EARTH SCIENCE: Avalanches

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6 MARCH 2,2015

Last April, tragedy struck on theslopes of Mount Everest, the worlds tallest peak. A group of Sherpas expedition guides native to Central Asias Himalayan mountainswas

in a treacherous area covered in blocks of fallen ice. They were working to prepare the route for hundreds of international climbers waiting below. They laid aluminum ladders over gaping cracks in the ice so the climbers could edge over them, and they placed ropes in strategic locations to aid their ascent.

Suddenly, the Sherpas heard a deep, terrifying rumble. Huge chunks of ice hurtled downhill toward them. A blast of frigid air hit the guides just before an avalanche of ice and snow swept over them. Sixteen climbers died that fateful day, making it the worst disaster in the history of the mountain.

Mount Everest and its neighboring peaks make up the Himalayasa mountain range

separating Nepal and China (see map, p. 8). In 1953, Edmund Hillary of New Zealand and a Sherpa named Tenzing Norgay became the first people known to reach Mount Everests towering summit. Since then, about 4,000 climbers have conquered the peakdespite its dangers.

Climbers tackling the worlds tallest peaks arent the only ones who risk getting swept up in an avalanche. Over the past 10 years, avalanches have killed an average of 28 people annually in the U.S. alone. Just this January, two members of the U.S. Ski Teamboth Olympic hopefulsdied when an avalanche in Austria buried them under 4.5 meters (15 feet) of cement-like snow.

These staggering stats have led experts to look for ways to keep climbers, skiers, and others safer on mountainsides.

TYPES OF AVALANCHESThere are two kinds of avalanches: ice

avalanches like the one that swept down Everest last April, and snow avalanches like

the one that took the lives of the two U.S. Ski Team members this year.

Most deadly avalanches are snow avalanches, which strike on

open slopesareas that are popular with skiers,

snowboarders, and snowmobilers.

To form, a snow avalanche needs a steep slope and an unstable*snowpack, which Continued on the next page >

AFTER THE AVALANCHERescuers, seen here just hours after the avalanche, found 11 of the 16 victims in this part of the route.

TOWERS OF ICEThe avalanche created chunks of ice the size of large buildings.

SITE OF THE AVALANCHE

Last year's avalanche on

M ount Everest s truck clim bers

in the Khumbu Icefall, one

of Everest's most dangerous

spots. The surface of the

slab o f ice that broke from a

nearby g lac ie r w as larger than

an N BA basketball court.

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camp.

MEET THE SHERPASFor centuries, Sherpas lived in the eastern

Himalayas as farmers, herders, and traders. Ever

s ince W estern explorers began tackling Everest in

the 1920s, they've hired Sherpas as porters and

guides. In 1953, Edmund H illary o f New Zealand

and a S herpa named Tenzing Norgay became the

firs t people known to reach the summit.

M ost people w ho clim b Everest depend on

S herpas d irectly as guides or ind irectly fo r setting

up the ropes and ladders along the clim bing route

each season. Their contribution to the sport is

a lm ost unm easurable," says c lim ber Peter Athans.

C lim bing is a big part o f the local econom y

in Nepal, but the w ork is risky. S ince the

1920s, 99 Sherpas and members

o f o ther local ethnic groups

have died on the slopes of

M ount Everest.

can form when snow accumulates in a mix of loosely packed and densely packed layers.

Every snowstorm is a little different, says Karl Birkeland, an avalanche scientist at the U.S. Forest Service National Avalanche Center.Each storm creates its own unique layer.

When a stronger, cohesive layer of snow rests on top of a weak, loose layer, the weaker layer can collapse, and a slab of the upper layer breaks away (see The Making of a Snow Avalanche, right).

It can fracture just like a pane of glass in a snap, says Ethan Greene, an avalanche scientist at the Colorado Avalanche Information Center. The slab slides downhill, picking up speed and pushing more snow along with it.

Under the right conditions, it doesnt take much to prompt a snow avalanche. Just the weight of a person on the slope can push it over

HEA VY LOAD: A porter carries m attresses down from Everest base

O N DUTY:Sherpasservebreakfast at

the edge and produce an avalanche, says Greene. Most avalanche victims in the U.S. either triggered the disaster themselves or were with someone who did. Last Aprils fatal ice avalanche on Everest, on the other hand, had a different trigger.

DEADLY ICE AVALANCHE

Sometimes, rivers of ice called glaciers creep down

8 MARCH 2, 2015

mountains and valleys.As a glacier inches over a cliff, its edge hangs until pieces of ice break off. Last April, a huge chunk of glacial ice tumbled from a cliff in a notori

ously dangerous section of Everest known as the

Khumbu Icefall, causing the deadly avalanche.Using before-and-after satellite

photos of the glacier, a National Geographic cartographer calculated the size of the ice chunk that fell. It weighed as much as 14.3 million kilograms (31.5 million pounds)about the weight of 657 fully loaded buses.

Peter Athans, a climber who has reached the summit of Mount

Everest seven times, knows firsthand

how quickly ice avalanches can strike. In 1991, he and a friend were passing through the Khumbu

Icefall when they got caught up in one. NA

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0 CROSSING THE GAP: Ladders, often installed by Sherpas, allow climbers to safely cross wide cracks in ice.

there. Ideas include eliminating unnecessary comfort supplies like espresso machines and dining tables and only accepting clients who are experienced climbers, since novices need extra equipment.

We heard a big crack and collapse, and a w hum pf type sound, he says. A blast of air enveloped the two in an icy cloud. I could only see him for another couple of seconds, and then the air went totally white. Both managed to surviveshaken but uninjured.

PREDICTING DANGER

REDUCING THE RISK

Unlike the unpredictable ice avalanche on Everest, snow avalanches often give warning signs. If a snow avalanche occurs, the snowpack on similar slopes nearby may be unstable tooso stay clear.

Other red flags: cracks that shoot across the snow and a w hum pf sound as the weaker snow layer collapses. If you see something like that on a flat slope, says Greene,

then you can be pretty confident that if you went up to a steeper slope, that would produce an avalanche.

There are efforts to help keep people safe. Local avalanche centers monitor snowpack and post advisories online. And manufacturers have developed emergency equipment, like beacons that help rescuers locate buried victims and inflatable avalanche bags that keep victims near the surface.

Still, no one can fully eliminate risk. If youre headed into potential avalanche terrain, get training, note warning signs, carry rescue equipmentand hope youre not in the wrong place at the wrong time. ^

Jacqueline Adams

C O R E Q U E S T I O N

H ow is an ice avalanche sim ilar

to a snow avalanche? H ow is it

d iffe ren t? C ite examples from

the text.

THE MAKING OF A SNOW AVALANCHE

Ice avalanches strike without warning, so people have almost no time to react. We dont know how to predict exactly when one of those blocks might fall, says Birkeland. You just have to try and minimize your time there, and hope it doesnt happen while youre there.

Theres no quick way through the spot where the 16 Sherpas died last April. Athans likens the stretch to an obstacle course. Sherpas may make dozens of trips through the treacherous area as they set up the route for international climbers and transport all of their supplies. Following the recent tragedy, people began to suggest ways to reduce the amount of time Sherpas spend

Som e avalanches, like the deadly one on M ount Everest last spring, are

ice avalanches. The kind that recently killed tw o U.S. O lym pic hopefu ls, on

the other hand, was a snow avalanche. The m ost dangerous snow avalanche

SNOWPACK:Layers of snow that pile up over time

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V mi.MOUNTAINAVALANCHE:Large amount of snow sliding down a mountain

is a slab avalanche. It happens when a th ick b lock o f

snow breaks free from a m ountainside.

A pe rson s w e igh t can be enough to

cause a w eak layer o f snow buried w ith in

a snow pack to give way, sending

layers o f snow roaring downhill.

WEAK LAYER: Loosely packed layer of snow

SLAB: Thick block of snow that breaks free and slides downhill

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