mud about 4 m below the low water mark on the east side of Bombay Island and on the Thirunelveli coast in Tamil Nadu. A part of the former town of Mahabalipuram near Chennai (Madras) and a large part of Gulf of Mannar and Palk Strait (shallow zone) have been submerged in geologically recent times.
II. Orogenetic Movements: It involves mountain building through severe folding and affecting long and narrow belts of the earth’s crust acting tangentially to the earth surface, as in plate tectonics. It can further be divided into Compressional and Tensional.
• Compressional forces result into folding (folds), crustal bending, local rise and subsidence because this type of force acts towards a point from two or more directions.
• Tensional forces create cracks, ruptures, features and faults, since this type of forces acts away from a point in two directions.
b) Sudden forces: These are the result of long period preparation deep within the earth. Only their cumulative effects on the earth’s surface are quick and sudden. Geologically, these sudden forces are termed as ‘constructive forces’ because these create certain relief features on the earth’s surface.
STRUCTURES PRODUCED BY ENDO-GENETIC FORCES
Endo-genetic forces create deformation in the earth’s crust. This distortion takes place more in sedimentary and metamorphic rocks than in igneous rocks as the former rocks are usually horizontal, a little deformation becomes easily discernible. The structures are discussed as follows:
(A) Folds: Fold is a bending of the Earth’s crust owing to compressional forces.
– According to shape, the folds are of many types:
o Symmetrical Folds—These are ordinary folds. The limbs of the folds are equally inclined on either side.
o Asymmetrical Fold—One of the limbs is more inclined than the other.
o Monoclinal Fold—In this fold, one limb makes a right angle with the surface but the other limb is ordinarily inclined.
o Isoclinal Fold—The two limbs are so much inclined in such a way that they appear equally inclined and parallel to each other.
o Recumbent Fold—In this fold the two limbs are so much inclined that they become horizontal.
o Overturned Fold—In this fold one limb is overturned over the other limb. The difference between the overturned and recumbent folds is that the overturned limbs are not horizontal like those of recumbent fold.
o Plunging Fold—If the axis of the fold is not parallel to the horizontal but makes an angle with it, it is known as Plunging Fold.
o Fan Fold—It is a great anticline which has many small anticlines and synclines. It is also known as Anticlinorium. A great syncline having many small anticlines and synclines is called Synclinorium.
o Open Fold—If the angle between the limbs of a fold is obtuse, the fold is called Open Fold.
o Closed Fold—If the angle between the limbs of a fold is acute, it is called Closed Fold.
(B) Fault: Fault is a rupture and fracture of rocks strata due to strain. Faults have also been defined as a surface along which a rock-body has broken and been displaced.
– Some of the important faults are as under.
• Normal Fault (Tension): The faults having mainly vertical movement are called normal faults. A normal faults results in a seep, straight cliff like feature.
• Reverse Fault (Compression): One side of the fault moves upward vertically in comparison to the other side.
• Strike-slip fault: A strike-slip fault is a fault on which the two blocks slide past one another. These faults are identified as either right-lateral or left lateral depending on whether the displacement of the far block is to the right or the left when viewed from either side. The San Andreas Fault in California is an example of a right lateral fault.
Strike-slip faults are either right-lateral or left-lateral. That means someone standing near the fault trace and looking across it would see the far side move to the right or to the left, respectively. The one in the picture is left-lateral.
Various types of landforms are being produced by faults:
• Rift Valley or Graben: When a block between two normal faults is depressed, the valley formed in the faults is known as Rift Valley or Graben.
• Horst or Block Mountain: When a block between two normal faults is raised up, the raised block with its plateau like surface and steep sloppy edge, is known as Block Mountain or an Horst
• Drag: When there is a movement in a fault, the rocks get bent. This bending gives direction to the movement. This bending of the rocks is called Drag.