Can the Himalayan Mountains grow forever?

Krishna: About 225 million years ago, India was a large island still situated off the Australian coast, and a vast ocean (called Tethys Sea) separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India began to forge northward. When India rammed into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas.

Himalayas formed when India and Eurasia, driven by plate movement, collided. Because both these continental landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging plates could only be relieved by thrusting skyward, contorting the collision zone, and forming the Himalayan peaks.

Image source: Google Images

The Himalayas and the Tibetan Plateau to the north have risen very rapidly. In just 50 million years, peaks such as Mt. Everest have risen to heights of more than 9 km. The impinging of the two landmasses has yet to end. The Himalayas continue to rise more than 1 cm a year -- a growth rate of 10 km in a million years! If that is so, why aren't the Himalayas even higher? Scientists believe that the Eurasian Plate may now be stretching out rather than thrusting up, and such stretching would result in some subsidence due to gravity.

At present, the movement of India continues to put enormous pressure on the Asian continent, and Tibet in turn presses on the landmass to the north that is hemming it in. The net effect of plate-tectonics forces acting on this geologically complicated region is to squeeze parts of Asia eastward toward the Pacific Ocean. One serious consequence of these processes is a deadly "domino" effect: tremendous stresses build up within the Earth's crust, which are relieved periodically by earthquakes along the numerous faults that scar the landscape. Some of the world's most destructive earthquakes in history are related to continuing tectonic processes that began some 50 million years ago when the Indian and Eurasian continents first met. (1)

But understanding how and why Everest's height changes is more complex than just this.

What stops our planet's mountains from growing forever? There are some factors that limit mountains' growth (3). The most important ones are Erosion and Gravity.

While plate tectonics push the summit higher into the sky, erosion claws away at it. This occurs because both glaciers and rivers, landslides, and avalanches were all kind of the processes that connected to tear the mountains down. (2) But as rivers erode material, their channels may become too steep. This can trigger landslides that carry material away from the mountain and limit its growth.

Weather can also cause significant erosion to a mountain.

For a tall mountain range, you can basically get to such a steep angle in the rock that it can't actually support ice, and snow, and then you start to get avalanches, and you get bare rock. Rock falls and land slides – a constant hazard on Mount Everest and the surrounding area – both play a role in shaving away at Everest's height, and rivers too. They have been estimated to be cutting gorges into the rock at a rate of between 4-8mm (0.2-0.3in) a year.

But there's some kind of equilibrium between how fast that landscape can erode and how high those peaks can get. The exact details of this equilibrium are still being explored. But in regions with tectonic activity, the tectonic force can be driving the mountains up slower, faster, or at around the same rate as the erosion is cutting it down. We don't fully understand all the drivers in those types of systems.

Another limiting factor is gravity. Many mountains form because of movements in Earth's surface layer known as plate tectonics; this theory describes the Earth's crust as mobile and dynamic, divided into large pieces that inch around with time. When two plates collide, the impact forces material from their touching edges to move upward. The plates keep pushing together and the mountains keep growing, until it becomes "too hard to do that work against gravity”. At some point the mountain becomes too heavy, and its own mass stops the upward growth caused by the crunching of those two plates. (3)

In other words, if Earth had less gravity, its mountains would grow higher. That is indeed what happened on Mars, where mountains loom much taller than on our planet.

Himalayas will stop growing in 20 million years, finds a study (4). The mountain building episode started with the collision of the Indian plate with the Eurasian plate will stop due to the strength of the underlying mantle , not the height of the Himalayas.

The rate at which the Indian sub-continent creeps toward Europe. is slowing exponentially
The convergence will halt — putting an end to one of the longest periods of mountain-building in recent geological history — in about 20 million years, according to the study (4).

So as long as Himalayas are geologically active, they grow. But at some point of time the limiting factors might take charge and limit or completely stop their growth. And who knows when the balance tilts towards the negative side, we might even hear the news that their height has even reduced?

Footnotes:

1. The Himalayas [This Dynamic Earth, USGS]
2. How tall will Mount Everest get before it stops growing?
3. Why Don't Mountains Grow Forever?
4. Growth of Himalayas slowing down, study finds