Chapter – 2

Structure and Physiography

In this post we have given the detailed notes of Class 11 Geography Book 2 Chapter 2 (Structure and Physiography) in English. These notes are useful for the students who are going to appear in Class 11 board exams.

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Chapter 2: Structure and Physiography

Introduction

• The Earth is approximately 4600 million years old and has undergone many changes due to endogenic and exogenic forces.  
• These forces have shaped the Earth’s surface and subsurface features.  
• The Indian plate was once south of the equator and much larger in size, including the Australian plate.  
• Over millions of years, the Indian plate broke up, and the Australian plate moved southeast, while the Indian plate moved north.  
• The northward movement of the Indian plate continues and has significant consequences for the Indian subcontinent’s physical environment.  

Geological Divisions of India

• Based on geological structure and formations, India can be divided into three regions:
  • The Peninsular Block  
  • The Himalayas and other Peninsular Mountains  
  • Indo-Ganga-Brahmaputra Plain  

The Peninsular Block

• The northern boundary of the Peninsular Block is an irregular line running from Kachchh along the western flank of the Aravali Range near Delhi and then roughly parallel to the Yamuna and the Ganga as far as the Rajmahal Hills and the Ganga delta.  
• The Peninsula is formed mainly of ancient gneisses and granites.  
• Since the Cambrian period, the Peninsula has been a rigid block, except for some submerged western coast and some parts changed by tectonic activity.  
• The Peninsular Block has undergone vertical movements and block faulting, resulting in rift valleys like the Narmada, Tapi, and Mahanadi.  
• The Peninsula mainly consists of relict and residual mountains like the Aravali hills, Nallamala hills, Javadi hills, Veliconda hills, Palkonda range, and Mahendragiri hills.  
• River valleys in the Peninsular Block are shallow with low gradients.  
• Most east-flowing rivers form deltas before entering the Bay of Bengal, such as the Mahanadi, Krishna, Kaveri, and Godavari deltas.  

The Himalayas and Other Peninsular Mountains

• The Himalayas and other Peninsular mountains are young, weak, and flexible in their geological structure, unlike the rigid Peninsular Block.  
• They are still subject to exogenic and endogenic forces, resulting in faults, folds, and thrust plains.  
• These mountains are tectonic, dissected by fast-flowing rivers in their youthful stage, forming landforms like gorges, V-shaped valleys, rapids, and waterfalls.  

Indo-Ganga-Brahmaputra Plain

• This plain is formed by the Indus, Ganga, and Brahmaputra rivers.  
• It was originally a geo-synclinal depression that formed during the third phase of Himalayan Mountain formation about 64 million years ago.  
• The plain has been filled by sediments from the Himalayan and Peninsular rivers, with an average depth of 1000-2000 m.  

Physiography

• Physiography is the outcome of structure, process, and stage of development.  
• India’s land is diverse in physical features, with rugged topography in the north, stable tableland in the south, and the vast North Indian plain in between.  

Physiographic Divisions of India

• Based on macro variations, India can be divided into the following physiographic divisions:
  • The Northern and Northeastern Mountains  
  • The Northern Plain  
  • The Peninsular Plateau  
  • The Indian Desert  
  • The Coastal Plains  
  • The Islands  

The Northern and Northeastern Mountains

• The Himalayas consist of a series of parallel mountain ranges, including the Greater Himalayan range, the Great Himalayas, and the Shiwalik.  
• The general orientation of these ranges is from northwest to southeast in the northwestern part of India.  
• The Himalayas in the Darjiling and Sikkim regions lie in an east-west direction, while in Arunachal Pradesh, they are from southwest to northwest.  
• In Nagaland, Manipur, and Mizoram, the Himalayas are in the north-south direction.  
• The Great Himalayan range is approximately 2500 km long and 160-400 km wide.  
• The Himalayas act as a physical, climatic, drainage, and cultural divide between the Indian subcontinent and Central and East Asian countries.  

The Northern Plains

• The northern plains are formed by the Indus, Ganga, and Brahmaputra rivers and extend approximately 3200 km from east to west.  
• The average width of these plains varies between 150-300 km, and the maximum depth of alluvium deposits is between 1000-2000 m.  
• From north to south, the plains can be divided into three zones: the Bhabar, the Tarai, and the alluvial plains.  
• The alluvial plains can be further divided into the Khadar and the Bhangar.  
• Bhabar is a narrow belt (8-10 km) parallel to the Shiwalik foothills where the slope breaks up.  
• Streams and rivers from the mountains deposit heavy materials like rocks and boulders in the Bhabar, sometimes disappearing in this zone.  
• South of the Bhabar is the Tarai belt (10-20 km wide), where most streams and rivers re-emerge without a demarcated channel, creating marshy conditions.  
• The Tarai has rich natural vegetation and diverse wildlife.  
• South of the Tarai is the Bhangar (old) and Khadar (new) alluvial deposits.  
• These plains has mature fluvial erosional and depositional landforms like sand bars, meanders, ox-bow lakes, and braided channels.  
• The Brahmaputra plains are known for riverine islands and sand bars.  
• Most of these areas experience periodic floods and shifting river courses, forming braided streams.  
• The mouths of these rivers form large deltas, such as the Sunderbans delta.  
• The Northern Plains have a general elevation of 50-150 m above sea level.  
• Haryana and Delhi form a water divide between the Indus and Ganga river systems.  
• The Brahmaputra flows from northeast to southwest before turning southward at Dhubri and entering Bangladesh.  
• The fertile alluvial soil supports various crops like wheat, rice, sugarcane, and jute, sustaining a large population.  

The Peninsular Plateau

• The Peninsular Plateau is an irregular triangle rising from 150 m to 600-900 m above the river plains.
• Its outer extent is formed by the Delhi ridge, Rajmahal hills, Gir range, and Cardamom hills, with an extension in the northeast as the Shillong and Karbi-Anglong plateau.  
• The plateau comprises a series of patland plateaus like the Hazaribagh, Palamu, Ranchi, Malwa, Coimbatore, and Karnataka plateaus.  
• It is one of the oldest and most stable landmasses in India.  
• The general elevation of the plateau increases from west to east, as seen in the flow of rivers.  
• Important physiographic features include tors, block mountains, rift valleys, spurs, bare rocky structures, hummocky hills, and quartzite dykes.  
• The western and northwestern parts have black soil.  
• The plateau has undergone phases of upliftment and submergence, accompanied by faulting and fractures, including the seismically active Bhima fault.  
• The northwestern part has complex relief with ravines and gorges, like those of Chambal, Bhind, and Morena.  
• The Peninsular Plateau can be divided into three groups:
  • The Deccan Plateau  
  • The Central Highlands  
  • The Northeastern Plateau  

The Deccan Plateau

• It is bordered by the Western Ghats in the west, Eastern Ghats in the east, and the Satpura, Maikal range, and Mahadeo hills in the north.  
• Western Ghats are known locally as Sahyadri, Nilgiri hills, Anaimalai hills, and Cardamom hills.  
• They are higher and more continuous than the Eastern Ghats, with an average elevation of 1500 m, increasing from north to south.  
• Anaimudi (2695 m) is the highest peak of the Peninsular Plateau, followed by Dodabetta (2637 m).  
• Most Peninsular rivers originate in the Western Ghats.  
• Eastern Ghats are discontinuous, low hills eroded by rivers like the Mahanadi, Godavari, Krishna, and Kaveri, and include ranges like the Javadi hills, Palconda range, Nallamala hills, and Mahendragiri hills.  
• The Eastern and Western Ghats meet at the Nilgiri hills.  

The Central Highlands

• They are bounded by the Aravali range in the west and the Satpura range in the south, forming the northernmost boundary of the Deccan Plateau.  
• The Central Highlands are relict mountains with an elevation of 600-900 m above sea level.  
• The Peninsular Plateau extends to Jaisalmer in the west, covered by sand ridges and barchans.  
• The region has undergone metamorphic processes, evident from the presence of rocks like marble, slate, and gneiss.  
• The general elevation of the Central Highlands is 700-1000 m above sea level, sloping towards the north and northeast.  
• Most tributaries of the Yamuna originate in the Vindhyan and Kaimur ranges.  
• The Banas is a significant tributary of the Chambal, originating from the Aravalli in the west.  
• The Rajmahal hills form an eastern extension of the Central Highlands, with the mineral-rich Chotanagpur plateau to the south.  

The Northeastern Plateau

• It is an extension of the main Peninsular Plateau, separated by a fault created during the Himalayan origin.  
• The Meghalaya and Karbi Anglong plateaus stand detached from the main Peninsular Block.  
• The Meghalaya Plateau is subdivided into the Garo, Khasi, and Jaintia Hills, named after the inhabiting tribal groups.  
• An extension is seen in the Karbi Anglong hills of Assam.  
• Like the Chotanagpur plateau, the Meghalaya plateau is rich in minerals like coal, iron ore, sillimanite, limestone, and uranium.  
• The area receives maximum rainfall from the southwest monsoon, resulting in a highly eroded surface.  
• Cherrapunji has a bare rocky surface with no permanent vegetation cover.  

The Indian Desert

• Northwest of the Aravali hills lies the Great Indian Desert, a land of undulating topography with longitudinal dunes and barchans.  
• It receives low rainfall (below 150 mm per year), resulting in an arid climate with low vegetation cover.  
• It is also known as Marusthali.  
• During the Mesozoic era, this region was under the sea, as evidenced by wood fossils at Aakal and marine deposits near Jaisalmer.  
• The underlying rock structure is an extension of the Peninsular Plateau, but its surface features are carved by physical weathering and wind action.  
• Prominent desert land features include mushroom rocks, shifting dunes, and oasis.  
• Based on orientation, the desert can be divided into two parts: the northern part sloping towards Sindh and the southern part towards the Rann of Kachchh.  
• Most rivers in this region are ephemeral, with the Luni river being significant in the southern part.  
• Low precipitation and high evaporation make it a water deficit region.  
• Some streams disappear after some distance, forming inland drainage by joining a lake or playa.  
• The lakes and playas have brackish water, which is the main source of salt.  

The Coastal Plains

• India has a long coastline that can be divided into two:
  • The western coastal plains  
  • The eastern coastal plains  
• The western coastal plains are a submerged coastal plain, believed to have once included the city of Dwaraka.  
• It is a narrow belt that provides natural conditions for developing ports and harbours like Kandla, Mazagaon, JLN port Navha Sheva, Marmagao, Mangalore, and Cochin.  
• The western coast extends from Gujarat to Kerala and can be divided into the Kachchh and Kathiawar coast, Konkan coast, Goan coast, and Malabar coast.  
• It is narrow in the middle and broader towards the north and south.  
• Rivers flowing through this plain do not form deltas.  
• The Malabar coast has ‘Kayals’ (backwaters) used for fishing, inland navigation, and tourism.  
• The eastern coastal plain is broader and is an emergent coast.  
• It has well-developed deltas formed by rivers flowing eastward into the Bay of Bengal, such as the Mahanadi, Godavari, Krishna, and Kaveri deltas.  
• Due to its emergent nature, it has fewer ports and harbours.  
• The continental shelf extends up to 500 km into the sea, making it difficult to develop good ports and harbours.  

The Islands

• India has two major island groups: one in the Bay of Bengal and the other in the Arabian Sea.  
• The Bay of Bengal island group consists of about 572 islands/islets located between 6∘N−14∘N and 92∘E−94∘E.  
• The two principal groups of islets are the Ritchie’s archipelago and the Labyrinth island.  
• The island group is divided into two categories: the Andaman in the north and the Nicobar in the south, separated by the Ten-degree channel.  
• These islands are believed to be elevated portions of submarine mountains, with some smaller islands being volcanic.  
• Barren island, India’s only active volcano, is in the Nicobar Islands.  
• Some important mountain peaks are Saddle peak (North Andaman 738 m), Mount Diavolo (Middle Andaman 515 m), Mount Koyob (South Andaman – 460 m), and Mount Thuiller (Great Nicobar – 642 m).  
• The coastline has coral deposits and beautiful beaches.  
• These islands receive convectional rainfall and have equatorial vegetation.  
• The Arabian Sea island group includes Lakshadweep and Minicoy, located between 8∘N−12∘N and 71∘E−74∘E longitude, at a distance of 220 km-440 km off the Kerala coast.
• The island group is built of coral deposits and comprises 36 islands, of which 11 are inhabited.  
• Minicoy is the largest island, with an area of 453 sq. km.  
• The Nine-degree channel divides the island group, with the Amini Island to the north and the Canannore Island to the south.  
• The islands have storm beaches with unconsolidated pebbles, shingles, cobbles, and boulders on the eastern seaboard.  

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