how and when did Mount Everest become the tallest mountain? And will it remain so?

Most people know that Mount Everest is the highest mountain, but I want to know how long it has been the highest, and for how long in the future it will remain so (…) What chain has preceded it ? (…) When will something else overtake him? – Nigel, 14, Christchurch

Nigel, thank you for this wonderful and interesting question. The answer is actually quite complex, since the height (or elevation) mountain ranges in the past can be difficult to know.

However, this is a very important question because mountains have a huge role in the environment. They can disrupt air circulation, affect global and regional climate and offer opportunities for plants and animals to evolve.

Understanding the history of mountain ranges

Geoscientists answer questions about ancient mountain heights by examining sedimentary basins in the mountain ranges. These are low areas where sediment materials such as pollen and plant leaves accumulate and minerals form in the soil.

A basin today can be much higher or lower than it was when the sediment entered it. Pollen, leaves and fossilized minerals that date back to when sediment was deposited can reveal how the elevation of the landscape has changed over time.

If we look at the fossilized pollen, we can find that it comes from plants that probably grew in a particular elevation range, and we can also notice the absence of certain other plants. (We can determine where ancient plants likely grew by looking at their modern parents.)

So, by dating the pollen that we find, we can calculate the possible range of rise in the past. We can conclude that the landscape was too high for plant A, high enough to plant B (which gave us the pollen), but not high enough to plant C.

This is a pretty powerful ability, especially if the elevation of the landscape has changed significantly since the initial deposition of sediment.

Podocarp (southern hemisphere conifer) pollen on the left grew in Timor about 2.5 million years ago. A modern parent is shown on the right. These plants have grown at elevations above 1.2 km and are not present in older sediments. Their sudden appearance in the sediment washed away by the waters of the ancient island indicates that parts of the island had reached at least 1.2 km high.
Author provided

We can also look at the different types (or isotopes) certain elements (in particular oxygen) contained in vegetable waxes, clays and carbonate minerals which are formed by chemical reactions in the soil. These plants and minerals incorporate rainwater.

When a band of rain hits a mountain range, water with heavier oxygen isotopes falls first. This means that rainwater at higher elevations contains lighter oxygen isotopes, which then pass into plants and minerals.

If we find sediments that were deposited in a low basin 30 million years ago, but are now much higher, they will still contain isotopes of oxygen that reveal the elevation at which they formed for the first time. We can measure these isotopes to estimate how much higher the landscape has become.

How long has Everest been the greatest?

Everest is part of the Himalayas, a mountain range that lies at the southern edge of the vast Tibetan plateau which is about 4-5 km above sea level. Scientists used the methods described above to understand the history of the plateau, which has evolved accordingly of several ancestral mountain ranges joining.

Parts of the modern plateau already exceeded 3.5 km by 26 million years ago. The southernmost of these chains was a large Andes-like mountain range, called the Gangdese Mountains.

These appear to have existed for over 50 million years at altitudes similar to the Andes today (around 4.5 km).

However, south of Gangdese, where today’s tallest mountains are found, geologists have found 34.5 million years old. sediments of a shallow sea just a few dozen kilometers east of Mount Everest (locally called Qomolangma).

This tells us that the part of the Himalayas that includes Everest, which now dominates the horizon, was not a mountain range at the time. In fact, it was at sea level. It has increased by more than 8 km in the last 30 million years.

Everest, now the big kid on the block, is currently over 100 meters taller than its closest rival. But a new winner will emerge over time.

What happens next?

To understand how Everest could lose its highest mountain status, we need to understand how mountain ranges are built. Today’s largest mountain belts were built from collisions between blocks of continental crust in the Earth’s outer layer, the lithosphere.

When these boulders collide, they crumple and slices of rock crust stack on top of each other, as shown in the right half of the cross section below. This gives rise to high mountains, which continuously rise and move and change as the collision continues.

General cross section of the lithosphere in the Himalayan region
This is a general cross section of the lithosphere in the Himalayan region. The lithosphere includes all of the crust and part of the mantle, up to the partially melted asthenosphere.
X, Author provided

The video below helps to visualize this process. It simulates the compression of a lithosphere block in the Himalayas. You can refer to the “Video Sandbox” part of the section above to see where this process would occur.

Students at the University of Melbourne’s School of Geography, Earth and Atmospheric Science squeeze layered sediment in a sandbox to see how it will warp.

You will notice that the mountains start to rise as soon as the collision begins. The sand pushing arm represents the already thickened crust of the high Himalayas and the pushed sand heap represents the Indian upper crust which lies below the mountain range.

The thickening moves to different places over time. While the youngest and smallest mountain is the farthest from the collision itself, the tallest peak is not always in the older part of the range (where the collision started).

Erosion and growth

Large mountain ranges “erode” as temperature changes, wind, and water break down the rock and eventually wash it away. Interestingly, erosion actually causes mountains to grow slowly over time.

It’s a fascinating process that geoscientists call “isostasis”Which can be measured using GPS. The diagram below shows how the process compares to blocks of wood floating in water.

If intact blocks of a certain type of wood are floating in a swimming pool, the same percentage of the overall volume will always protrude above the surface. So if material is removed from a block, that block will increase.

This diagram shows how mountain erosion – similar to cutting slits in wooden blocks – causes peaks to increase in elevation.
X, Author provided

We can compare these wooden columns to lithospheric blocks. As erosion occurs, the mountain surface increases in elevation. This allows deeply buried rocks to rise up into the mountain range.

Hard to beat

Despite 82,350 km of converging borders on Earth (where two plates meet and come together), it is unlikely that other mountain ranges will soon exceed the height of the Himalayas.

Indeed, the Himalayas were built by the collision of two large continents made up of rocks of lower than average density. They are therefore located higher than the oceanic lithosphere.

Someday in the distant future a new frontier will form somewhere and the forces creating the Himalayas will be removed.

The range will then collapse and eventually erode to become like modern times Appalachians in North America, which was an active mountain belt between 325 and 260 million years ago.



Read more: Curious Kids: how are mountains formed?


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