Structure and Physiography

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (India: Physical Environment), Chapter 2.

Was India once south of the equator?

Yes. Millions of years ago the Indian plate lay south of the equator. It was much larger then, and the Australian plate was part of it. Over time the plate broke into pieces. The Australian plate moved towards the south-east, and the Indian plate drifted north. That northward push has never stopped, and it explains why the Himalayas keep rising, why earthquakes strike the north, and why the land of India looks so different from region to region.

The earth is about 4,600 million years old, and its surface has been shaped by two kinds of forces. Endogenic forces work from inside the earth, such as folding, faulting and the movement of plates. Exogenic forces work on the surface, such as rivers, wind and weathering. India's present structure and landforms come from the interplay of the two, along with the sideways movement of plates.

Structure: three geological divisions

Rocks of different age and behaviour produce different land. Geologists therefore divide India into three parts, which follow the main physical features.

The Peninsular Block. Its northern edge is an irregular line from Kachchh, along the western side of the Aravali Range near Delhi, then roughly parallel to the Yamuna and Ganga up to the Rajmahal Hills and the Ganga delta. The Meghalaya Plateau and Karbi Anglong in the north-east, and the desert of Rajasthan lying over this block, are also extensions of it. The Malda fault in West Bengal separates the north-eastern parts from the Chotanagpur Plateau.

Though rigid, the block has not been unmoved. As part of the Indo-Australian plate it has been pushed by vertical movements and broken by block faulting. The rift valleys of the Narmada, Tapi and Mahanadi and the Satpura block mountains are examples. Most of its hills, such as the Aravali, Nallamala, Javadi, Veliconda, Palkonda and Mahendragiri, are relict (left over) and residual (what remains after erosion). Its rivers flow in shallow valleys with low gradients, and most east-flowing rivers build deltas before entering the Bay of Bengal. The deltas of the Mahanadi, Krishna, Kaveri and Godavari are important.

The Himalayas and other Peninsular mountains. These mountains are tectonic in origin and are still adjusting, so faults, folds and thrust plains keep forming. Fast-flowing rivers in their youthful stage cut them up, and gorges, V-shaped valleys, rapids and waterfalls are the signs of that stage.

The Indo-Ganga-Brahmaputra Plain. It began as a geosynclinal depression (a long, sinking trough). That depression reached its greatest development in the third phase of Himalayan mountain building, about 64 million years ago. Since then Himalayan and Peninsular rivers have been filling it with sediment.

Why the contrast matters. Cause and effect run in one line: a rigid old block means worn-down hills and gentle rivers; a young flexible mountain belt means steep land and powerful rivers; a sinking trough means deep, fertile plains. When a question asks why rivers behave differently in the north and south, the answer is structure.

Pause and predict: which of the three divisions would you expect to have rapids and waterfalls, and which shallow valleys with low gradients?

Check your answer: rapids and waterfalls belong to the Himalayas, where youthful rivers are still cutting down. Shallow valleys with low gradients belong to the Peninsular Block, which is old, stable and already worn down.

Physiography: structure, process and stage

Physiography is the result of three things working together: the structure, the geomorphic process acting on it, and the stage of development. In the north there is a vast stretch of rugged mountains with peaks, valleys and deep gorges. In the south is a stable table land with dissected plateaus, bare rocks and scarps. Between them lies the great plain. On this basis India has six physiographic divisions.

The Northern and North-eastern Mountains

The Himalayas are a series of parallel ranges. The Greater Himalayan range, also called the central axial range, is about 2,500 km long from east to west, with a width of 160 to 400 km from north to south. Their direction changes along the arc: they run north-west to south-east in the north-western part, east to west in Darjiling and Sikkim, south-west to north-east in Arunachal Pradesh, and north to south in Nagaland, Manipur and Mizoram.

The Himalayas stand like a strong, long wall between the Indian subcontinent and Central and East Asia. They are a physical barrier, and also a climatic, drainage and cultural divide. They stop cold winds from the north, make monsoon winds shed their moisture, and give birth to the great rivers.

Beyond the textbook. For the exam: the Himalayas form a series of parallel ranges, with the Great Himalayas and the Shiwalik among the important ones. In reality: geographers usually describe three parallel belts from north to south: the Greater Himalaya (Himadri), the Lesser Himalaya (Himachal) and the outer Shiwalik. Each has its own height, rock type and age, and the Shiwalik is the youngest and lowest. Why it matters: knowing this order helps you answer a question that places a river, valley or peak in the right belt.

The Northern Plain

The Indus, Ganga and Brahmaputra and their tributaries built this plain from the alluvium they carried. It stretches about 3,200 km from east to west, with an average width of 150 to 300 km and alluvium 1,000 to 2,000 m deep. From north to south it falls into three zones, and the alluvial zone is further divided.

Why the streams disappear. Rivers rush down steep Himalayan slopes carrying heavy rock. When the slope suddenly flattens they lose speed and dump the heavy load. The loose boulder-and-gravel belt is so porous that water sinks in and flows underground. Further south, where the ground is finer, the water comes back to the surface and spreads out without clear channels, which creates the marshy Tarai.

The mature stage of river action shows in sand bars, meanders, ox-bow lakes and braided channels. The Brahmaputra plains are known for riverine islands and sand bars, and many areas are flooded often as rivers shift course. The Sunderbans delta is one of the largest in the world. Most of the plain is featureless, at 50 to 150 m above sea level. Haryana and Delhi form the water divide between the Indus and Ganga systems. The Brahmaputra flows from the north-east to the south-west and makes an almost 90 degree turn to the south at Dhubri before entering Bangladesh. The fertile soil grows wheat, rice, sugarcane and jute and supports a large population.

Beyond the textbook. For the exam: Bhangar is old alluvium and Khadar is new alluvium. In reality: Bhangar usually lies on slightly higher ground, farther from the river, and often contains lime nodules known as kankar, while the Khadar is low-lying, sandy and renewed by floods almost every year. Why it matters: Khadar land gains fresh silt but risks floods, while Bhangar land is safer from floods but is not renewed by them.

The Peninsular Plateau

The Peninsular Plateau is an irregular triangle rising from 150 m above the river plains to 600 to 900 m. The Delhi ridge (an extension of the Aravali) in the north-west, the Rajmahal Hills in the east, the Gir range in the west and the Cardamom Hills in the south mark its outer edge, with an extension in the north-east as the Shillong and Karbi Anglong plateaus. It is made of a series of plateaus, such as the Hazaribagh, Palamu, Ranchi, Malwa, Coimbatore and Karnataka plateaus. It is one of the oldest and most stable landmasses of India. The land slopes from west to east, and the river pattern shows this.

Typical features are tors, block mountains, rift valleys, spurs, bare rocky structures, hummocky hills and wall-like quartzite dykes that make natural sites for water storage. Black soil is common in the west and north-west. The plateau has seen repeated upliftment and submergence with faulting. The Bhima fault is noted for repeated seismic activity. The north-west has complex relief of ravines and gorges, such as the Chambal, Bhind and Morena ravines.

The plateau has three broad groups, and the table compares them.

The Ghats side by side. The Western and Eastern Ghats look similar on a map but are very different.

The two ranges meet at the Nilgiri hills. Anaimudi (2,695 m) on the Anaimalai hills is the highest peak of the Peninsular plateau, followed by Dodabetta (2,637 m) in the Nilgiris.

Pause and predict: most Peninsular rivers rise in the Western Ghats, which are close to the west coast. Which sea do you expect them to flow into, and why?

Check your answer: the Bay of Bengal. The plateau slopes from west to east, so water starting near the west coast flows across the whole peninsula. The Narmada and Tapi, which run west through rift valleys, are the exceptions.

The Central Highlands. The Satpura, a series of scarped plateaus at 600 to 900 m, forms the northern edge of the Deccan. It is a classic relict mountain, highly denuded and in discontinuous ranges. The plateau extends as far as Jaisalmer, where it is covered by longitudinal sand ridges and crescent-shaped dunes called barchans. Most tributaries of the Yamuna rise in the Vindhyan and Kaimur ranges. The Banas is the only significant Chambal tributary rising in the Aravali. In the east the Rajmahal Hills continue the highlands, and south of them lies the Chotanagpur plateau with large mineral reserves.

The Northeastern Plateau. The force of the north-eastward movement of the Indian plate at the time the Himalayas rose created a huge fault between the Rajmahal Hills and the Meghalaya plateau. Rivers later filled it with sediment, so Meghalaya and Karbi Anglong now stand apart from the main block. The Meghalaya plateau has three parts, named for tribal groups: the Garo, Khasi and Jaintia hills. It gets the highest rainfall from the south-west monsoon, so its surface is highly eroded, and Cherrapunji shows a bare rocky surface with no permanent vegetation.

Beyond the textbook. For the exam: the Western Ghats are the higher, continuous range on the west edge of the Deccan. In reality: the Western Ghats are the steep, eroded western edge (escarpment) of the plateau, rather than a fold mountain like the Himalayas. They face the sea on one side and slope gently inland on the other. Why it matters: this is why west-flowing rivers are short and swift, and east-flowing rivers are long.

The Indian Desert

North-west of the Aravali lies the Great Indian Desert, or Marusthali, a land of undulating topography with longitudinal dunes and barchans. Rainfall is below 150 mm a year, so the climate is arid and vegetation thin. The region was under the sea in the Mesozoic era. Wood fossils at Aakal and marine deposits near Brahmsar, Jaisalmer, support this, and the fossils are estimated to be about 180 million years old.

The Coastal Plains

India's coast is divided into the western and eastern plains. They look alike on a map but formed in opposite ways.

The western coast has natural ports such as Kandla, Mazagaon, JLN Port Nhava Sheva, Marmagao, Mangalore and Cochin. It is divided into the Kachchh and Kathiawar coast (Gujarat), the Konkan coast (Maharashtra), the Goan coast and the Malabar coast (Karnataka and Kerala). The kayals are used for fishing, inland navigation and tourism, and the Nehru Trophy Vallamkali (boat race) is held every year in the Punnamada Kayal in Kerala. The textbook notes the belief that the city of Dwaraka, once on the west coast of the mainland, now lies under water.

Cause and effect. Because the sea has drowned the western edge, the land is narrow and the coast has deep sheltered inlets, which are ideal for ports. Because the eastern coast has risen out of the sea, the plain is broad and the shelf wide, so rivers build deltas but harbours are fewer.

The Islands

India has two major island groups, one in the Bay of Bengal and the other in the Arabian Sea.

The two principal groups of islets in the Andaman and Nicobar chain are Ritchie's archipelago and the Labyrinth island. Other peaks are Mount Diavolo (515 m, Middle Andaman), Mount Koyob (460 m, South Andaman) and Mount Thuillier (642 m, Great Nicobar). The coastline has coral deposits and beaches, and the islands receive convectional rainfall and have equatorial vegetation. On 26 December 2004 the islands suffered a devastating tsunami that also hit other coasts of the Indian Ocean.

Beyond the textbook. For the exam: Barren Island is the only active volcano in India. In reality: Narcondam, also in the Andaman Sea, is classed as a dormant volcano, and Barren Island has erupted several times in recent decades. Why it matters: the words "active" and "dormant" are what the question really tests, so read them carefully.

Exam traps

60-second revision

Where this chapter connects

• Previous: India gave the location and size that frame these divisions.
• Next: Drainage System explains how rivers flow across the Himalayas, plateau and plain described here.
• Background: The Origin and Evolution of the Earth and Distribution of Oceans and Continents explain plate movement, and Geomorphic Processes explains endogenic and exogenic forces.