Why is Mount Everest so big? New research highlights rogue river, but deeper forces at work

Mount Everest (also known as Chomolungma or Sagarmāthā) is renowned for being the highest mountain in the Himalayas and even on Earth. But why?

At 8,849 meters above sea level, Everest is approximately 250 meters higher than the other major peaks of the Himalayas. It also grows about 2 mm each year, about twice as fast as its long-term average growth.

In an article published in Natural geosciencesa team of Chinese and English scientists say Everest’s abnormal height and growth was influenced by the Arun River, which flows through the Himalayas. They say the river’s course changed about 90,000 years ago, eroding the rocks that weighed down Everest, and that the mountain rebounded in response, by 15 to 50 m.

The authors argue for the river’s contribution, but they acknowledge that the “fundamental cause” of the peak’s size lies in the tectonic processes that create the mountains. To understand what’s going on, we need to understand the forces that created the Himalayas in the first place, as well as the movements that allowed it to rise so high.

Tibetan gout

In the 19th century, British surveyors showed that the southern boundary of the Himalayan Mountains accurately describes an arc that aligns precisely with a small circle on Earth. It’s quite astonishing.

The only rational way to explain it would be to have the Eurasian tectonic plate to the north, the Indian plate to the south, and in between a viscous mass (Tibet) extending southward slowly collapsing under the force of gravity.

Basically, the Tibetan Plateau must be like a hot syrup, with a cold crust in the upper levels that bears witness to faults and earthquakes, pushed by the slow northward advance of the Indian tectonic plate. The exact nature and depth of this hot syrup is the subject of some debate, with geologists comparing it variously to crème brûlée and a jelly sandwich.

Overall, the collision between India and Eurasia is marked by a “mega-thrust fault,” where the Indian plate gradually slides beneath the Eurasian plate. The whole mega-thrust doesn’t move at the same time. Typically, it moves forward little by little in a series of “thrust earthquakes.”

Where the mass of Tibet comes into contact with India, we observe a narrow band of these thrust earthquakes. It is what happens in this narrow band that ultimately determines the elevation of the world’s highest mountain.

How Mountains Rise (and Fall)

Why is the Tibetan Plateau, north of Everest, so flat, yet mountains abound alongside this narrow band of earthquakes, where the collapsing mass couples with the advancing Indian subcontinent ?

The answer lies in how the mass of a mountain is supported.

Imagine a mountain as a pile of rubble on a thin plastic table. The table top has no inherent strength, so it sags downward and the pile of rubble sinks. Just like an iceberg, only part of the mass stands up.

Now imagine a thicker, more sturdy plate at the edge of the table. Here the pile of rubble is supported by the bending resistance of the plate, so that it can rise much higher above the surface. The mountains here can be much higher. This is what happens when one tectonic plate slides over another, because the sliding plate creates a stronger region.

Naturally, there is a balance. When the movement of tectonic plates causes earthquakes, mountain peaks can break off and giant avalanches move fallen rocks into adjacent river systems.

The fall of this rubble could reduce the absolute height of the mountains, as well as their relative height compared to neighboring valleys, although this depends on how efficiently rivers move the debris downstream.

In turn, as this rock mass moves further downstream, the upstream areas will be a little lighter. In our plastic table model, we might expect the table surface to tilt less and the peak of the rubble to rise a little higher.

That’s what the new research supports, but basically it’s earthquakes that push mountains higher. When the megathrust breaks up, where tectonic plates meet, mountains rise, although their height depends on the strength of the rock supporting them.

What is special about Everest?

The crucial question (as the authors acknowledge) is why does Everest stand out?

The boundary between collapsing Tibet and advancing India is defined by a giant mega-rift. Parts of this fault have not ruptured for a very long time, perhaps several centuries or more. It is likely that a lot of tension has built up in these areas, and when it finally breaks down, the result will be catastrophic.

However, the part of the megathrust beneath Everest appears to break up regularly, perhaps once or twice a century. The last big earthquake partly involved an existing rupture.

With each break, Everest is likely to grow a little higher. It is therefore not surprising that Everest is able to maintain its superiority over peaks located in the quieter parts of the mega-thrust.

As new research suggests, rogue rivers may well play a role in Everest’s size, but most of the mountain’s height still appears to be due to the pattern of earthquakes along the Himalayan fault.

The difficulty for the scientists involved is how to separate individual contributions to the extent of different factors. One of these is rebound erosion, as the new research suggests, but there are also tectonic processes such as the movement of the main central thrust or the slow slip of the South Tibetan Detachment fault , under which the highest mountain on Earth was unearthed.

This article is republished from The Conversation under a Creative Commons license. Read the original article.