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  • Every spring,

  • hundreds of adventure-seekers dream of climbing Qomolangma,

  • also known as Mount Everest.

  • At base camp, they hunker down for months

  • waiting for the chance to scale the mountain's lofty, lethal peak.

  • But why do people risk life and limb to climb Everest?

  • Is it the challenge?

  • The view?

  • The chance to touch the sky?

  • For many, the draw is Everest's status as the highest mountain on Earth.

  • There's an important distinction to make here.

  • Mauna Kea is actually the tallest from base to summit,

  • but at 8850 meters above sea level,

  • Everest has the highest altitude on the planet.

  • To understand how this towering formation was born,

  • we have to peer deep into our planet's crust,

  • where continental plates collide.

  • The Earth's surface is like an armadillo's armor.

  • Pieces of crust constantly move over,

  • under,

  • and around each other.

  • For such huge continental plates, the motion is relatively quick.

  • They move two to four centimeters per year,

  • about as fast as fingernails grow.

  • When two plates collide,

  • one pushes into or underneath the other, buckling at the margins,

  • and causing what's known as uplift to accommodate the extra crust.

  • That's how Everest came about.

  • 50 million years ago, the Earth's Indian Plate drifted north,

  • bumped into the bigger Eurasian Plate,

  • and the crust crumpled, creating huge uplift.

  • Mountain Everest lies at the heart of this action,

  • on the edge of the Indian-Eurasian collision zone.

  • But mountains are shaped by forces other than uplift.

  • As the land is pushed up, air masses are forced to rise as well.

  • Rising air cools, causing any water vapor within it to condense

  • and form rain or snow.

  • As that falls, it wears down the landscape,

  • dissolving rocks or breaking them down in a process known as weathering.

  • Water moving downhill carries the weathered material

  • and erodes the landscape,

  • carving out deep valleys and jagged peaks.

  • This balance between uplift and erosion gives a mountain its shape.

  • But compare the celestial peaks of the Himalayas

  • to the comforting hills of Appalachia.

  • Clearly, all mountains are not alike.

  • That's because time comes into the equation, too.

  • When continental plates first collide, uplift happens fast.

  • The peaks grow tall with steep slopes.

  • Over time, however, gravity and water wear them down.

  • Eventually, erosion overtakes uplift,

  • wearing down peaks faster than they're pushed up.

  • A third factor shapes mountains: climate.

  • In subzero temperatures, some snowfall doesn't completely melt away,

  • instead slowly compacting until it becomes ice.

  • That forms the snowline, which occurs at different heights around the planet

  • depending on climate.

  • At the freezing poles, the snowline is at sea level.

  • Near the equator, you have to climb five kilometers before it gets cold enough

  • for ice to form.

  • Gathered ice starts flowing under its own immense weight

  • forming a slow-moving frozen river known as a glacier,

  • which grinds the rocks below.

  • The steeper the mountains, the faster ice flows,

  • and the quicker it carves the underlying rock.

  • Glaciers can erode landscapes swifter than rain and rivers.

  • Where glaciers cling to mountain peaks, they sand them down so fast,

  • they lop the tops off like giant snowy buzz saws.

  • So then, how did the icy Mount Everest come to be so tall?

  • The cataclysmic continental clash from which it arose

  • made it huge to begin with.

  • Secondly, the mountain lies near the tropics,

  • so the snowline is high, and the glaciers relatively small,

  • barely big enough to whittle it down.

  • The mountain exists in a perfect storm of conditions

  • that maintain its impressive stature.

  • But that won't always be the case.

  • We live in a changing world where the continental plates,

  • Earth's climate,

  • and the planet's erosive power

  • might one day conspire to cut Mount Everest down to size.

  • For now, at least, it remains legendary in the minds of hikers,

  • adventurers,

  • and dreamers alike.

Every spring,

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B2 US TED-Ed everest uplift continental mountain mount everest

【TED-Ed】Why is Mount Everest so tall? - Michele Koppes

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    Yuan Jui Yu posted on 2016/06/18
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