Most individuals know that Mount Everest is the tallest mountain however I wish to know for the way lengthy it has been the tallest, and for the way lengthy sooner or later it’s going to stay so (…) Which vary preceded it? (…) When will one thing else overtake it? — Nigel, age 14, Christchurch
Nigel, thanks for this excellent and insightful query. The reply is definitely fairly complicated, because the peak (or elevation) of mountain ranges previously might be troublesome to know.
Nevertheless, it’s a essential query as mountains have an enormous function within the atmosphere. They will disturb air stream, have an effect on world and regional local weather and supply alternatives for crops and animals to evolve.
Understanding the historical past of mountain ranges
Geoscientists tackle questions on historical mountain heights by taking a look at sedimentary basins inside mountain ranges. These are low areas the place sediment supplies comparable to pollen and plant leaves accumulate and minerals kind within the soil.
A basin right now could also be a lot increased or decrease than it was when sediment entered it. The fossilised pollen, leaves and minerals that date again to the time when the sediment was deposited can reveal how the panorama’s elevation modified over time.
If we take a look at fossilised pollen, we might discover it comes from crops which seemingly grew in a selected vary of elevation, and we may additionally discover the absence of sure different crops. (We will work out the place historical crops seemingly grew by taking a look at their fashionable family members.)
So by relationship the pollen we discover, we are able to calculate the panorama’s doable vary of elevation previously. We will conclude the panorama was too excessive for plant A, excessive sufficient for plant B (which gave us the pollen), however not excessive sufficient for plant C.
That may be a fairly highly effective functionality, particularly if the elevation of the panorama has modified considerably because the sediment was first deposited.
We will additionally take a look at the totally different varieties (or isotopes) of sure components (notably oxygen) contained in plant waxes, clays and carbonate minerals that kind by chemical reactions within the soil. These crops and minerals incorporate rainwater.
As a band of rain reaches a mountain vary, water with heavier oxygen isotopes falls out first. This implies rainwater at increased elevations incorporates lighter oxygen isotopes, which then go into the crops and minerals there.
If we discover sediment that was deposited right into a low basin 30 million years in the past, however is now a lot increased, it’s going to nonetheless include oxygen isotopes that reveal the elevation at which it first fashioned. We will measure these isotopes to estimate how a lot increased the panorama has develop into.
How lengthy has Everest been the tallest?
Everest is a part of the Himalayas, a mountain vary that stands on the southern fringe of the huge Tibetan Plateau which is round 4-5km above sea stage. Scientists have used the strategies described above to grasp the historical past of the plateau, which developed on account of a number of ancestral mountain ranges becoming a member of up.
Components of the fashionable plateau have been already increased than 3.5km by 26 million years in the past. The southernmost of these ranges was an amazing, Andes-like mountain vary referred to as the Gangdese mountains.
These appear to have existed for greater than 50 million years at elevations just like these of the Andes right now (about 4.5km).
Nevertheless, south of the Gangdese, the place we have now right now’s highest mountains, geologists discovered 34.5-million-year-old sediments from a shallow sea only some dozen kilometres east of Mount Everest (domestically referred to as Qomolangma).
This tells us the a part of the Himalayas that features Everest, which now dominates the skyline, was not a mountain vary again then. The truth is, it was at sea stage. It has grown greater than 8km within the final 30 million years.
Everest, now the massive child on the block, is presently greater than 100 metres increased than its closest rival. However a brand new victor will emerge with time.
What occurs subsequent?
To grasp how Everest would possibly lose its highest mountain standing, we have to perceive how mountain ranges are constructed. The most important mountain belts right now have been constructed from collisions between blocks of continental crust in Earth’s outer layer, the lithosphere.
As these blocks collide, they crumple and slices of rocky crust get stacked on each other, as seen in the correct half of the cross part under. This offers start to excessive mountains, which repeatedly rise and shift and alter because the collision continues.
x, Creator supplied
The video under helps visualise this course of. It simulates the squeezing of a block of lithosphere within the Himalayas. You’ll be able to check with the “Sandbox Video” a part of the cross part above to see the place this course of would happen.
You’ll discover the mountains start to rise as quickly because the collision begins. The arm pushing the sand represents the already thickened crust of the excessive Himalayas and the sand pile being pushed represents the Indian higher crust which lies under the mountain vary.
The thickening strikes to totally different spots over time. Whereas the youngest and smallest mountain is furthest from the collision itself, the best peak isn’t all the time within the oldest a part of the vary (the place the collision started).
Eroding and rising
Giant mountain ranges “erode” when modifications in temperature, wind and water break down the rock and finally carry it away. Curiously, erosion really causes mountains to slowly develop over time.
This can be a fascinating course of geoscientists name “isostasy” which might be measured utilizing GPS. The diagram under exhibits how the method is corresponding to blocks of wooden floating in water.
If intact blocks of a sure kind of wooden float in a pool, the identical share of the general quantity will all the time protrude above the floor. So, if materials is faraway from one block, that block will rise.
x, Creator supplied
We will examine these columns of wooden to lithospheric blocks. As extra erosion happens, the mountain’s floor will increase in elevation. This offers a means for deeply buried rocks to rise inside the mountain vary.
Exhausting to beat
Regardless of having 82,350km of convergent boundaries on Earth (the place two plates meet and push collectively), it’s unlikely different mountain ranges will surpass the peak of the Himalayas anytime quickly.
It is because the Himalayas have been constructed by the collision of two massive continents composed of rocks with decrease than common density. They subsequently sit increased than the oceanic lithosphere.
Sooner or later within the distant future a brand new boundary will kind someplace and the forces creating the Himalayas will likely be eliminated.
The vary will then collapse and finally erode to develop into just like the modern-day Appalachians in North America, which was an energetic mountain belt from between 325 and 260 million years in the past.
Curious Youngsters: how do mountains kind?
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