Structure and Physiography

Introduction

The Earth has a long and complex history, with its current landforms evolving over approximately 4600 million years. The physical features we observe today are the result of continuous geological processes driven by endogenic (internal) and exogenic (external) forces. Endogenic forces include tectonic plate movements, volcanic activity, and mountain building, while exogenic forces encompass weathering, erosion, and sedimentation. The Indian subcontinent's geological evolution is closely linked to the movement of the Indian plate, which was once located south of the equator and was much larger, including what is now the Australian plate. Over millions of years, the Indian plate drifted northwards, colliding with the Eurasian plate, a movement that continues today and has profound effects on the region's physical environment, including the formation of the Himalayas. This chapter introduces the geological structure and physiographic divisions of India, explaining how these features have been shaped by geological processes and plate tectonics. India’s geological structure is broadly divided into three major divisions: the Peninsular Block, the Himalayas and other Peninsular mountains, and the Indo-Ganga-Brahmaputra Plain. Each division has distinct geological characteristics and physical features.

• Earth is approximately 4600 million years old with evolving landforms.
• Endogenic and exogenic forces shape Earth's surface features.
• Indian plate was south of the equator and larger in size millions of years ago.
• Northward movement of Indian plate continues, influencing India's geology.
• India’s geological divisions: Peninsular Block, Himalayas, Indo-Ganga-Brahmaputra Plain.
• Geological structure underpins India’s physiographic features.
• 📌 Endogenic forces: Internal geological processes like tectonic movements.
• 📌 Exogenic forces: External processes like weathering and erosion.
• 📌 Indian plate: A tectonic plate that includes the Indian subcontinent.

Geological Structure of India

India’s geological structure forms the foundation for its diverse physiographic features. The country can be divided into three broad geological divisions based on rock formations and tectonic history: (i) The Peninsular Block, (ii) The Himalayas and other Peninsular Mountains, and (iii) The Indo-Ganga-Brahmaputra Plain. The Peninsular Block is an ancient, stable landmass composed mainly of crystalline and granitic rocks dating back to the Cambrian period and earlier. It extends from Kachchh in the west, along the Aravali range near Delhi, parallel to the Yamuna and Ganga rivers, up to the Rajmahal hills and the Ganga delta in the east. Extensions of this block include the Karbi Anglong and Meghalaya plateaus in the northeast and parts of Rajasthan in the west. The Peninsular Block has undergone vertical movements and block faulting, leading to features such as rift valleys (Narmada, Tapi, Mahanadi) and block mountains (Satpura range). The Himalayas and other Peninsular mountains, in contrast, are geologically young, weak, and flexible, formed by tectonic folding and faulting due to the ongoing collision of the Indian and Eurasian plates. These mountains are dissected by fast-flowing rivers, exhibiting youthful landforms like gorges, V-shaped valleys, rapids, and waterfalls. The Indo-Ganga-Brahmaputra Plain is a vast alluvial plain formed by sediments deposited in a geo-synclinal depression created during the Himalayan orogeny about 64 million years ago. The alluvial deposits here are very thick, ranging from 1000 to 2000 meters in depth. This geological diversity results in significant variations in relief and physiography across India.

• India’s geological divisions: Peninsular Block, Himalayas, Indo-Ganga-Brahmaputra Plain.
• Peninsular Block is ancient, stable, composed of gneisses and granites.
• Peninsular Block boundaries run from Kachchh to Rajmahal hills and Ganga delta.
• Himalayas are young, tectonic fold mountains, geologically weak and flexible.
• Indo-Ganga-Brahmaputra Plain formed by sedimentation in a geo-synclinal depression.
• Alluvial deposits in plains are 1000-2000 m deep.
• 📌 Peninsular Block: Ancient, stable geological region of India.
• 📌 Rift valley: A valley formed by the downward displacement of a block of the earth’s crust.
• 📌 Geo-synclinal depression: A large-scale depression in the earth’s crust filled with sediments.