The mountains that are still rising

The highest mountains on Earth are the result of a collision. Not a sudden crash, but one that has been going on for tens of millions of years — and is still happening as you read this.

A continent on the move

The surface of the Earth is broken into large plates that slowly move. Most of the time we do not notice, because the movement is measured in centimetres per year. But over millions of years, centimetres add up to entire oceans opened and closed.

India’s story is one of the most dramatic examples. According to the U.S. Geological Survey (USGS), around 80 million years ago India lay far to the south of where it is today, and it was moving north at about 9 metres per century — fast, by the standards of plate tectonics. Between India and Asia lay an ocean, the Tethys.

Over the following tens of millions of years, India travelled more than 6,000 kilometres northward.

The collision

Somewhere around 40 to 50 million years ago, the USGS explains, India met Asia. The ocean between them had closed. And because both landmasses are made of continental crust, which is relatively light and does not easily sink back into the Earth’s interior, neither could slide neatly beneath the other.

Instead, the crust crumpled, thickened and was pushed upward. India’s northward movement slowed to about half its earlier speed — but it did not stop. The result of that ongoing push is the Himalaya and, to the north, the Tibetan Plateau, which the USGS describes as having risen very rapidly in geological terms.

Traces of an old ocean

Rocks in the region still carry memories of the vanished Tethys Sea. The USGS describes marine fossils in sandstone about 50 kilometres north of Lhasa, in Tibet, deposited when the sea flooded the area. Sediments that once lay on an ocean floor are now part of the roof of the world.

Still growing

The collision is not over. India is still pushing into Asia, and the USGS notes that the Himalaya continue to rise by more than one centimetre a year. That may sound tiny, but over a million years it would add up to more than ten kilometres of uplift.

The mountains do not simply keep getting higher forever, though. At the same time as they are pushed up, wind, water and ice wear them down. Rivers cut deep valleys and glaciers grind away at the rock. The height of the Himalaya is a balance between these forces.

Mountains and earthquakes

A collision of this size also builds up enormous stress in the rocks. When that stress is released, the ground shakes. The Himalayan region regularly experiences earthquakes, some of them very large — a reminder that the forces that built these mountains are still active.

Mountains that shape the weather

High mountains affect far more than the land beneath them. The USGS links the uplift of the Himalaya and Tibet to changes in the climate of the region, including a drier climate in Tibet. Mountains this high block and redirect moving air, which shapes where rain falls across a whole continent.

Why it matters

The Himalaya are not a finished landscape but a process in motion. Standing in front of them, you are looking at a collision between two continents, frozen into rock but still slowly continuing. Few places show so clearly that the Earth beneath our feet is alive.