Why does the height of Everest change?

An ancient geological collision gave rise to the Himalayan mountain range, a collision that continues to this day.

Climbers who reach the summit of Everest may not know it, but beneath the blanket of snow lies a stretch of mottled gray rocks that once lay on the ocean floor.

The rocks arrived at this unusual location, nearly 9,000 meters above sea level, thanks to the slow movement of tectonic plates, massive slabs of solid rock that make up Earth’s outer and fractured layer. These plates are in constant competition for position, shaping the wide range of formations visible on the surface. In some places, the plates pull apart, creating valleys on land. In others, they collide, raising mountains into the sky.

Mount Everest, located on the border between Tibet and Nepal, was formed due to a violent tectonic collision between the Indian and Eurasian tectonic plates tens of millions of years ago. This collision crumpled the landscape, raising mountains along a roughly 2,400-kilometer stretch, a range now known as the Himalayas. Although many mysteries still surround the exact steps this continental collision followed, the collision continues today, partly explaining why the height of Everest is still changing.

The story of the Himalayas begins about 200 million years ago when the supercontinent Pangaea began to split apart. The Indian plate broke free and moved northward toward the landmass we now know as Asia. The Indian plate moved at an astonishing pace, geologically speaking, traveling nearly nine meters or more every century.

At that time, the vast Tethys Ocean filled the gap between India and Eurasia. But as the Indian plate moved north, the ocean began to close. The submerged plate, composed of dense oceanic crust, dove beneath the southern edge of the buoyant rocks of the Eurasian continental plate, creating a subduction zone. The slow sinking of the oceanic plate into the mantle scraped a thick layer of seafloor sediments off the ocean floor and stacked them at the edge of the Eurasian plate. Later, this sandy layer would be compressed to form rock and end up on the tops of mountain peaks.

About 50 million years ago, the speed of the Indian plate plummeted, a shift many scientists interpreted as the early stages of the plate’s collision with Eurasia. Other evidence from marine sediments suggests that the final stretch of the Tethys Ocean closed roughly between 50 and 60 million years ago.

Unlike an oceanic plate, which is cold and dense, the Indian continental plate is thick and buoyant. So, as the continents compressed and India was shoved beneath Asia, the surface gave way, and the crust thickened, forming what would eventually become the towering Himalayan mountain range. Or, at least, that is the most widely accepted version of the story.

But as scientists scrutinize every twist, crack, and rock of this system, many mysteries have emerged. Studying the ancient magnetic patterns of the rock allows researchers to map the position of a continent over time, and recent work using this method has revealed that when the collision that formed the mountains supposedly took place, about 55 million years ago, India would have been well south of Eurasia. This would leave a mysterious gap between the two continents.

Did the Indian plate initially collide with a vanished continental mass that lay between two larger continental blocks? Could the northern end of the Indian plate have extended much farther than previously thought? Why did the Indian plate move so swiftly before impact? These are some of the many puzzles that scientists are trying to unravel.

The view from the North Base Camp of Everest shows the route to the higher camps on the way to the summit of the mountain.

Regardless of when it began, the collision that formed Everest continues today. India is moving northward at a rate of about four centimeters per year, and scientists estimate that the impact with Eurasia could cause the mountains to reach higher elevations, with an estimated average uplift of about 10 millimeters per year in the northwestern sections of the range and nearly one millimeter per year on Everest.

Growth can occur in fits and starts, triggered by more violent changes in the landscape. As India dives beneath Eurasia, it doesn’t always do so smoothly. When the land is compressed, pressure builds up until it reaches a critical point. Blocks of land can suddenly move, shaking the ground in an earthquake.

However, the mountain doesn’t necessarily grow during earthquakes. Depending on how and where the ground shifts, earthquakes can make the mountain grow or shrink in small amounts. This might have happened during the 2015 earthquake in Nepal, according to satellite data.

Also, as the rocks rise toward the sky, erosion acts against the upward progression. Wind and water gradually wear away the surface, carrying sediment into downstream rivers. In the Himalayas, much of the sediment flows into the Ganges and Brahmaputra rivers. The sand settles out of the water as the slope decreases at the foot of the mountain in the world’s largest river delta, forming the land beneath much of Bangladesh and the Indian state of West Bengal.

Although erosion and gravity keep these towering mountains in check, the tectonic plates continue their geological dance, and Everest will keep its pace.

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