Physical Geography of IndiaMountains, Plains, Plateau & Coasts

Northern and North-eastern mountains

• ▹Himalayas are the young fold mountains. This is the highest mountain range of the world.
• ▹Himalayas act as natural barrier.
• ▹They run from west-east direction from Indus to Brahmaputra along the northern boundary of India covering a distance of 2500 KM.
• ▹Their width varies from 400 km in the west and 150 KM in the East.

Northern plains

• ▹The Northern Plains are located between south of the Himalayas and north of the Peninsular plateau.
• ▹It is formed by the deposition of the sediments brought by three main river systems namely: the Indus, the Ganga and the Brahmaputra.
• ▹These plains extend approximately 3,200 km from the east to the west.
• ▹The average width of these plains varies between 150-300 km.
• ▹The maximum depth of alluvium deposits varies between 1,000-2,000 m.
• ▹It mainly includes the states of Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal and Assam.
• ▹This plain is very fertile due to alluvial sediments brought by the rivers from the Himalayas. This plain is one of the largest and most fertile plains of the world.
  • Major crops such as wheat, rice, sugarcane, pulses, oil seeds and jute are grown here.

Peninsular plateau

• ▹The Peninsular plateau is a triangular shaped table land. It is part of ancient land mass called Gondwana land.
• ▹It is spread over the states of Gujarat, Maharashtra, Bihar, Karnataka and Andhra Pradesh.
• ▹Delhi ridge in the northwest, the Rajmahal hills in the east, Gir range in the west and the Cardamom hills in the south constitute the outer extent of the Peninsular plateau.
  • An extension of this is also seen in the northeast, in the form of Shillong and Karbi-Anglong plateau.
• ▹The Peninsular India is made up of a series of patland plateaus such as the Hazaribagh plateau, the Palamu plateau, the Ranchi plateau, the Malwa plateau, the Coimbatore plateau and the Karnataka plateau, etc.
• ▹This is one of the oldest and the most stable landmass of India.
• ▹The general elevation of the plateau is from the west to the east, which is also proved by the pattern of the flow of rivers.
• ▹This Peninsular plateau has undergone recurrent phases of upliftment and submergence accompanied by crustal faulting and fractures.
• ▹The valley of Narmada lies between the Vindhyas and the satpura which flows east to west and joins the Arabian sea.

The structure of the Peninsular India

• ▹Roughly inverted triangle in shape.
• ▹It comprises the diverse topographical and climatic patterns and is an aggregation of several smaller plateaus, hill ranges interspersed with river basins and valleys.
• ▹The structure of Peninsular India can be divided into: The Great Indian Plateau:
  • The Great Indian Plateau lies to the South of the Great Northern Plains.
  • This is the largest physiographic division of our country. It covers an area of about 16 lakh square km, i.e., about half of the total area of the country.
  • It is an old rocky plateau region.
  • The topography consists of a series of plateaus and hill ranges interspersed with river valleys.
  • Aravalli hills mark the north-western boundary of the plateau region. Its northern and north-eastern boundary is marked by the northern edge of the Bundelkhand Plateau, Kaimur and Rajmahal Hills.
  • The Western Ghats (Sahyadri) and the Eastern Ghats mark the western and eastern boundaries respectively of this Great Plateau.
  • Most of the area of the plateau has a height of more than 400 metres above sea level. The highest point of plateau region is the Anaimudi peak (2965 m).
  • The general slope of this plateau is towards east. It is indicated by the flow of the peninsular rivers flow west to east.
  • Narmada-Tapti are the exceptions which flow from east to west in a rift.
  • River Narmada divides the peninsular block of India into two parts.

  Example: A rift is a long trough formed where a block of land sinks between two faults. The Narmada and Tapi flow west through such troughs instead of following the plateau's eastward slope.

The Indian Desert

• ▹To the northwest of the Aravali hills lies the Great Indian desert.
• ▹It is also called Thar Desert. It is the ninth largest desert in the world. It spreads over the states of Gujarat and Rajasthan.
• ▹This region has semi-arid and arid weather conditions. It receives less than 150 mm of rainfall per year. The vegetation cover is low with thorny bushes.
• ▹Luni is the main river in this area. All other streams appear only at the time of rainfall otherwise they disappear into the sand.
• ▹During the Mesozoic era, this region was under the sea. This can be corroborated by the evidence available at wood fossils park at Aakal and marine deposits around Brahmsar, near Jaisalmer.
• ▹Though the underlying rock structure of the desert is an extension of the Peninsular plateau, yet, due to extreme arid conditions, its surface features have been carved by physical weathering and wind actions.
• ▹On the basis of the orientation, the desert can be divided into two parts: the northern part is sloping towards Sindh and the southern towards the Rann of Kachchh.
• ▹There are some streams which disappear after flowing for some distance and present a typical case of inland drainage by joining a lake or playa.

Islands

There are two major island groups in India – one in the Bay of Bengal and the other in the Arabian Sea.

• ▹The Bay of Bengal island groups consist of about 572 islands/islets.
  • The entire group of island is divided into two broad categories – the Andaman in the north and the Nicobar in the south. They are separated by a water body which is called the Ten degree channel.
• ▹The islands of the Arabian sea include Lakshadweep and Minicoy.
  • There are approximately 36 islands of which 11 are inhabited. Minicoy is the largest island with an area of 453 sq. km.
  • The entire group of islands is broadly divided by the Eleventh degree channel, north of which is the Amini Island and to the south of the Canannore Island.

The evolution of the peninsular India

• ▹Largely the Peninsular India has been a stable landmass but it has gone through some major changes with time.
• ▹The Great Plateau is the part of very ancient landmass, called Gondwana land. It was formed due to the breaking and drifting of Gondwana land.
  • From the earliest time it has been above the level of the sea. Therefore, it has been subjected to large scale denudation.
  • Its mountains are generally of relic type. They are composed of very hard rocks, which have withstood the ravages of denudation more effectively than the surrounding regions.
  • Because of their old age, all the rivers have almost attained their base level and have built up broad and shallow valleys. The dominant rock formations, especially those in the southern parts, are of metamorphic origin with frequent occurrences of granites.

  Example: Denudation is the slow wearing down of land by rain, rivers, wind and weathering. Over hundreds of millions of years it has rounded the Peninsula's once-tall mountains into low, worn ranges like the Aravallis.

Reasons for eastward tilt

• ▹when India broke apart from Gondwana there was lowering of the eastern side of the shield.
• ▹the Deccan volcanic flow tilted to east due to the Western margin undergoing uplift.
  • The erosion has stripped material away from the Western margin. All that eroded sediment has been deposited within basins in the Arabian sea. This removal of weight has led to the crust rebounding and tilting in an easterly direction.
• ▹The oceanic lithosphere in the Bay of Bengal is colder and denser and it is sinking and dragging down the peninsular region with it.
• ▹The Bengal sediment fan i.e. the pile of sediment eroded from the Himalayas throughout the Cenozoic and deposited in the Bay of Bengal is weighing the crust down and exerting a dragging down effect on the Indian peninsular region.

Rifts, lava and the Ghats

• ▹The faults or rifts between the Vindhyas and the Satpuras through which the Narmada and Tapi river flow were probably caused when the Himalayas began to emerge from the Tethys Sea of the olden times.
• ▹The Deccan Traps was formed during the movement of Indian plates towards the Asian plates by the outpouring of lava. This started somewhere around 60 million years ago and continued for a long period of time.
  • The volcanic hotspot that produced the Deccan traps is considered to lie under the present day island of Reunion in the Indian Ocean.
• ▹The Western Ghats are the mountainous faulted and eroded edge of the Deccan Plateau. It evolved due to a combination of factors:
  • Geologic evidence indicates that they were formed during the break-up of the supercontinent of Gondwana some 150 million years ago. I.e. the Crustal doming and rifting which preceded the drifting of the Indian subcontinent resulted in an initial escarpment along the western margin of India.
  • Recession of the Ghats to the east due to marine erosion. The erosion has stripped material away from the Western margin forming the coastal plain and the steep Western Ghat escarpment.
  • Development of westerly drainage.
  • Gradual post glacial rise in sea level resulting in drowned valleys, estuaries and lagoons.
• ▹The eastern Ghat:
  • It is older than the Western Ghats and have a complex geologic history related to the assembly and breakup of the ancient supercontinent of Rodinia and the assembly of the Gondwana supercontinent.
  • The breakup of Gondwanaland produced Permian-Triassic rift basins along what is now the eastern margin of India.
  • The river deltas has created many passes and broader coastal plain.
• ▹The lakes (Chilka, Pulicat and Koluru) have been formed by enclosing small parts of the Bay of Bengal behind sand bars.

The origin of the Himalayas with the support of Plate Tectonics Theory

• ▹225 million years ago (Ma) India was a large island situated off the Australian coast and separated from Asia by the Tethys Ocean.
• ▹The supercontinent Pangea began to break up 200 Ma and India started a northward drift towards Asia.
• ▹80 Ma India was 6,400 km south of the Asian continent but moving towards it at a rate of between 9 and 16 cm per year. At this time Tethys Ocean floor would have been subducting northwards beneath Asia and the plate margin would have been a Convergent oceanic-continental one just like the Andes today.
  • However not all of the Tethys Ocean floor was completely subducted, most of the thick sediments on the Indian margin of the ocean were scraped off and accreted onto the Eurasian continent in what is known as an accretionary wedge.
  • These scraped-off sediments now form the Himalayan mountain range.

  Example: Subduction is when one plate slides beneath another into the mantle. Like a bulldozer blade, the upper plate scrapes the soft sediments off the sinking plate and piles them up — an accretionary wedge.

• ▹From about 50-40 Ma the rate of northward drift of the Indian continental plate slowed to around 4-6 cm per year. This slowdown is interpreted to mark the beginning of the collision between the Eurasian and Indian continental plates, the closing of the former Tethys Ocean, and the initiation of Himalayan uplift.
• ▹The Eurasian plate was partly crumpled and buckled up above the Indian plate but due to their low density/high buoyancy neither continental plate could be subducted.
• ▹This caused the continental crust to thicken due to folding and faulting by compressional forces pushing up the Himalaya and the Tibetan Plateau.
• ▹The continental crust here is twice the average thickness at around 75 km.
  • The thickening of the continental crust marked the end of volcanic activity in the region as any magma moving upwards would solidify before it could reach the surface.