Why is Everest the world's highest mountain?

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1. Discovery of Peak XV

During British rule in India, surveyors decided to measure the height of all the peaks in the Himalayas. They became interested in one in particular which towered above the others – Peak XV. The summit was calculated to be almost 28,000 ft – nearly 9,000 metres above sea-level – making it the world’s tallest mountain.

In 1865, it was renamed Mount Everest in honour of George Everest, surveyor of much of the Indian sub-continent.

Since then, climbers have faced death to conquer Everest so they can say they have stood on the world’s highest mountain. But there are other tall mountains around the world, so does Everest still retain its title?

2. How to measure a mountain

Everest is the world's highest mountain from sea level. However, several others including Kilimanjaro and Mauna Kea are higher if measured in different ways

Everest is the world’s highest mountain, at nearly 9,000 metres if you measure from sea level. However, many mountains are taller than Everest when measured in different ways. Kilimanjaro and Mauna Kea are taller, if measured from the base on which they sit. Kilimanjaro sits on the African plains which is close to sea level and Mauna Kea is taller but sits at the bottom of the sea.

3. The world’s biggest collision

So Everest is only the world's tallest mountain when measured from sea level.

The secret of Everest’s height is not found at the summit but deep underground. Because of how it was formed it sits on higher ground.

Fifty million years ago, India’s continental plate crashed into Asia – the biggest collision on Earth in the last 400 million years. The collision was so violent that India’s plate did not just crumple, it pushed under Asia – raising the land mass high into the sky.

Plates collide all around the world, but what happened below Everest is unique.

The towering Himalayas are the result. But this is not the only reason why Everest is so tall.

4. Older mountains are shrinking

The Himalayas are a youthful 50 million years old. Other mountain ranges are hundreds of millions of years older than this.

Due to the plates pushing the Indian plate northward and under Asia, the Himalayas continue to rise. At the moment the forces upward outweigh erosion, which means that not only is Everest the tallest mountain in the world, it is still growing.

What's more, the lower parts of Mount Everest consist of granite, one of the toughest rocks in the world. This lets Everest better withstand erosion compared to other, less hard mountains.

After the recent earthquake in Nepal, the mountains north of Kathmandu in Nepal rose by over a metre and Everest may actually have decreased elevation very slightly.

At some point, the rate of erosion will exceed the rate by which Everest is pushed up by the plates and it will be worn down.

So the the world's highest mountain could lose its crown eventually, though probably not for many millions of years.

5. Everest: The solar system’s highest mountain?

Click/tap to reveal how Earth's neighbours' highest mountains compare to Everest.

Mount Everest (Earth)

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Mount Everest (Earth)

At just under 9km, Mount Everest isn't even close to mountains on Mars and Venus. Earth's gravity is too powerful to allow higher mountains.

Maxwell Montes (Venus)

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Maxwell Montes (Venus)

Maxwell Montes is just over 2km higher than Everest at around 11km. It would be a warmer but deadlier climb with temperatures of 500 degrees.

Olympus Mons (Mars)

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Olympus Mons (Mars)

An active volcano with a larger land area than the UK, Olympus Mons towers over Everest at 22km.