WEBVTT

00:00:07.196 --> 00:00:08.265
Every spring,

00:00:08.265 --> 00:00:12.185
hundreds of adventure-seekers dream
of climbing Qomolangma,

00:00:12.185 --> 00:00:14.702
also known as Mount Everest.

00:00:14.702 --> 00:00:17.209
At base camp, they hunker down for months

00:00:17.209 --> 00:00:22.011
waiting for the chance to scale 
the mountain's lofty, lethal peak.

00:00:22.011 --> 00:00:26.163
But why do people risk life and limb
to climb Everest?

00:00:26.163 --> 00:00:27.582
Is it the challenge?

00:00:27.582 --> 00:00:28.667
The view?

00:00:28.667 --> 00:00:32.017
The chance to touch the sky?

00:00:32.017 --> 00:00:37.563
For many, the draw is Everest's status
as the highest mountain on Earth.

00:00:37.563 --> 00:00:40.328
There's an important distinction
to make here.

00:00:40.328 --> 00:00:44.511
Mauna Kea is actually the tallest
from base to summit,

00:00:44.511 --> 00:00:47.597
but at 8850 meters above sea level,

00:00:47.597 --> 00:00:51.171
Everest has the highest altitude 
on the planet.

00:00:51.171 --> 00:00:54.199
To understand how 
this towering formation was born,

00:00:54.199 --> 00:00:57.612
we have to peer deep 
into our planet's crust,

00:00:57.612 --> 00:01:00.022
where continental plates collide.

00:01:00.022 --> 00:01:03.410
The Earth's surface 
is like an armadillo's armor.

00:01:03.410 --> 00:01:06.171
Pieces of crust constantly move over,

00:01:06.171 --> 00:01:07.011
under,

00:01:07.011 --> 00:01:08.998
and around each other.

00:01:08.998 --> 00:01:13.612
For such huge continental plates,
the motion is relatively quick.

00:01:13.612 --> 00:01:16.507
They move two to four 
centimeters per year,

00:01:16.507 --> 00:01:18.919
about as fast as fingernails grow.

00:01:18.919 --> 00:01:20.524
When two plates collide,

00:01:20.524 --> 00:01:25.035
one pushes into or underneath the other,
buckling at the margins,

00:01:25.035 --> 00:01:29.963
and causing what's known as uplift
to accomodate the extra crust.

00:01:29.963 --> 00:01:32.083
That's how Everest came about.

00:01:32.083 --> 00:01:36.811
50 million years ago, the Earth's 
Indian Plate drifted north,

00:01:36.811 --> 00:01:38.911
bumped into the bigger Eurasian Plate,

00:01:38.911 --> 00:01:42.869
and the crust crumpled,
creating huge uplift.

00:01:42.869 --> 00:01:45.541
Mountain Everest lies at the heart
of this action,

00:01:45.541 --> 00:01:49.120
on the edge of the Indian-Eurasian 
collision zone.

00:01:49.120 --> 00:01:52.879
But mountains are shaped by forces
other than uplift.

00:01:52.879 --> 00:01:58.286
As the land is pushed up, 
air masses are forced to rise as well.

00:01:58.286 --> 00:02:02.731
Rising air cools, causing any water
vapor within it to condense

00:02:02.731 --> 00:02:05.016
and form rain or snow.

00:02:05.016 --> 00:02:07.630
As that falls, 
it wears down the landscape,

00:02:07.630 --> 00:02:12.679
dissolving rocks or breaking them down
in a process known as weathering.

00:02:12.679 --> 00:02:15.468
Water moving downhill carries
the weathered material

00:02:15.468 --> 00:02:17.404
and erodes the landscape,

00:02:17.404 --> 00:02:20.655
carving out deep valleys and jagged peaks.

00:02:20.655 --> 00:02:25.582
This balance between uplift and erosion
gives a mountain its shape.

00:02:25.582 --> 00:02:28.027
But compare the celestial peaks
of the Himalayas

00:02:28.027 --> 00:02:30.375
to the comforting hills of Appalachia.

00:02:30.375 --> 00:02:33.019
Clearly, all mountains are not alike.

00:02:33.019 --> 00:02:35.981
That's because time 
comes into the equation, too.

00:02:35.981 --> 00:02:40.290
When continental plates first collide,
uplift happens fast.

00:02:40.290 --> 00:02:43.157
The peaks grow tall with steep slopes.

00:02:43.157 --> 00:02:46.800
Over time, however, gravity and water
wear them down.

00:02:46.800 --> 00:02:49.463
Eventually, erosion overtakes uplift,

00:02:49.463 --> 00:02:52.525
wearing down peaks 
faster than they're pushed up.

00:02:52.525 --> 00:02:55.965
A third factor shapes mountains: climate.

00:02:55.965 --> 00:03:00.768
In subzero temperatures, some snowfall
doesn't completely melt away,

00:03:00.768 --> 00:03:03.861
instead slowly compacting 
until it becomes ice.

00:03:03.861 --> 00:03:08.944
That forms the snowline, which occurs
at different heights around the planet

00:03:08.944 --> 00:03:11.229
depending on climate.

00:03:11.229 --> 00:03:14.926
At the freezing poles,
the snowline is at sea level.

00:03:14.926 --> 00:03:19.339
Near the equator, you have to climb
five kilometers before it gets cold enough

00:03:19.339 --> 00:03:21.398
for ice to form.

00:03:21.398 --> 00:03:24.807
Gathered ice starts flowing under
its own immense weight

00:03:24.807 --> 00:03:28.736
forming a slow-moving frozen river
known as a glacier,

00:03:28.736 --> 00:03:30.896
which grinds the rocks below.

00:03:30.896 --> 00:03:33.642
The steeper the mountains, 
the faster ice flows,

00:03:33.642 --> 00:03:37.276
and the quicker it carves 
the underlying rock.

00:03:37.276 --> 00:03:41.028
Glaciers can erode landscapes
swifter than rain and rivers.

00:03:41.028 --> 00:03:45.129
Where glaciers cling to mountain peaks,
they sand them down so fast,

00:03:45.129 --> 00:03:49.663
they lop the tops off
like giant snowy buzzsaws.

00:03:49.663 --> 00:03:54.364
So then, how did the icy Mount Everest
come to be so tall?

00:03:54.364 --> 00:03:57.802
The cataclysmic continental clash
from which it arose

00:03:57.802 --> 00:04:00.541
made it huge to begin with.

00:04:00.541 --> 00:04:03.156
Secondly, the mountain lies 
near the tropics,

00:04:03.156 --> 00:04:07.714
so the snowline is high,
and the glaciers relatively small,

00:04:07.714 --> 00:04:10.266
barely big enough to widdle it down.

00:04:10.266 --> 00:04:13.252
The mountain exists in a perfect storm
of conditions

00:04:13.252 --> 00:04:15.702
that maintain its impressive stature.

00:04:15.702 --> 00:04:17.699
But that won't always be the case.

00:04:17.699 --> 00:04:20.495
We live in a changing world 
where the continental plates,

00:04:20.495 --> 00:04:22.077
Earth's climate,

00:04:22.077 --> 00:04:23.971
and the planet's erosive power

00:04:23.971 --> 00:04:28.128
might one day conspire to cut 
Mount Everest down to size.

00:04:28.128 --> 00:04:32.406
For now, at least, it remains legendary
in the minds of hikers,

00:04:32.406 --> 00:04:33.498
adventurers,

00:04:33.498 --> 00:04:35.137
and dreamers alike.