Chapter Notes

Structure and Physiography

Our planet Earth has a long history, and the landforms we see today, like mountains and plains, have been shaped over millions of years. The Earth is estimated to be about 4600 million years old. Its surface features are the result of two types of forces:

• Endogenic forces: Forces that originate from within the Earth, such as plate movements and volcanic activity.
• Exogenic forces: Forces that act on the surface of the Earth, such as weathering by wind, water, and ice.

The landmass of India has been significantly shaped by these forces, especially by the movement of the Indian plate. Millions of years ago, the Indian plate was part of a much larger landmass (which also included the Australian plate) and was located south of the equator. Over time, this plate broke apart. The Indian plate began moving northward, a journey that is still continuing today. This movement has had massive consequences, leading to the formation of major physical features like the Himalayas.

Based on its geological structure, India can be divided into three main divisions:

1. The Peninsular Block
2. The Himalayas and other Peninsular Mountains
3. The Indo-Ganga-Brahmaputra Plain

The Peninsular Block

The Peninsular Block is the ancient, core landmass of India. Its northern boundary is an irregular line running from Kachchh in the west, along the Aravali Range near Delhi, and then parallel to the Yamuna and Ganga rivers up to the Rajmahal Hills.

• Composition: It is a massive complex of very old gneisses and granites.
• Stability: Since the Cambrian period, it has acted as a rigid block, meaning it is very stable and hasn't undergone the kind of folding that created the Himalayas.
• Tectonic Activity: While stable, it has experienced vertical movements and block faulting. This process created the rift valleys of the Narmada and Tapi rivers and block mountains like the Satpura range.
• Extensions: The Karbi Anglong and the Meghalaya Plateau in the northeast are extensions of this block, separated from the Chotanagpur plateau by the Malda fault.
• Mountains: The mountains here are mostly relict and residual, meaning they are old mountains that have been worn down over millions of years. Examples include the Aravali hills, the Nallamala hills, and the Mahendragiri hills.
• Rivers: The river valleys in the Peninsula are generally shallow with low gradients. Most rivers that flow eastwards, like the Mahanadi, Krishna, Kaveri, and Godavari, form large deltas before entering the Bay of Bengal.

The Himalayas and other Peninsular Mountains

In stark contrast to the stable Peninsular Block, the Himalayas are geologically young, weak, and flexible.

• Origin: These mountains are tectonic in origin, formed by the collision of the Indian plate with the Eurasian plate.
• Instability: Because they are young, they are still subject to both endogenic and exogenic forces. This results in features like faults, folds, and thrust plains, and makes the region prone to earthquakes.
• River Systems: The rivers that flow through the Himalayas are in their youthful stage. This is evident from the landforms they create, such as deep gorges, V-shaped valleys, rapids, and waterfalls.

Indo-Ganga-Brahmaputra Plain

This is the third major geological division of India, comprising the vast plains formed by three major river systems: the Indus, the Ganga, and the Brahmaputra.

• Formation: Originally, this area was a massive depression, known as a geo-synclinal depression. This depression formed during the third phase of the Himalayan mountain building, about 64 million years ago.
• Sedimentation: Over millions of years, this depression was gradually filled with sediments (silt, sand, and clay) brought down by the rivers flowing from both the Himalayas and the Peninsula.
• Depth: The average depth of these alluvial deposits is immense, ranging from 1,000 to 2,000 meters.

Physiography

'Physiography' refers to the physical features of an area, which are a result of its geological structure, the processes acting upon it, and its stage of development. India has a great diversity of physical features, from the rugged mountains in the north to the stable tableland in the south, with a vast plain in between.

Based on these variations, India is divided into six physiographic divisions:

1. The Northern and North-eastern Mountains
2. The Northern Plain
3. The Peninsular Plateau
4. The Indian Desert
5. The Coastal Plains
6. The Islands

The North and Northeastern Mountains

This division includes the mighty Himalayas and the hills of the Northeast.

• Himalayan Ranges: The Himalayas consist of a series of parallel mountain ranges, including the Great Himalayas and the Shiwalik.
• Orientation: The orientation of these ranges varies. They run from northwest to southeast in the western part of India, east-west in the Darjiling and Sikkim regions, and north-south in Nagaland, Manipur, and Mizoram.
• Dimensions: The Great Himalayan range is approximately 2,500 km long from east to west, and its width varies from 160 to 400 km.
• Significance: The Himalayas act as more than just a physical barrier. They are also a climatic, drainage, and cultural divide, separating the Indian subcontinent from the rest of Asia.

The Northern Plains

These vast plains are formed by the alluvial deposits brought by the Indus, Ganga, and Brahmaputra rivers.

• Dimensions: The plains stretch for about 3,200 km from east to west, with an average width of 150-300 km.
• Zones of the Plains: From north to south, the plains can be divided into three zones:
  • Bhabar: A narrow belt (8-10 km wide) at the foothills of the Shiwaliks. Here, rivers deposit heavy rocks and boulders, and smaller streams often disappear underground.
  • Tarai: Located south of the Bhabar, this is a 10-20 km wide belt where the streams re-emerge. This creates marshy and swampy conditions, supporting dense forests and diverse wildlife.
  • Alluvial Plains: South of the Tarai, this belt consists of older alluvium called Bhangar and newer alluvium called Khadar.
• Landforms: The plains are in a mature stage of river development, featuring landforms like sand bars, meanders, oxbow lakes, and braided channels. The Brahmaputra plains are particularly known for their riverine islands.
• Economic Importance: The fertile alluvial soil of these plains supports a variety of crops like wheat, rice, sugarcane, and jute, making this region one of the most densely populated in the world. The Sunderbans delta, formed at the mouth of the Ganga and Brahmaputra, is one of the largest in the world.

The Peninsular Plateau

This is an irregularly shaped tableland that rises from the northern plains. It is one of the oldest and most stable landmasses in India.

• Boundaries: It is bordered by the Delhi ridge in the northwest, the Rajmahal hills in the east, the Gir range in the west, and the Cardamom hills in the south.
• Elevation: Its elevation ranges from 150 m to 600-900 m, with a general slope from west to east.
• Features: The plateau is marked by various features like tors (rocky outcrops), block mountains, rift valleys, ravines, and bare rocky structures. The northwestern part of the plateau has a large presence of black soil.
• Sub-divisions: The Peninsular Plateau is broadly divided into three groups:
  1. The Deccan Plateau
  2. The Central Highlands
  3. The Northeastern Plateau

The Deccan Plateau

This is the large southern part of the plateau.

• Borders: It is bordered by the Western Ghats in the west, the Eastern Ghats in the east, and the Satpura and Maikal ranges in the north.
• Western Ghats: These are continuous, high mountains (average elevation 1,500 m) that run parallel to the west coast. Their height increases from north to south. They are known by different local names like Sahyadri in Maharashtra, Nilgiri hills in Karnataka and Tamil Nadu, and Anaimalai hills and Cardamom hills in Kerala. The highest peak of the Peninsular plateau, 'Anaimudi' (2,695 m), is located on the Anaimalai hills.
• Eastern Ghats: These are discontinuous, lower hills that have been heavily eroded by rivers like the Mahanadi, Godavari, and Krishna. Important ranges include the Javadi hills and the Nallamala hills.
• Meeting Point: The Eastern and Western Ghats meet at the Nilgiri hills.

The Central Highlands

This part of the plateau lies to the north of the Deccan Plateau.

• Boundaries: It is bounded by the Aravali range to the west and the Satpura range to the south. The Aravalis are a classic example of relict mountains, meaning they are ancient mountains that have been heavily worn down.
• Geology: This region has undergone metamorphic processes, evidenced by the presence of rocks like marble, slate, and gneiss.
• Slope: The Central Highlands slope towards the north and northeast. Many tributaries of the Yamuna river originate here.
• Eastern Extension: The Rajmahal hills form its eastern extension, south of which lies the mineral-rich Chotanagpur plateau.

The Northeastern Plateau

This is an extension of the main Peninsular plateau.

• Formation: It is believed that during the formation of the Himalayas, the force from the northeastward movement of the Indian plate created a huge fault between the Rajmahal hills and the Meghalaya plateau. This depression was later filled by river deposits, separating the Northeastern Plateau from the main block.
• Sub-divisions: The Meghalaya plateau is subdivided into the Garo Hills, Khasi Hills, and Jaintia Hills. The Karbi Anglong hills in Assam are also an extension.
• Resources and Climate: Like the Chotanagpur plateau, it is rich in minerals such as coal, iron ore, and uranium. This area receives very heavy rainfall from the southwest monsoon, which has led to a highly eroded surface.

The Indian Desert

Located to the northwest of the Aravali hills, this is the Great Indian Desert, also known as Marusthali.

• Topography: It is a land of undulating topography, featuring longitudinal dunes and crescent-shaped dunes called barchans.
• Climate: It receives very low rainfall (below 150 mm per year), resulting in an arid climate with sparse vegetation.
• Geological Past: Evidence from wood fossils suggests that this region was under the sea during the Mesozoic era.
• Landforms: Prominent desert landforms include mushroom rocks, shifting dunes, and oases.
• Drainage: Most rivers here are ephemeral (they flow only during the rainy season). The Luni is the most significant river in the southern part. Many streams exhibit inland drainage, disappearing into salt lakes called playas.

The Coastal Plains

India has a long coastline, which can be divided into two main plains.

The Western Coastal Plains

• Formation: These are an example of a submerged coastal plain. This submergence has created a narrow belt, which provides excellent natural conditions for ports and harbours like Kandla, Mangalore, and Cochin.
• Divisions: The coast is divided into the Kachchh and Kathiawar coast in Gujarat, the Konkan coast in Maharashtra, the Goan coast, and the Malabar coast in Karnataka and Kerala.
• Features: The rivers flowing through this plain are fast and do not form deltas. A distinguishing feature of the Malabar coast is the presence of 'Kayals' (backwaters), which are used for fishing, navigation, and tourism.

The Eastern Coastal Plains

• Formation: In contrast to the west, this is an emergent coast, which means the land has risen from the sea. This has resulted in a broader plain.
• Deltas: The coast is marked by well-developed deltas formed by the major east-flowing rivers: the Mahanadi, Godavari, Krishna, and Kaveri.
• Ports: Due to its emergent nature and a wide continental shelf (extending up to 500 km into the sea), it is difficult to develop good natural ports here.

The Islands

India has two major island groups.

The Bay of Bengal Islands

• Composition: This group consists of about 572 islands, collectively known as the Andaman and Nicobar Islands.
• Division: The group is divided into the Andaman in the north and the Nicobar in the south, separated by the Ten Degree Channel.
• Origin: It is believed that these islands are the elevated portions of submarine mountains. Some smaller islands, like Barren island (India's only active volcano), are volcanic in origin.
• Features: Important peaks include Saddle Peak (North Andaman). The islands have coral deposits, beautiful beaches, and receive convectional rainfall, supporting an equatorial type of vegetation.

The Islands of the Arabian Sea

• Composition: This group includes Lakshadweep and Minicoy. These islands are located 220-440 km off the Kerala coast.
• Origin: The entire island group is built of coral deposits. There are approximately 36 islands, of which 11 are inhabited.
• Division: The Nine Degree Channel broadly divides the island group. Minicoy is the largest island in the archipelago.
• Features: The islands have storm beaches on their eastern side, consisting of unconsolidated pebbles, shingles, and cobbles.