Endogenic Geomorphic Movements: Epeirogenic & Orogenic

• Earth’s crust and its surface are constantly evolving (changing) due to various forces emanating from below (endogenic forces) as well as above the surface of the earth (exogenic forces).
• These forces cause physical and chemical changes to the geomorphic structure (earth’s surface).
• Some of these changes are imperceptibly slow (e.g. weathering, folding), some others are gradual (e.g. erosion) while the remaining are quite sudden (earthquakes, volcanic eruptions).
• Geomorphic: relating to the form of the landscape and other natural features of the earth’s surface.
• Geomorphic agents: mobile medium (like running water, moving ice masses or glaciers, wind, waves, currents etc.) which removes, transports and deposits earth materials.
• Geomorphic processes: physical and chemical processes that take place on the earth’s surface (folding, faulting, weathering, erosion, etc.) due to endogenic and exogenic forces.
• Geomorphic movements: large scale physical and chemical changes that take place on the earth’s surface due to geomorphic processes.

Endogenic Geomorphic Movements

• The large-scale movements on the earth’s crust or its surface brought down by the forces emanating from deep below the earth’s surface are called as endogenic geomorphic movements or simply endogenic movements (endo: internal; genic: origin; geo: earth; morphic: form).
• The geomorphic processes that are driven by the forces emanating from deep below the earth’s surface are called endogenic geomorphic processes (folding, faulting, etc.).

The force behind Endogenic Movements

• The ultimate source of energy behind forces that drive endogenic movements is earth’s internal heat.
• Earth’s internal heat is a result of mainly radioactive decay (50% of the earth’s internal heat) and gravitation (causes pressure gradients).
• Differences in temperature and pressure (temperature gradients or geothermal gradients and pressure gradients) among various layers of the earth give rise to density differences and these density differences give rise to conventional currents.
• Convectional currents in the mantle drive the lithospheric plates (crust and upper mantle) and the movement of the lithospheric plates (tectonics) is the cause behind endogenic movements.
• The Earth’s rotation (Coriolis effect) can influence where convection currents travel.
• The destination of convection currents determines the nature and location of the endogenic movements.

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Classification of Endogenic movements

• Endogenic movements are divided into diastrophic movements and sudden movements.
• Diastrophism refers to deformation of the Earth’s crust.
• Diastrophic movements are gradual and might stretch for thousands of years.
• On the other hand, sudden movements like earthquakes and volcanic eruptions occur in a very short period.
• Diastrophic movements are further classified into epeirogenic movements (continent forming ― subsidence, upliftment) and orogenic movements (mountain building ― folding, faulting).

Diastrophism

• Diastrophism refers to deformation of the Earth’s crust due to diastrophic movements (deforming movements) such as folding, faulting, warping (bending or twisting of a large area) and fracturing.
• 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 (crust is severely deformed into folds) 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 (simple deformation);
• earthquakes and volcanism involving local relatively minor movements;
• plate tectonics involving horizontal movements of crustal plates.
• The most obvious evidence of diastrophic movement can be seen where sedimentary rocks have been bent, broken or tilted.

Epeirogenic or continent forming movements

• Epeirogenic or continent forming movements are radial movements (act along the radius of the earth).
• Their direction may be towards (subsidence) or away (uplift) from the centre.
• They cause upheavals or depressions of land exhibiting undulations (wavy surface) of long wavelengths and little folding.
• The broad central parts of continents are called cratons and are subject to epeirogeny, hence the name continent forming movements.

Uplift

• Raised beaches, elevated wave-cut terraces, sea caves and fossiliferous beds above sea level are evidence of upliftment.

Uplifted landforms

• In India, raised beaches occur at several places along the Kathiawar, Nellore, and Tirunelveli coasts.
• Several places which were on the sea some centuries ago are now a few miles inland due to upliftment.
• For example, Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Tirunelveli, were all flourishing seaports about 1,000 to 2,000 years ago.

Subsidence

• Submerged forests and valleys, as well as buildings, are evidence 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 Tirunelveli 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.

Arakan coast (Highlighted part)

• On the east side of Bombay island, trees have been found embedded in the mud about 4 m below low water mark. A similar submerged forest has also been noticed on the Tirunelveli 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 is submerged in the sea.

Orogenic or the mountain-forming movements

• In contrast to epeirogenic movement, the orogenic movement is a more complicated deformation of the Earth’s crust, associated with crustal thickening (due to the convergence of tectonic plates).
• Such plate convergence forms orogenic belts that are characterised by “the folding and faulting of layers of rock, by the intrusion of magma, and by volcanism.
• Orogenic or the mountain-forming movements act tangentially to the earth surface, as in plate tectonics.
• Tension 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).

Fissure and Fold

Sudden Movements

• Sudden geomorphic movements occur mostly at the lithospheric plate margins (tectonic plate margins).
• The plate margins are highly unstable regions due to pressure created by pushing and pulling of magma in the mantle (convectional currents).
• These movements cause considerable deformation over a short period.

Earthquakes

• Earthquakes occur when the surplus accumulated stress in rocks in the earth’s interior due to folding, faulting or other physical changes is relieved through the weak zones over the earth’s surface in the form of kinetic energy (seismic waves).
• Such movements may result in uplift or subsidence 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 up to 3 metres.
• An earthquake in Japan (1891) caused subsidence of up to 6 metres.
• Earthquakes may cause a change in contours, change in river courses, shoreline changes, glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.

Volcanoes

• Volcanism includes the movement of molten rock (magma) onto or towards the earth’s surface through narrow volcanic vents or fissures.
• 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 etc.) and pyroclastic material (cloud of ash, lava fragments carried through the air, and vapour).
• Depending on the chemical composition and viscosity of the lava, a volcano may take various forms.