India comprises the bulk of the Indian subcontinent and lies atop the minor Indian tectonic plate, which in turn belongs to the Indo-Australian Plate. India's defining geological processes commenced 75 million years ago when the Indian subcontinent, then part of the southern supercontinent Gondwana, began a north-eastward drift across the then-unformed Indian Ocean that lasted fifty million years. The subcontinent's subsequent collision with, and subduction under, the Eurasian Plate bore aloft the planet's highest mountains, the Himalayas. They abut India in the north and the north-east. In the former seabed immediately south of the emerging Himalayas, plate movement created a vast trough that has gradually filled with river-borne sediment; it now forms the Indo-Gangetic Plain. To the west lies the Thar Desert, which is cut off by the Aravalli Range.
The original Indian plate survives as peninsular India, the oldest and geologically most stable part of India and extends as far north as the Satpura and Vindhya ranges in central India. These parallel ranges run from the Arabian Sea coast in Gujarat in the west to the coal-rich Chota Nagpur Plateau in Jharkhand in the east. To the south the remaining peninsular landmass, the Deccan Plateau, is flanked on the west and east by the coastal ranges, the Western and Eastern Ghats respectively; the plateau contains the nation's oldest rock formations, some over one billion years old. Constituted in such fashion, India lies to the north of the equator between 6° 44' and 35° 30' north latitude[e] and 68° 7' and 97° 25' east longitude.
India's coast is 7,517 kilometres (4,700 mi) long; of this distance, 5,423 kilometres (3,400 mi) belong to peninsular India and 2,094 kilometres (1,300 mi) to the Andaman, Nicobar, and Lakshadweep Islands. According to the Indian naval hydrographic charts, the mainland coast consists of the following: 43% sandy beaches, 11% rocky coast including cliffs, and 46% mudflats or marshy coast. Major Himalayan-origin rivers that substantially flow through India include the Ganges (Ganga) and the Brahmaputra, both of which drain into the Bay of Bengal. Important tributaries of the Ganges include the Yamuna and the Kosi; the latter's extremely low gradient causes disastrous floods every year. Major peninsular rivers, whose steeper gradients prevent their waters from flooding, include the Godavari, the Mahanadi, the Kaveri, and the Krishna, which also drain into the Bay of Bengal; and the Narmada and the Tapti, which drain into the Arabian Sea. Among notable coastal features of India are the marshy Rann of Kutch in western India, and the alluvial Sundarbans delta, which India shares with Bangladesh. India has two archipelagos: the Lakshadweep, coral atolls off India's south-western coast; and the Andaman and Nicobar Islands, a volcanic chain in the Andaman Sea. The Indian climate is strongly influenced by the Himalayas and the Thar Desert, both of which drive the economically and culturally pivotal summer and winter monsoons. The Himalayas prevent cold Central Asian katabatic winds from blowing in, keeping the bulk of the Indian subcontinent warmer than most locations at similar latitudes. The Thar Desert plays a crucial role in attracting the moisture-laden south-west summer monsoon winds that, between June and October, provide the majority of India's rainfall. Four major climatic groupings predominate in India: tropical wet, tropical dry, subtropical humid, and montane.