Researchers have discovered why Everest is so tall

Everest is 8,849 metres above sea level, about 250 metres higher than the other great peaks of the Himalayas. Interestingly, the mountain continues to grow at a rate of about 2 millimetres per year, double its long-term average growth rate.

In a paper published in the journal Nature Geoscience, a team of Chinese and British scientists claim that the abnormal height and growth of Everest was due to the influence of the Arun River, which flows through the Himalayas. They believe that about 90,000 years ago, the course of the river changed, causing erosion of the rocks that were holding back the mountain's growth. As a result, Everest "jumped" upwards by 15-50 metres.

While the authors emphasise the role of the river, they acknowledge that the "fundamental cause" of the peak's size is the tectonic processes that create the mountain. To understand what is happening, it is necessary to understand the forces that shaped the Himalayas and the movements of the earth's crust that allowed the mountains to reach such heights.

The Tibetan "drop."

In the 19th century, British explorers noticed that the southern boundary of the Himalayas precisely describes an arc that coincides with the Earth's small circle. This is an astonishing observation.

The only rational explanation is that to the north is the Eurasian tectonic plate, to the south is the Indian plate, and in between is a viscous mass (Tibet) that is slowly spreading southwards by gravity.

Deep below the surface, the Tibetan plateau is like a hot syrup with a cold crust on top, where faults and earthquakes occur due to the slow northward movement of the Indian plate. The exact structure of this "syrupy" mass is a matter of debate, with geologists comparing it either to a crème brûlée or a jelly sandwich.

In general, the collision between India and Eurasia is characterised by a "megathrust fault" where the Indian plate is gradually sinking beneath the Eurasian plate. This fault does not move simultaneously along its entire length, but shifts in stages in a series of 'thrust earthquakes'.

Where the spreading mass of Tibet contacts India, we see a narrow band of such earthquakes. It is what happens in this narrow band that ultimately determines the height of the world's tallest mountain.

How mountains grow (and shrink)

Why is the Tibetan plateau north of Everest so flat, while mountains rise next to the narrow band of earthquakes?

The answer lies in the way the mass of the mountain is supported.

Imagine the mountain as a pile of rubble on a thin plastic table. The table top has no strength, so it sags and the mountain "sinks." Like an iceberg, only part of the mass protrudes above the surface.

Now imagine a thicker, sturdier plate at the edge of the table. Here the pile of rubble is supported by the flexural strength of the plate and can rise much higher above the surface. This is what happens where one tectonic plate slides under another, creating a stronger area.

Naturally, there is a balance. When plate movement causes earthquakes, mountain tops can collapse and huge avalanches carry debris into neighbouring river systems.

The fall of this debris can reduce the absolute height of the mountains and the relative height compared to neighbouring valleys, although this depends on how efficiently the rivers carry the sediment downstream.

In turn, when this mass is removed, the upper regions become slightly lighter. In our model with the plastic table, the table surface may sag less and the top of the mountain rise a little higher.

This is exactly what the new study claims. However, the growth of mountains is based on earthquakes that lift them up. When a megathrust fault ruptures, mountains rise, but how high depends on the strength of the supporting rocks below.

What's special about Everest?

The key question (which the authors themselves recognise) is why exactly does Everest stand out from other peaks?

The boundary between "falling" Tibet and advancing India is defined by a giant megathrust fault. Some parts of this fault have not moved for a very long time, perhaps several centuries or more. It is likely that a great deal of stress has built up in these areas, and when they finally rupture, the consequences will be catastrophic.

However, part of the megathrust fault beneath Everest appears to rupture regularly, perhaps once or twice a century. The last big earthquake there partially affected an existing fault.

With each such rupture, Everest probably rises a little higher. So it's not surprising that the mountain maintains its superiority compared to peaks in the calmer parts of the megathrust.

As the new study suggests, "runaway" rivers may play a role in Everest's size, but the main reason for its greater height appears to be related to the pattern of earthquakes along the Himalayan rift.

Challenges for scientists

The challenge for scientists is to separate out the individual contributions of different factors to the mountain's height. One is uplift due to erosion, as the new study suggests. But there are also tectonic processes, such as movement along the Main Central Thrust or slow slip along the South Tibetan Detachment, under which Earth's highest mountain was exposed.