Chapter 2 Structure And Physiography

The Earth has a long and complex history, and its current landforms have evolved over millions of years. The Earth is estimated to be approximately 4.6 billion years old. Geological changes, including the formation of continents and oceans, have been shaped by the continuous interplay of endogenic (internal) and exogenic (external) forces and the lateral movements of tectonic plates. These forces and processes have created the diverse surface and subsurface features we see today.

Millions of years ago, the Indian plate was situated south of the equator and was much larger, forming part of the Indo-Australian plate. This supercontinent, Gondwanaland, began to break apart. The Indian plate embarked on a long northward journey, while the Australian plate moved southeastward. The northward movement of the Indian plate is still ongoing and has had profound consequences on the physical environment of the Indian subcontinent.

Significant consequences of the Indian plate's northward movement include:

• Formation of the Himalayas: The collision of the Indian plate with the Eurasian plate resulted in the folding and uplift of the Tethys Sea sediments, creating the young, fold mountains of the Himalayas. This process continues, contributing to the ongoing rise of the Himalayas.
• Formation of the Indo-Ganga-Brahmaputra Plain: The depression created in front of the rising Himalayas was filled with sediments brought by rivers, forming the vast northern plains.
• Volcanic Activity: The movement over a hotspot is believed to have contributed to the formation of the Deccan Traps.
• Seismic Activity: The convergence and collision zone along the Himalayas is highly active seismically, leading to frequent earthquakes.

Based on the variations in its geological structure and formation history, India can be broadly divided into three major geological divisions, which correspond closely to its major physical features:

1. The Peninsular Block
2. The Himalayas and other Peninsular Mountains (referring to the younger, folded mountains like the Himalayas)
3. Indo-Ganga-Brahmaputra Plain

The Peninsular Block

The Peninsular Block is the oldest and most stable landmass of India. Its northern boundary is generally considered to run from the Rann of Kachchh eastward along the western flank of the Aravali range near Delhi, then roughly parallel to the Yamuna and Ganga rivers up to the Rajmahal Hills, and extending into the Ganga delta. Extensions of this block are also found in the northeast (Karbi Anglong and Meghalaya Plateau, separated by the Malda fault) and in the west (parts of Rajasthan). In Rajasthan, the underlying Peninsular block is covered by desert features.

Geologically, the Peninsula is formed from ancient, rigid crystalline rocks, primarily gneisses and granites. It has remained a stable, rigid block since the Cambrian period, largely unaffected by major folding, although some parts have undergone vertical movements and block faulting (e.g., rift valleys of the Narmada, Tapi, Mahanadi, and fault-block mountains like the Satpura range). Parts of the western coast have also been submerged beneath the sea.

The mountains in the Peninsula, such as the Aravali hills, Nallamala hills, Javadi hills, Veliconda hills, Palkonda range, and Mahendragiri hills, are typically relict and residual mountains – remnants of much older mountain systems that have been heavily eroded over long periods. River valleys in the Peninsula are generally shallow with gentle gradients compared to the steep, young valleys of the Himalayas. Most rivers flowing eastward across the Peninsula form deltas before draining into the Bay of Bengal (e.g., Mahanadi, Krishna, Godavari, Kaveri deltas).

The Himalayas And Other Peninsular Mountains

This geological division primarily refers to the Himalayas and associated younger fold mountain ranges. In contrast to the stable Peninsular Block, the Himalayas are geologically young, weak, and flexible mountains formed relatively recently by tectonic activity (the collision of the Indian and Eurasian plates). They are still actively being shaped by both endogenic (tectonic forces) and exogenic (erosion, weathering) processes, resulting in the presence of faults, folds, and thrust faults.

The Himalayan rivers are in their youthful stage of development. They flow rapidly over steep gradients and are actively downcutting, creating characteristic landforms like deep gorges (Figure 2.1 shows a gorge), V-shaped valleys, rapids, and waterfalls.

Indo-Ganga-Brahmaputra Plain

This vast alluvial plain, also known as the North Indian Plain, is the third major geological division. It was formed by the deposition of sediments from the three major river systems: the Indus, the Ganga, and the Brahmaputra. The plain developed in a large depression (a geo-syncline) that formed in front of the Himalayas as they were uplifted, primarily during the third phase of Himalayan mountain formation, starting around 64 million years ago. Since then, this depression has been progressively filled with alluvial deposits transported by the Himalayan rivers and, to a lesser extent, Peninsular rivers.

The average depth of alluvial deposits in these plains is substantial, ranging from 1,000 to 2,000 meters. This geological history has resulted in significant variations in structure and relief across India, profoundly influencing its physiography and other geographical characteristics.

The Northern And North-eastern Mountains

The physiography of an area is shaped by its underlying geological structure, the processes acting upon it, and the stage of its geomorphological development. India's physical landscape exhibits great diversity. The north features rugged mountains, while the south is a stable plateau. In between lies the extensive northern plain.

Based on these macro-level variations, India is divided into six major 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 Northern and North-eastern Mountains system primarily comprises the Himalayas and the associated hills in the northeast. The Himalayas are a series of parallel mountain ranges. The main ranges include the Greater Himalayas (or Himadri), the Lesser Himalayas (or Himachal), and the Shiwaliks (or Outer Himalayas). The orientation of these ranges varies regionally: generally northwest-southeast in the northwest, east-west in the Darjiling and Sikkim regions, southwest-northeast in Arunachal Pradesh, and north-south in Nagaland, Manipur, and Mizoram.

The Great Himalayan range (the central axial range) is approximately 2,500 km long from east to west and varies in width from 160 to 400 km from north to south. The Himalayas form a significant physical barrier between the Indian subcontinent and Central/East Asia, influencing climate, drainage patterns, and cultural interactions.

The Himalayas exhibit considerable regional variations in relief and geomorphological features, leading to their subdivision into:

Kashmir Or Northwestern Himalayas

This region includes prominent ranges like the Karakoram, Ladakh, Zaskar, and Pir Panjal. The area between the Greater Himalayas and the Karakoram is a cold desert. Between the Greater Himalayas and the Pir Panjal range lies the famous Kashmir valley with Dal Lake. Significant glaciers like Baltoro and Siachen are found here. The Kashmir Himalayas are known for 'Karewa' formations, thick deposits of glacial clay embedded with moraines, suitable for saffron cultivation. Important passes include Zoji La, Banihal, Photu La, and Khardung La. The region has freshwater lakes (Dal, Wular) and saltwater lakes (Pangong Tso, Tso Moriri). The main rivers are the Indus and its tributaries Jhelum and Chenab. Srinagar, on the Jhelum, is notable as the Jhelum river in the valley forms meanders despite being in its youth stage, due to a local base level provided by a former larger lake. This region is a major tourist and pilgrimage destination (Vaishno Devi, Amarnath Cave).

The Himachal And Uttarakhand Himalayas

Located between the Ravi river in the west and the Kali river in the east, this region is drained by the Indus (Ravi, Beas, Satluj) and Ganga (Yamuna, Ghaghara) river systems. It includes an extension of the Ladakh cold desert in the Spiti subdivision. All three Himalayan ranges (Great Himalayas, Lesser Himalayas, Shiwaliks) are present. The Lesser Himalayas here are known as Dhaoladhar (Himachal) and Nagtibha (Uttarakhand). Hill stations like Shimla, Mussoorie, Nainital, and cantonment towns developed in the Lesser Himalayas (1,000-2,000 m altitude). The Shiwaliks and 'Dun formations' (longitudinal valleys) are characteristic features, with Dehra Dun being the largest. The Great Himalayan valleys are home to nomadic groups like the Bhotia's. The 'Valley of Flowers' and pilgrimage sites (Gangotri, Yamunotri, Kedarnath, Badrinath) are located here. The region features famous river confluences (Prayags).

The Darjiling And Sikkim Himalayas

Situated between the Nepal and Bhutan Himalayas, this small but significant region features fast-flowing rivers (like Tista), high peaks (Kanchenjunga), and deep valleys. It has a mixed population, including Lepchas in the higher reaches. The British introduced tea plantations here due to favorable conditions (moderate slope, rich soil, well-distributed rainfall). Unlike other sections, the Shiwaliks are absent; instead, 'duar formations' (alluvial plains at the foothills) are important and used for tea gardens. The region is known for its scenic beauty, rich flora, and fauna, especially orchids.

The Arunachal Himalayas

Extending from east of the Bhutan Himalayas to the Diphu Pass, these ranges are generally aligned southwest to northeast. Important peaks include Kangtu and Namcha Barwa. Fast-flowing rivers (Kameng, Subansiri, Dihang, Dibang, Lohit) cut deep gorges (e.g., Brahmaputra after Namcha Barwa) and have high hydroelectric potential. The region is inhabited by various ethnic tribal communities (Monpa, Abor, Mishmi, Nyishi, Nagas) who traditionally practice Jhumming (shifting cultivation). Rich in biodiversity preserved by indigenous communities. Inter-valley transportation is limited due to rugged terrain; most interactions occur through the 'duar' region along the Assam border.

The Eastern Hills And Mountains

Located in northeastern India, these hills are part of the Himalayan system, running mainly north-south. They are known by local names: Patkai Bum, Naga hills, Manipur hills, and Mizo or Lushai hills (Figure 2.8 shows Mizo Hills). These are relatively low hills compared to the Himalayas. They are home to numerous tribal groups practicing Jhum cultivation and are separated by small rivers. The Barak river is important in Manipur and Mizoram. Manipur is notable for Loktak lake (Figure 2.9), a large lake in the center surrounded by mountains. Mizoram is known as the 'Molassis basin' due to its soft, unconsolidated deposits. Rivers in Nagaland feed the Brahmaputra, while rivers in eastern Manipur are tributaries of the Irrawady in Myanmar.

Image showing the relatively low, rounded hills characteristic of the Mizo Hills in northeastern India.

Image showing Loktak Lake in Manipur, famous for its floating biomass islands (phumdis).

The Northern Plain

The Northern Plain (Indo-Ganga-Brahmaputra Plain) is a vast, fertile lowland formed by the alluvial deposits of the Indus, Ganga, and Brahmaputra river systems (Figure 2.10). It extends about 3,200 km from east to west, with an average width of 150-300 km. The alluvial deposits are very deep, reaching 1,000-2,000 m.

Satellite image showing the extensive, flat Northern Plain of India formed by river sediments.

Based on relief and sediment characteristics, the northern plain can be divided into three zones from north to south:

• Bhabar: A narrow belt (8-10 km wide) parallel to the Shiwalik foothills. Rivers descending from the mountains deposit coarse sediments (boulders, pebbles) here, often causing streams to disappear underground.
• Tarai: South of the Bhabar belt (10-20 km wide). Streams re-emerge here without well-defined channels, creating marshy and swampy conditions. Historically characterized by dense forests and rich wildlife, much of it is now cleared for agriculture.
• Alluvial Plains: South of the Tarai, comprising older and newer alluvial deposits.
  • Bhangar: Older alluvium, forms slightly elevated terraces above flood levels. Less fertile but stable.
  • Khadar: Newer alluvium deposited by rivers during floods, found in lower areas close to river channels. Highly fertile but prone to flooding.

These plains exhibit features of mature river stages, such as sand bars, meanders, oxbow lakes, and braided channels. The Brahmaputra plains are known for riverine islands (like Majuli, the largest river island) and sand bars. Many areas are flood-prone, leading to shifting river courses (braiding). The mouths of the Ganga and Brahmaputra form the world's largest delta, the Sunderbans.

The plain is largely featureless, with a gentle slope and elevation of 50-150 m above sea level. The states of Haryana and Delhi form a water divide separating the Indus and Ganga river systems. The Brahmaputra river flows northeast to southwest through Assam before turning south into Bangladesh. The fertile alluvial soils of the Northern Plain support intensive agriculture (wheat, rice, sugarcane, jute) and a large population.

The Peninsular Plateau

The Peninsular Plateau is a large, irregular triangular tableland rising above the northern plains, with elevations typically ranging from 150 m to 600-900 m (Figure 2.11). It is the oldest and most stable landmass of India, composed primarily of ancient crystalline igneous and metamorphic rocks.

Satellite image showing part of the vast, undulating tableland characteristic of the Peninsular Plateau in India.

Its outer boundaries are marked by the Delhi ridge (extension of Aravalis) in the northwest, Rajmahal hills in the east, Gir range in the west, and Cardamom hills in the south. Extensions are also found in the northeast (Shillong and Karbi Anglong plateau), separated from the main block by the Malda fault.

The plateau consists of several smaller plateaus (patland plateaus) like Hazaribagh, Palamu, Ranchi, Malwa, Coimbatore, and Karnataka plateaus. The general slope of the plateau is from west to east, influencing the drainage pattern of most Peninsular rivers, which flow eastwards into the Bay of Bengal (e.g., Mahanadi, Godavari, Krishna, Kaveri, which form deltas). Rivers flowing westwards into the Arabian Sea are fewer (e.g., Narmada, Tapi) and typically flow through rift valleys and do not form deltas.

Physiographic features of the plateau include tors (isolated rock outcrops), block mountains, rift valleys, spurs, bare rocky surfaces, hummocky hills, and quartzite dykes suitable for water storage. Black soil is prominent in the western and northwestern parts (Deccan Trap region).

The plateau has experienced repeated episodes of uplift and subsidence with associated faulting. This geological history contributes to the varied relief. The northwest part features complex ravine and gorge topography (e.g., ravines of Chambal). Despite being stable, some fault zones (like the Bhima fault) have shown recurrent seismic activity.

Based on prominent relief features, the Peninsular plateau is divided into three broad groups:

The Deccan Plateau

This is the largest part of the Peninsular plateau, bordered by the Western Ghats in the west, Eastern Ghats in the east, and the Satpura, Maikal range, and Mahadeo hills in the north. The Western Ghats (Sahyadri in Maharashtra, Nilgiri in Karnataka/Tamil Nadu, Anaimalai and Cardamom hills in Kerala) are higher (average 1,500 m) and more continuous than the Eastern Ghats and are the origin of most Peninsular rivers. Anaimudi (2,695 m) in the Anaimalai hills is the highest peak of the Peninsular plateau, followed by Dodabetta (2,637 m) in the Nilgiri hills. The Eastern Ghats are discontinuous, lower hills (Javadi, Palconda, Nallamala, Mahendragiri ranges) that have been heavily eroded by eastward-flowing rivers. The two mountain ranges meet at the Nilgiri hills.

The Central Highlands

Located north of the Deccan Plateau, bounded by the Aravali range in the west and the Satpura range to the south (forming the northern boundary of the Deccan). The Satpura range is a relict mountain system (600-900 m elevation), highly eroded and forming discontinuous ranges. Extensions of the Peninsular plateau in the west reach Jaisalmer, covered by sand dunes. This region shows evidence of metamorphic processes, with rocks like marble, slate, and gneiss. The general elevation of the Central Highlands is 700-1,000 m, sloping towards the north and northeast. Rivers originating from the Vindhyan and Kaimur ranges are tributaries of the Yamuna (e.g., Chambal, Betwa, Ken). The Banas river, a Chambal tributary, originates in the Aravalis. The eastern part extends to the Rajmahal hills and the resource-rich Chotanagpur plateau to the south.

The Northeastern Plateau

This is an extension of the main Peninsular plateau, consisting of the Meghalaya and Karbi Anglong plateaus. It is believed to have been separated from the main Peninsular block by a large fault (Malda fault) formed due to the stress of the Indian plate's northward movement. This fault trough was later filled with river sediments. The Meghalaya plateau is further subdivided into the Garo, Khasi, and Jaintia Hills (named after tribal groups). The region is rich in minerals (coal, iron ore, limestone, uranium). It receives very high rainfall from the southwest monsoon, leading to a highly eroded surface (e.g., bare rocky surface at Cherrapunji). The Karbi Anglong plateau is a similar detached extension in Assam.

The Indian Desert

The Great Indian Desert, or Thar Desert (also known as Marusthali), lies northwest of the Aravali hills (Figure 2.12). It is an undulating sandy plain characterized by longitudinal dunes and crescent-shaped barchans. This region receives very low rainfall (below 150 mm annually) and has an arid climate with sparse vegetation.

Image showing sand dunes, typical of desert landscapes, with wavy or curved crests (could be longitudinal dunes or barchans depending on the shape).

Geological evidence suggests the region was submerged under the sea during the Mesozoic era (around 180 million years ago), supported by wood fossils found at Aakal and marine deposits near Jaisalmer. Although the underlying structure is an extension of the Peninsular plateau, the extreme arid conditions have led to surface features being primarily shaped by physical weathering and wind action, resulting in mushroom rocks, shifting sand dunes, and oases.

The desert is broadly divided into a northern part sloping towards Sindh (Pakistan) and a southern part sloping towards the Rann of Kachchh. Most rivers are ephemeral (short-lived). The Luni river in the southern part is the only significant river, though many streams disappear into the sand, contributing to inland drainage and forming temporary lakes or playas with brackish water, a source of salt. Low rainfall and high evaporation make this a severely water-deficit region.

The Coastal Plains

India has a long coastline, divided into two broad sections based on location and geomorphological processes (Figure 2.13):

Map illustrating the location and extent of the Western and Eastern Coastal Plains and the major island groups of India.

• Western Coastal Plains: Located between the Western Ghats and the Arabian Sea. This is considered an example of a submerged coastal plain, possibly due to the submergence of former land (like the mythical city of Dwaraka). This submergence has created a narrow belt along the coast and provided natural conditions for the formation of excellent natural ports and harbors (Kandla, Mumbai, JNPT, Marmagao, Mangalore, Cochin). The western coast extends from the Gujarat coast southwards. It is divided into the Kachchh and Kathiawar coast (Gujarat), Konkan coast (Maharashtra), Goan coast, and Malabar coast (Karnataka and Kerala). The plains are narrower in the middle and wider in the north and south. Rivers here are short and swift and do not form deltas. The Malabar coast is known for its 'Kayals' (backwaters), which are used for fishing, inland navigation, and tourism (e.g., Nehru Trophy boat race in Punnamada Kayal, Kerala).
• Eastern Coastal Plains: Located between the Eastern Ghats and the Bay of Bengal. This is considered an example of an emergent coast, where land has been uplifted or sea level has dropped relative to the land. The eastern coastal plain is broader than the western plain. It features well-developed deltas formed by large east-flowing rivers (Mahanadi, Godavari, Krishna, Kaveri) that deposit vast amounts of sediment before entering the Bay of Bengal. Due to its emergent nature and the presence of extensive shallow areas from deltaic deposition and a continental shelf extending up to 500 km into the sea, this coast has fewer natural harbors compared to the west coast. Important ports include Visakhapatnam, Paradip, Chennai, Tuticorin.

The Islands

India has two major groups of islands (Figure 2.14 shows a conceptual island):

Image showing a typical island surrounded by water.

• Bay of Bengal Islands (Andaman and Nicobar Islands): Located roughly between $6^\circ$N and $14^\circ$N latitude and $92^\circ$E and $94^\circ$E longitude. This group consists of about 572 islands and islets. They are believed to be the elevated peaks of submerged submarine mountains (extensions of the Arakan Yoma range of Myanmar). Some smaller islands, however, are volcanic in origin, including Barren Island, India's only active volcano, located in the Nicobar Islands. The group is broadly divided into the Andaman Islands in the north and the Nicobar Islands in the south, separated by the Ten Degree Channel ($10^\circ$ N latitude). The topography is rugged with hills; important peaks include Saddle Peak (North Andaman, 738m) and Mount Thuiller (Great Nicobar, 642m). The islands have coral deposits, beautiful beaches, receive convectional rainfall, and support equatorial type vegetation (tropical rainforest). These islands were severely impacted by the Indian Ocean Tsunami on 26 December 2004, which caused significant loss of life, habitat destruction, and geological changes like subsidence in some areas (including the lowering of Indira Point).
• Arabian Sea Islands (Lakshadweep Islands and Minicoy): Located off the coast of Kerala, scattered between $8^\circ$N and $12^\circ$N latitude and $71^\circ$E and $74^\circ$E longitude, about 280-480 km from the coast. This archipelago is made up entirely of coral deposits, representing atolls. There are about 36 islands, of which only 11 are inhabited. Minicoy is the largest island in terms of area (4.53 sq km). The entire group is broadly divided by the Eleven Degree Channel ($11^\circ$ N latitude), with Amini Island group to the north and Canannore Island group to the south. The islands often have storm beaches on their eastern side, composed of unconsolidated pebbles and boulders. They have a tropical climate and vegetation.

Comparison of the island groups:

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