Table of Contents
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- Our earth is undergoing deformations imperceptibly [so slight, gradual, or subtle as not to be perceived] but continuously.
- These deformations are caused by the movements generated by various factors like
- The heat generated by the radioactive elements in earth’s interior.
- Movement of the crustal plates due to tectogenesis.
- Forces generated by rotation of the earth.
- Climatic factors like winds, precipitation, pressure belts etc.
- Isostacy ==> According to this concepts, blocks of the earth’s crust, because of variations in density would rise to different levels and appear on the surface as mountains, plateau, plains or ocean basins
- Tectonic ==> relating to the structure of the earth’s crust and the large-scale processes which take place within it.
- Geomorphic == relating to the form of the landscape and other natural features of the earth’s surface.
- The endogenic and exogenic forces causing physical and chemical changes on earth surface are known as geomorphic processes.
- Diastrophism and volcanism are endogenic geomorphic processes.
- Weathering, mass wasting, erosion and deposition are exogenic geomorphic processes.
- Geomorphic agent == mobile medium (like running water, moving ice masses, wind, waves and currents etc.) which removes, transports and deposits earth materials.
Earth Movements – Endogenetic Movements
- The interaction of matter and temperature generates these forces or movements inside the earth’s crust. The earth movements are mainly of two types: diastrophism and the sudden movements.
- The energy emanating from within the earth is the main force behind endogenic geomorphic processes.
- This energy is mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth. This energy due to geothermal gradients and heat flow from within induces diastrophism and volcanism in the lithosphere.
- Diastrophism is the general term applied to slow bending, folding, warping and fracturing.
- Wrap == make or become bent or twisted out of shape, typically from the action of heat or damp; make abnormal; distort.
- All processes that move, elevate or build up portions of the earth’s crust come under diastrophism. They include:
- orogenic processes involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust;
- epeirogenic processes involving uplift or warping of large parts of the earth’s crust;
- earthquakes involving local relatively minor movements;
- plate tectonics involving horizontal movements of crustal plates.
- In the process of orogeny, the crust is severely deformed into folds. Due to epeirogeny, there may be simple deformation. Orogeny is a mountain building process whereas epeirogeny is continental building process.
- Through the processes of orogeny, epeirogeny, earthquakes and plate tectonics, there can be faulting and fracturing of the crust. All these processes cause pressure, volume and temperature (PVT) changes which in turn induce metamorphism of rocks.
Epeirogenic or continent forming movements
- In geology, Epeirogenic movement refers to upheavals or depressions of land exhibiting long wavelengths [undulations] and little folding.
- The broad central parts of continents are called cratons, and are subject to epeirogeny.
- The movement is caused by a set of forces acting along an Earth radius, such as those contributing to Isostacy and Faulting in the lithosphere
- Epeirogenic or continent forming movements act along the radius of the earth; therefore, they are also called radial movements. Their direction may be towards (subsidence) or away (uplift) from the center. The results of such movements may be clearly defined in the relief.
- Raised beaches, elevated wave-cut terraces, sea caves and fossiliferous beds above sea level are evidences of uplift.
- Raised beaches, some of them elevated as much as 15 m to 30 m above the present sea level, occur at several places along the Kathiawar, Nellore, and Thirunelveli coasts.
- Several places which were on the sea some centuries ago are now a few miles inland. For example, Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Thirunelveli, were all flourishing sea ports about 1,000 to 2,000 years ago.
- Submerged forests and valleys as well as buildings are evidences of subsidence.
- In 1819, a part of the Rann of Kachchh was submerged as a result of an earthquake.
- Presence of peat and lignite beds below the sea level in Thirunelveli and the Sunderbans is an example of subsidence.
- The Andamans and Nicobars have been isolated from the Arakan coast by submergence of the intervening land.
- On the east side of Bombay island, trees have been found embedded in mud about 4 m below low water mark. A similar submerged forest has also been noticed on the Thirunelveli coast in Tamil Nadu.
- A large part of the Gulf of Mannar and Palk Strait is very shallow and has been submerged in geologically recent times. A part of the former town of Mahabalipuram near Chennai (Madras) is submerged in the sea.
Orogenic or the mountain-forming movements
- Orogenic or the mountain-forming movements act tangentially to the earth surface, as in plate tectonics.
- Tensions produces fissures (since this type of force acts away from a point in two directions) and compression produces folds (because this type of force acts towards a point from two or more directions). In the landforms so produced, the structurally identifiable units are difficult to recognise.
- In general, diastrophic forces which have uplifted lands have predominated over forces which have lowered them.
- These movements cause considerable deformation over a short span of time, and may be of two types.
- It occurs when the surplus accumulated stress in rocks in the earth’s interior is relieved through the weak zones over the earth’s surface in form of kinetic energy of wave motion causing vibrations (at times devastating) on the earth’s surface. Such movements may result in uplift in coastal areas.
- An earthquake in Chile (1822) caused a one-metre uplift in coastal areas.
- An earthquake in New Zealand (1885) caused an uplift of upto 3 metres in some areas while some areas in Japan (1891) subsided by 6 metres after an earthquake.
- Earthquakes may cause change in contours, change in river courses, ‘tsunamis’ (seismic waves created in sea by an earthquake, as they are called in Japan) which may cause shoreline changes, spectacular glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.
- Volcanism includes the movement of molten rock (magma) onto or toward the earth’s surface and also formation of many intrusive and extrusive volcanic forms.
- A volcano is formed when the molten magma in the earth’s interior escapes through the crust by vents and fissures in the crust, accompanied by steam, gases (hydrogen sulphide, sulphur dioxide, hydrogen chloride, carbon dioxide) and pyroclastic material. Depending on chemical composition and viscosity of the lava, a volcano may take various forms.
- Pyroclastic ==> adjective of or denoting rock fragments or ash erupted by a volcano, especially as a hot, dense, destructive flow.