Geomorphic process, be it endogenic or exogenic create a variety of landforms, on the solid surface of the earth which includes Mountains, Plateaus and Plains.
The existing six Major plates, nine Minor plates and other plates on the earth are in constant movement causing three types of motion (i) Separation or Divergent, (ii) Closing together or Convergent and (iii) Frictional and Shearing resulting into the formation of various landforms.
Geosyncline: A mobile zone varied in space and time, usually elongated, basin like depression along the edge of a continent, in which a thick sequence of sediments and volcanic deposits has accumulated.
The various rocks which got deposited in sinking seas forming the elongated mountain like structures. All the mountains have come out of geosynclines.
There were 4 geosynclines in the past (E. Haug)
1) Rockies Geosynclines
2) Ural Geosynclines
3) Tethys Geosynclines
4) Circum-Pacific Geosynclines
The geosyncline theory of mountain formation states that the sediments were squeezed and folded into mountain ranges due to compressive forces from rigid masses.
• A portion of land rising considerably above the surrounding country either as a single eminence (Kilimanjaro) or in range (Himalayas, Rockies, Andes), is known as ‘mountain’.
• Some authorities regard elevations above 600m (2000 feet) as mountains, and those below being referred to as hills.
• The existing mountains are the result of folding and faulting (endogenic forces), and that of the agents of denudation.
• The processes which involve in the building of mountains are known as Orogensis-from Greek word Oros meaning mountain, and genesis for meaning to come into being’.
• Orogeny (Orogenesis): A period of mountain building involving the process of intense upward displacement of the earth’s crust, usually associated with folding, thrust faulting and other compressional processes.
• An orogeny extends in time for some tens of millions of year. There appears to be an average interval of 200 to 300 million years between 2 orogenies.
• Some of the important theories about the mountains building are:
1) Radio-active Theory by J. Joly (1925)
2) The Geosynclinal Theory by Kober.
3) The Thermal Contraction Theory by Jeffreys.
4) Theory of Sliding Continents by R.A. Daly.
5) The Continental Drift Theory of Wegener.
6) The Theory of Sea-Floor Spreading by H.H. Hees.
7) The Theory of Plate Tectonics by W.J. Morgan.
• The first four theories attempt to explain the origin of folded mountains of Tertiary Period but they do not throw any light on the mountains older than Tertiary Period. Hereby, we are explaining the Theory of Plate Tectonics by W.J. Morgan of mountain building.
Plate Tectonics and Mountains Building:
• The theory of plate tectonics has a close relationship with mountain building which is based on the concept of ‘Sea Floor Spreading’ advocated by Harry H. Hess. It is an improvement over Wegener’s Theory of Continental Drift.
• The term ‘Plate Tectonics’ was coined by Tuzo Wilson (Univ. of Toronto) in 1965 and was propounded by W. J. Morgan (Princeton University) in 1967.
• The theory of Plate Tectonics is a comprehensive theory which explains
• Mountain Building,
• Folding and Faulting,
• Continental Drift,
• Seismic events (earthquakes) etc.
The process of plate tectonics:
• The Plates which are floating over the liquid Asthenosphere move in different directions due to the forces produced in them.
• In the opinion of Morgan – the propounder of Plate Tectonic Theory, the mountain building takes place because of compressive forces caused by the collision of two convergent or destructive plates.
• It is destructive plate boundaries which builds the mountain. In this process of collision of two plates, the plates having relatively denser material is subducted under the other plate boundary of relatively lighter materials. For example: The ‘Benioff Zone’ or the ‘Ring of Fire’ and the ‘Ocean Trenches’ which are the subduction zone.
The theory of Plate Tectonics identifies the three types of margins of the major and minor plates, i.e. (convergent, divergent and conservative margins).
The collision of convergent plates may be:
1) Collision of two oceanic plates: Both the plates have denser materials. They result in the formation of folded mountain ranges and islands arcs – e.g. Japanese Islands, Philippines.
2) Collision of two Continental Plates: Like the Eurasian Plate and the Indian Plate, the Alps and the Himalayas (Tethys Sea – 70-65 million years ago). The Himalayas and Alps came into existence about 30-20 million years ago.
3) Collision of one oceanic and one continental plate. These mountains were formed due to the subduction of Pacific Plate under the American Continental Plate-e.g. the Rockies and the Andes.
There is a general consensus among the Geologists, Geomorphologists and Geophysicists that the theory of Plate Tectonics has given a scientific explanation about the origin of continents, ocean basins and mountain building. It explains correctly the causes of volcanic eruptions, earthquakes, folding, faulting and proved the Wegener’s Continental Drift. It also explains the cyclic pattern of orogeny process of Mountain Building.
Classification of Mountains:
On the basis of their orogeny and formation, Mountains are of:
• Fold Mountains: Formed due to the force of compression originating from the endogenic or internal forces resulting into synclines (trough) and anticlines (crest) which are the part of Fold Mountains.
For Example: The Himalayas, the Rockies in North America, Andes in South America and the Alps in Europe are the Fold Mountains.
Since these are formed in very recent periods hence, are also known as Young Fold Mountains.
• Block Mountains: Formed due to the force of tension originating inside the earth resulting into down-lifting or uplifting of land in between two parallel faults forming a block mountain.
The uplifted blocks are termed as horsts and the lowered blocks are called graben.
For Example: Black Forest Mountains in Germany, Sierra Nevada in North America. The Great African Rift Valley (valley floor is graben), The Rhine Valley and the Vosges Mountain in Europe are examples.
• Dome Mountains: (Mountains of Accumulations): Formed by the uplifting of land surface due to push factor of magma accumulation beneath the surface without the lava erupting out.
For example: Half Dome Mountain in the Sierra Nevada range in California, Dark Mountain in British Columbia, Canada.
• Volcanic Mountains or Accumulated Mountains: Formed when molten magma deep within the earth erupts and piles upon the surface, cooling and building a cone of rock one upon the other.
For example: Mount Mauna Loa in Hawaii Island, Mount Popa in Myanmar, Mount Fuji Yama in Japan.
• Residual Mountains or Relict Mountains: During the process of weathering (exogenic process) of an elevated area, when some hardened rocks escape from being worn away leaving behind the structure (made up of hard materials) known as Relict Mountain, while at the same time the surrounding area gets eroded
For Example: Hill like Satpura, Vindhya, Aravali, Parasnath, Rajmahal Hills, Nilgiri, Palkonda etc. are the examples of Relict Mountains.
• A broad area of relatively flat land, covering more than one-third of the world’s land area is a major landforms or types of land on Earth. For Example: the Eurasian Plains and the Russian Steppes.
• Classification of Plains: On the basis of various processes of formations, plains can be classified as:
A. Structural Plains: Formed by uplift or subsidence of land due to Diastrophic force causing upliftment of a portion of land beneath the ocean water or may cause submergence of coastal land under ocean water.
In the other word, it is formed by the upliftment of a part of the sea floor or continental shelf due to endogenic forces, located on the borders of almost all the major continents.
For Example: the Great Plains of the USA surrounded by Mississippi-Missus plains in the east and Rockies in the west and the Coromandal plains in India which is the result of mild subsidence followed by sedimentation.
B. Erosional Plains: These plains are formed by the agents of erosion after millions of years even high mountains are reduced to low undulating plains these include the following.
• Pene Plains: Formed by eroding the high mountains or upliftment where wild plains predominate with some resistant peak remaining. It literally means almost a plain. For Example: Niagara Plain in the USA, Lorrain in France and Southern England Plains are some examples.
• Plains of Glacial Erosion: These plains have rounded peaks and worn down ice scoured shield lands. Glaciated plains are those of Kashmir in India, northern part of North America and North western Eurasia.
• Wind-eroded Plains: These are called Reg, Seris and Hamada in the Sahara desert; strong winds pick up weathered materials and deposit them elsewhere. Their formations are also called Pedi-plains.
• Karst Plains: There plains have corrugated and undulating surfaces produced by chemical weathering of limestone landscape by ground water. For Example: Karst region of Yugoslavia.
C. Depositional Plains: Formed by the depositional activities of various geomorphic agents brought by various agents of transportation.
• River Deposition: The most widespread of these are alluvial plains like the northern plains of India, the flood plains like the Mississippi Plains in the USA deltaic plains in Egypt and India.
• At the foot hills of mountain are formed Piedmont alluvial Plains.
• The flood plain described above is also alluvial plains. They are found along the rivers like Yangtze, Mekong, Salween etc. and are regarded as granaries of the world. In India flood Plains are divided into Khadar and Bhangar Plains.
• Wind Deposition: Most such plains are sandy deserts like the Sahara in Africa and the desert in India. They have irregular and undulating surface made by existence of sand dunes and fallows. Loess plains in china are formed from deserts.
Marine Deposition: These plains are found in coastal regions. Examples are the east coast of India, the estuarine banks in Netherlands coast of Germany and Denmark. In Denmark we find polders and dykes. A polder is a piece of land reclaimed from the sea or lake. The submerged land is surrounded by an embankment and drained by pumping water into canals.
It is an elevated area (compared to surroundings) with nearly even surface on its top having large area on its top and a steep slope on its sides. They are also known as high plains or table lands covering around 18% of the earth’s land surface.
Classification of Plateaus:
Broadly plateaus are of two types: Dissected Plateaus and Volcanic Plateaus.
A. Dissected Plateaus: Formed as a result of upward movement in the earth’s crust caused by slow collision of tectonic plates. The Colorado Plateau, in the western United States, has been rising about .03 centimeter (.01 inch) a year for more than 10 million years.
B. Volcanic Plateaus: Formed by numerous small volcanic eruptions that slowly builds up over the time, forming a plateau from the resulting lava flows. For example: Mount Tongariro, Mount Ngauruhoe, Mount Ruapehu and The North Island Volcanic Plateau covers most of the central part of the North Island of New Zealand.
On the basis of their geographical location and structure of rocks, plateaus can be classified as:
A. Intermontane Plateau: The word Intermontane means between the mountain, the plateaus which are bordering the fold mountain range or are partly or fully enclosed within them.
Vertical movement raises these extensive landforms of nearly horizontal rocks to thousands of meters above sea level. For example: Tibet plateau (4500 m high) which is surrounded by folded mountains like Himalaya, Karakoram, Kunlun, Tien-Shian etc. eg: the plateau of Colorado, Mexico, Bolovia and Iran etc.
B. Piedmont Plateaus: Also known as Plateaus of denudation as being formed due to various agents of erosion reducing the level of mountain. The word Piedmont means foot of a mountain.
Such mountains are situated at the foot of a mountain and are locked on the other side by a plain or a sea/ocean. For Example: Malwa Plateau in India, those of Patagonia facing the Atlantic ocean and the Appallachian situated between the Appalachian Mountain and the Atlantic Coastal Plain in U.S.A.
C. Continental Plateaus: Also known as the plateaus of Accumulation, formed either by an extensive continental upliftment or by the spread of horizontal basic lava (less viscous) sheets completely covering the original topography.
The volcanic lava covered plateau of Maharashtra in India, Snake River Plateau in North West USA are the examples of this type.
D. Volcanic Plateaus: Produced by volcanic activity diverging into lava plateau and pyroclastic plateau.
• Lava plateau: Formed by numerous successive eruptions of highly fluid basaltic lava through numerous vents without any violent explosions.
• Pyroclastic Plateau: Formed by violent eruptions of massive pyroclastic flows, underlain by pyroclastic rocks.
• For example: Columbia Plateau in USA, Peninsular Plateau in India etc.