Geography: Class 11 (Part 1) — Chapter 2: Structure and Physiography

Geological Divisions of India

Based on the variations in its geological structure and formations, India can be divided into three major geological divisions which broadly follow the physical features of the subcontinent.

1. The Peninsular Block

The Peninsular Block is an ancient, rigid, and stable landmass primarily formed by a great complex of very ancient gneisses and granites.

Boundaries: Runs from Kachchh along the western flank of the Aravali Range near Delhi, then roughly parallel to the Yamuna and Ganga up to the Rajmahal Hills and the Ganga delta. Extensions include the Karbi Anglong and the Meghalaya Plateau in the northeast, and Rajasthan in the west.
Structure: Standing as a rigid block since the Cambrian period, with exceptions of western coast submergence and minor tectonic activities.
Features: Subjected to vertical movements and block faulting, creating rift valleys of the Narmada, Tapi, and Mahanadi, and the Satpura block mountains. It contains relict and residual mountains like the Aravali, Nallamala, Javadi, Veliconda, and Palkonda ranges.

The gradient of Peninsular rivers is generally low, and the valleys are shallow. The formula for calculating river gradient is:

$$\text{Gradient} = \frac{\text{Vertical Drop (m)}}{\text{Horizontal Distance (km)}}$$

2. The Himalayas and Other Peninsular Mountains

Unlike the rigid Peninsular Block, the Himalayas and other Peninsular mountains are young, weak, and flexible in geological structure.

Origin: Tectonic in origin, formed by the collision of tectonic plates.
Geomorphic Features: Dissected by fast-flowing rivers in their youthful stage, resulting in landforms like gorges, V-shaped valleys, rapids, and waterfalls.

3. Indo-Ganga-Brahmaputra Plain

The third geological division comprises the vast plains formed by the river systems of the Indus, the Ganga, and the Brahmaputra.

Origin: Originally a geo-synclinal depression that attained maximum development during the third phase of Himalayan mountain formation (approx. 64 million years ago).
Deposits: Gradually filled by alluvial sediments brought by Himalayan and Peninsular rivers, with an average depth of alluvial deposits ranging from 1,000 to 2,000 meters.

Physiographic Divisions of India

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

The Northern and North-eastern Mountains
The Northern Plain
The Peninsular Plateau
The Indian Desert
The Coastal Plains
The Islands

1. The Northern and North-eastern Mountains

Consists of the Himalayas and the Northeastern hills, forming a continuous physical, climatic, and cultural barrier.

Himalayan Ranges: Include parallel ranges such as the Greater Himalayas (Himadri), Himachal, and Shiwalik.
Orientation: Northwest to southeast in the north; east-west in Darjiling and Sikkim; southwest to northwest in Arunachal Pradesh; and north-south in Nagaland, Manipur, and Mizoram.
Dimensions: Approximate length of the Great Himalayan range is 2,500 km from east to west, with width varying between 160 and 400 km.

2. The Northern Plains

Formed by the alluvial deposits of the Indus, Ganga, and Brahmaputra rivers, extending approximately 3,200 km from east to west with a width of 150-300 km.

The plains are divided from north to south into three major zones:

Bhabar: A narrow belt (8-10 km wide) parallel to the Shiwalik foothills where rivers deposit heavy rocks and boulders, often disappearing underground.
Tarai: A marshy and swampy belt (10-20 km wide) south of Bhabar where underground streams re-emerge, supporting dense forests and wildlife.
Alluvial Plains: Divided into Bhangar (older alluvium forming terraces) and Khadar (newer, fertile alluvium deposited by annual floods).

3. The Peninsular Plateau

An irregular triangle rising from 150 m to an elevation of 600-900 m, representing one of the oldest and most stable landmasses of India.

It is divided into three broad groups:

The Deccan Plateau: Bordered by the Western Ghats (Sahyadri, Nilgiri, Anaimalai, Cardamom hills) and Eastern Ghats. The highest peak is Anaimudi (2,695 m).
The Central Highlands: Bounded by the Aravali range, containing scarped plateaus, metamorphic rocks, and sand dunes called barchans.
The Northeastern Plateau: An extension of the main Peninsular block, comprising the Meghalaya (Garo, Khasi, Jaintia Hills) and Karbi Anglong plateaus, rich in mineral resources.

4. The Indian Desert

Located northwest of the Aravali hills, also known as Marusthali.

Climate: Arid climate with low rainfall (below 150 mm per year) and low vegetation cover.
Features: Dotted with longitudinal dunes, barchans, mushroom rocks, and oasis formations.
Drainage: Rivers are ephemeral (e.g., Luni River). Inland drainage is common, terminating in brackish lakes or playas.

5. The Coastal Plains

India has a long coastline divided into the Western Coastal Plains and the Eastern Coastal Plains based on location and geomorphic processes.

Feature	Western Coastal Plain	Eastern Coastal Plain
Nature	Submerged coast, narrow belt	Emergent coast, broader belt
Divisions	Kachchh, Kathiawar, Konkan, Goan, and Malabar Coast	Northern Circars and Coromandel Coast
Deltas	No major deltas; rivers form estuaries	Large deltas formed by Mahanadi, Godavari, Krishna, Kaveri
Special Features	Kayals (backwaters) used for fishing and tourism	Continental shelf extends up to 500 km into the sea

6. The Islands

India has two major island groups:

Bay of Bengal (Andaman and Nicobar Islands): Consists of about 572 islands/islets situated between 6°N-14°N and 92°E-94°E. Separated by the Ten Degree Channel. Contains Barren Island, the only active volcano in India.
Arabian Sea (Lakshadweep and Minicoy Islands): Scattered between 8°N-12°N and 71°E-74°E. Built entirely of coral deposits. Minicoy is the largest island. Separated by the Nine Degree Channel.