Chapter 2 — Structure and Physiography

India's structure and physiography arise from three geological divisions — the Peninsular Block, the Himalayas, and the Indo-Ganga-Brahmaputra Plain — which together produce six physiographic divisions: Northern and Northeastern Mountains, Northern Plains, Peninsular Plateau, Indian Desert, Coastal Plains, and Islands.

India's landforms evolved over 4,600 million years through endogenic and exogenic forces. The Indian plate, once south of the equator, drifted northward, shaping the Himalayas. India has three geological divisions: the ancient, rigid Peninsular Block formed by gneisses and granites, stable since the Cambrian period; the young, tectonically active Himalayas and Peninsular Mountains; and the Indo-Ganga-Brahmaputra Plain, a former geo-synclinal depression filled with alluvial deposits 1,000–2,000 m deep. These divisions yield six physiographic units: the Northern and Northeastern Mountains, the Northern Plains (3,200 km long), the Peninsular Plateau, the Indian Desert (Marusthali), the Coastal Plains, and the Islands — including the Andaman and Nicobar group and Lakshadweep.

1. 01India has three geological divisions: the Peninsular Block, the Himalayas and other Peninsular Mountains, and the Indo-Ganga-Brahmaputra Plain.
2. 02The Peninsular Block is the oldest and most stable landmass, composed mainly of ancient gneisses and granites, and has stood as a rigid block since the Cambrian period.
3. 03The Himalayas are young, weak, and flexible in geological structure — tectonic in origin with landforms such as gorges, V-shaped valleys, rapids, and waterfalls formed by fast-flowing youthful rivers.
4. 04The Northern Plains extend approximately 3,200 km east to west with an average width of 150–300 km; alluvial deposits reach 1,000–2,000 m deep, and from north to south the plains are subdivided into Bhabar, Tarai, Bhangar, and Khadar zones.
5. 05The Peninsular Plateau is divided into three parts — Deccan Plateau, Central Highlands, and Northeastern Plateau; Anaimudi (2,695 m) on the Anaimalai hills of the Western Ghats is the highest peak of the Peninsular plateau.
6. 06The Great Indian Desert (Marusthali) receives below 150 mm of rainfall annually and features barchans, longitudinal dunes, mushroom rocks, and mostly ephemeral rivers; marine deposits near Jaisalmer indicate it was under the sea during the Mesozoic era.
7. 07The western coastal plain is a submerged, narrow coast with no deltas; the Malabar coast has kayals (backwaters) used for fishing and navigation; the eastern coastal plain is broader and emergent with well-developed deltas of the Mahanadi, Godavari, Krishna, and Kaveri.
8. 08The Bay of Bengal island group has about 572 islands (Andaman and Nicobar), separated by the Ten Degree Channel; Lakshadweep in the Arabian Sea is built entirely of coral deposits, with Minicoy as the largest island at 453 sq. km.