India's geography: a plate that collided and never stopped

What the six regions are

India lies between 8°4' north on the mainland and 37°6' north, and between 68°7' and 97°25' east, covering 3,287,263 square kilometres.

The country divides into six physiographic regions, and the division is worth learning because nothing else describes the place as efficiently.

The Northern Mountains are the Himalayas. The Indo-Gangetic Plain, also called the Northern Plains, lies below them. The Peninsular Plateau occupies most of the south and contains the Aravalli, Vindhya and Satpura ranges, the Eastern and Western Ghats, and the Malwa, Chhota Nagpur and Deccan plateaus. The Thar Desert sits in the northwest. The Coastal Plains run along both flanks of the peninsula. And the islands, the Andaman and Nicobar group in the Bay of Bengal and Lakshadweep in the Arabian Sea, sit offshore.

Why the collision explains the rest

India sits entirely on the Indian Plate, which formed when it split from Gondwana, the southern part of the ancient supercontinent, and moved north.

An arc of mountains comprising the Himalayas, the Hindu Kush and the Patkai ranges defines the northern frontier of the subcontinent, and those ranges were formed by the ongoing collision of the Indian plate with the Eurasian one. The process has not stopped. The mountains are still rising, and the northern part of the country is seismically active as a permanent condition rather than an occasional one.

The collision produced three things at once. It raised the mountain arc. It depressed the ground immediately south of that arc into a long trough. And it supplied, through erosion of the rising mountains, the sediment that has filled that trough to enormous depth.

The Indo-Gangetic Plain is therefore both flat and deep: alluvium accumulated over millions of years, watered by rivers descending from the range that produced it. It is among the most agriculturally productive ground on earth, and it exists because of the mountains above it. A plain that would be unremarkable if it had formed any other way is exceptional because it was assembled out of a rising mountain range by the rivers coming off it.

What the rivers do

The Ganges is the longest river originating in India, and the Ganges-Brahmaputra system occupies most of northern, central and eastern India.

That system is the reason the north is settled as densely as it is. A plain with deep alluvial soil, a monsoon climate and two great rivers running through it can support agriculture at a density that mountains and plateaus cannot, and the population distribution of India is largely a map of where that combination holds.

The southern rivers behave differently. The Deccan Plateau occupies most of southern India, and the four major rivers of the south, the Mahanadi, the Godavari, the Krishna and one more, run east across it to the Bay of Bengal. In doing so they have eroded and quadrisected the Eastern Ghats, which is why that range is discontinuous while its western counterpart is not.

Why the two coasts differ

The peninsula has a range down each side and they are not alike.

The Western Ghats, also called the Sahyadri, run along the western edge of the Deccan Plateau and separate it from a narrow coastal plain along the Arabian Sea. They are continuous, and being continuous they intercept the monsoon arriving from the southwest, which makes their seaward slopes among the wettest places in the country and the plateau behind them markedly drier.

The Eastern Ghats are discontinuous, broken into segments by the four southern rivers cutting through them on their way to the sea. The eastern coastal plain behind them is wider than the western one, and the rivers arrive there having crossed the whole plateau.

The deltas are the consequence worth naming. Rivers that have crossed an entire plateau arrive on the eastern coast carrying the material they eroded on the way, and they deposit it where they meet the Bay of Bengal. That has built some of the largest deltaic country anywhere, ground that is extremely fertile, densely settled and, being barely above sea level at the mouth of rivers draining a subcontinent, exposed to both flood and cyclone.

So one coast has a wall and a narrow shelf, the other has gaps and deltas. The ports and the historical trade patterns of the two sides differ accordingly.

What the monsoon does to all of it

One weather system governs the agricultural year across the whole country, and its path is set by the terrain described above.

Moist air arrives from the southwest and meets the Western Ghats first. Being continuous, the range forces that air upward and takes most of its water on the seaward slopes, which is why the narrow western coastal plain and the hills behind it are among the wettest ground in India and the Deccan Plateau immediately beyond them is markedly drier.

What crosses the peninsula continues north and northeast, and what reaches the Indo-Gangetic Plain runs into the Himalayan wall, which stops it and drops it. The plain therefore receives rain from a system that has already given up part of its load in the south, and the mountains that produced the plain's soil also produce its water.

The northwest is what is left over. By the time the system reaches the western end of the plain there is very little moisture in it, and the Thar Desert is the result. The same monsoon that makes the eastern plain among the most productive ground on earth leaves the western end of the same plain arid.

That single mechanism explains the country's agricultural map, its population distribution and the fact that a bad monsoon is a national event rather than a regional one.

What the desert is

The Thar Desert occupies the northwest, and its composition is not what the word suggests.

Sand dunes make up only about 10% of the region. The other 90% of it is craggy rock, compacted salt-lake bottoms, and interdunal and fixed-dune areas. It is arid country rather than a sand sea, and the distinction determines what can be done with it: fixed dunes and rock support grazing and some cultivation, and moving sand supports neither. The salt-lake bottoms are a separate matter again, useful for extraction and for nothing that grows.

Its position is the other significant thing about it. The desert sits at the western end of the Indo-Gangetic Plain, so the same plain that is among the most productive ground on earth at its eastern end is desert at its western one. The gradient between those two states is rainfall, and the rainfall is the monsoon.

What the plateau is made of

The Peninsular Plateau is the oldest part of India and it behaves differently from everything north of it.

It carries three mountain systems that are not the Himalayas: the Aravalli, the Vindhya and the Satpura ranges, all far older and far more eroded than the northern arc. It carries the Malwa, Chhota Nagpur, Deccan and Kutch Kathiawar plateaus, and the southern granulite terrain. And it is bounded on both sides by the Ghats.

Being ancient and stable, it is mineral-rich where the northern plain is not, which is why India's heavy industry and its mining sit largely on the plateau while its agriculture sits largely on the plain. Two of the country's economic geographies are stacked in different places for reasons that are geological rather than historical.

The plateau is also the part of the country that was never underwater in the recent geological past and never buried in alluvium, so its soils are thinner, its rivers run in older valleys, and its agriculture depends more directly on the monsoon reaching it across the Western Ghats.

The two island groups sit in different seas and belong to different systems.

The Andaman and Nicobar islands lie in the Bay of Bengal, far to the east of the mainland and much closer to Southeast Asia than to peninsular India. They are the emergent tops of a submarine ridge on the same tectonic boundary that produces the seismicity of the region.

Lakshadweep lies in the Arabian Sea to the west and is coral, low and small.

Between them they extend the country's maritime reach a long way in both directions from a coastline that is otherwise a single peninsula. A country whose mainland runs from 8° to 37° north acquires, through those two groups, a presence in two separate seas and an exposure to both.

How the borders were drawn

India's frontiers are a mixture of physical features and lines drawn by people, and the difference is legible on the map.

The northern frontiers are defined largely by the Himalayan range, where the country borders China, Bhutan and Nepal. The western border with Pakistan lies in the Karakoram and the western Himalaya at its northern end.

Elsewhere the lines are recent and administrative. The borders with Pakistan and Bangladesh were delineated according to the Radcliffe Line, created in 1947 during the partition described in the history article. The western border with Pakistan extends up to 3,323 kilometres. The Line of Actual Control, the effective border with China, traverses 4,057 kilometres along Ladakh, Himachal Pradesh, Uttarakhand and the states to the east.

The catalogue records that several of these lines are disputed and takes no position on any of them. What is worth noting geographically is that the country's longest frontiers run along and through the highest mountains on earth. That makes them difficult to demarcate, difficult to patrol and difficult to agree, and it means the northern border is a line drawn across terrain where the ground itself is still moving.

What the geography settles

Three things follow from the arrangement above, and they run through the rest of this catalogue's articles on India.

The population is where the alluvium and the water are. The Indo-Gangetic Plain carries a density that the plateau, the mountains and the desert cannot, and that has been true for as long as there has been agriculture here.

The country's climate is a single system with a date. The monsoon arriving from the southwest is intercepted by the Western Ghats, crosses the peninsula, and reaches the northern plain, and the agricultural year of a country of this size turns on when it arrives and how much it brings. No other single variable matters comparably.

The north is geologically active and will remain so. A collision that is still in progress means seismicity along the whole northern arc, and the highest elevations in the country, including Kangchenjunga at 8,586 metres, stand on ground that is still moving.

The low point is Kuttanad in Kerala, two metres below sea level, on the wet southwestern coast behind the Western Ghats. Between it and the northern peaks lies almost every terrestrial environment except the polar.