Mountains can be defined as a large natural elevation of the earth’s surface rising abruptly from the surrounding level.
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).
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.).
Endogenic processes are divided into diastrophic process and sudden process.
- Diastrophism refers to deformationof the Earth’s crust.
- Diastrophic movements are gradual and might stretch for thousands of years.
- Diastrophic processes are further classified into epeirogenic processes (continent forming ― subsidence, upliftment)and orogenic processes (mountain building ― folding, faulting).
- On the other hand, sudden processes like earthquakes and volcanic eruptions occur in a very short period.
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. 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 Thirunelveliwere all flourishing seaports 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 Kachchhwas submerged as a result of an earthquake.
- The presence of peat and lignitebeds below the sea level in Thirunelveli and the Sunderbans is an example of subsidence.
- The Andamans and Nicobarshave been isolated from the Arakan coast by the submergence of the
- On the east side of Bombay island, trees have been found embedded in the mud about 4 m below the low watermark. A similar submerged forest has also been noticed on the Thirunelveli coast in Tamil Nadu.
- A large part of the Gulf of Mannarand 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.
Orogenesis, the process of mountain building, occurs when two tectonic plates collide – either forcing material upwards to form mountain belts such as the Alps or Himalayas or causing one plate to be subducted below the other, resulting in volcanic mountain chains such as the Andes.
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 characterized by “the folding and faulting of layers of rock, by the intrusion of magma, and by volcanism.
Orogenic or mountain-forming movements act tangentially to the earth’s 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).
On the basis of mode of origin, mountains are classified as
- Original or Tectonic mountains
- Circum-erosional or Relict or Residual mountains