India forms the northwestern portion of the Indian-Australian plate.
 
 
 

About 500 million years ago, the Indian landmass was part of the super-continent of Gondwanaland. When the Gondwanaland started to break up, the Indian-Australian plate with the Indian landmass started to move towards the Eurasian plate and collided with the latter about 50 million years ago. When the two plates collided, the northern edge of the Indo-Australian plate was thrust under the Eurasian plate at a low angle thus leading to the formation of the Himalayas.
The rock from the top of the underthrusting Indo-Australian plate was sliced off and these slices were thrown back onto the northern edge of the Indo-Australian plate leading to the formation of much of the Himalayas. The northern edge of the Indo-Australian plate sank under the heavy Himalayas. The present-day Indian landmass has three basic structural units: the Himalayas to the north, the Deccan plateau to the south and the Indo-Gangetic plain in the middle.

 
The Himalayas:

In Sanskrit hima means "snow", and alaya means "abode".

Indians have all the reason to be proud of the most magnificent, the largest and the highest mountain range in the whole world. It is 2500 Km long stretching from the Nanga Parbat in the east to the Namcha Barwa Peak in the Tibet. The width of the Himalayas ranges from 250-350 Km. The Himalayas can be divided into three longitudinal belts: the greater, the lesser and the outer Himalayas. Because of the continued movement of the Indo-Australian Plate under the Eurasian plate, the Himalayas continue to rise and subsequently/unfortunately earthquakes are/should be common.

The outer Himalayas or the Shiwalik hills: These are the southernmost of the three mountain belts described above. Their height ranges from 3000 to 5000 feet. The range tapers off in West Bengal. The Siwaliks are immediately north to the Indo-Gangetic plain and provide much of the sediment deposited onto the plain.

The lesser Himalayas: These are immediately north to the Siwaliks and are seperated from the Siwaliks by a fault. They are also known as the ‘Himachals’ which in Sanskrit means hima or ‘snow’ and acal or ‘mountain’. The height of the lesser Himalayas is about 12000 to 15000 feet.

The great Himalayas: Known in Sanskrit as Himadri. The height of the peaks is generally above 16000 feet. Between the Great and Lesser Himalayas are many fertile longitudinal vales. The most famous and the largest of them is the Kashmir vale which is about 4400 square km. The Pir Panjal Range forms the southwest flank of the Kashmir vale. Some of the world’s highest peaks are in the great Himalayas. Mount Everest (29028 feet) should be in India but unfortunately is on the border of Nepal and China. But India has the second highest peak in the world, which is Kanchenjunga (28208 feet).
 
 
 
 

The Indo-Gangetic plain:

This plain comprises the youngest land of the Indian subcontinent. The plain was originally under water (there was a shallow sea in the area). However due to continued erosion of the Himalayas by rain and snow, the rivers have deposited a large alluvial plain up to 6000 feet in depth in the region of Sindh, Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal, Assam and Bangladesh. Hence the present day Indo-Gangetic plain is above sea level.

The Ganges basin forms the central and principal part of this basin. Towards the east is the Brahmaputra plain and to the west is the Indus plain. The Indo-Gangetic plain has a gentle slope towards the oceans. The Ganges basin has a slope of about 10cm a km. Due to this slope the rivers flow towards the ocean.
The Ganges and Brahmaputra both form a delta which is mostly situated in Bangladesh but a part of it is also present in the state of West Bengal. This region is susceptible to periodic flooding due to the heavy rains in the monsoon season in the Gangetic plain.
 
 
 
 
 

The Deccan Plateau:

The Vindhya Range is considered as the divide between the Indo-Gangetic plain and the Deccan (in Sanskrit: daksina or ‘south’). The Deccan is the oldest landmass of the Indian subcontinent.

During the time of the Gondwananland, the above sea portion of the Indo-Australian plate was the Deccan and the present day Australia. Everything north of the Deccan plateau was under shallow sea. The plateau is mainly between 1000 and 2500 feet above sea level and slopes towards the east.Therefore, Godavari and Krishna rivers flow towards the east.
The plateau has a chain of hills called the western Ghats along the west coast and a similar chain along the eastern coast called the eastern Ghats.
The highest peak in the western Ghats is the Anai Mudi (8842 feet) and the highest peak in the eastern Ghats is the Arma Konda (5512 feet).
The western and eastern Ghats merge near the southern tip of the peninsula.
The rivers draining into the bay of Bengal are the Mahanadi, Godavari, Krishna and Kaveri. All of these except the Mahanadi arise in the western Ghats and flow east into the bay of Bengal.