During an avalanche, a mass of snow, rock, ice, soil, and other material slides swiftly down a mountainside. Avalanches of rocks or soil are often called landslides. Snowslides, the most common kind of avalanche, can sweep downhill faster than the fastest skier.
A snow avalanche begins when an unstable mass of snow breaks away from a slope. The snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles that rises high into the air. The moving mass picks up even more snow as it rushes downhill. A large, fully developed avalanche can weigh as much as a million tons. It can travel faster than 320 kilometers (200 miles) per hour.
Avalanches occur as layers in a snowpack slide off. A snowpack is simply layers of snow that build up in an area, such as the side of a mountain. In winter, repeated snowfalls build a snowpack dozens of meters thick. The layers vary in thickness and texture.
The bonds between the layers of a snowpack may be weak. Melted snow that refreezes may cause a slick coating of ice to form on the surface of a layer. A new snowfall may not stick to this slippery layer, and it may slide off. During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery. Added weight or vibration can easily send the top layers of a snowpack hurtling downhill.
Sluffs and Slabs
There are two main types of snow avalanches—sluffs and slabs. Sluff avalanches occur when the weak layer of a snowpack is on the top. A sluff is a small slide of dry, powdery snow that moves as a formless mass. Sluffs are much less dangerous than slab avalanches.
A slab avalanche occurs when the weak layer lies lower down in a snowpack. This layer is covered with other layers of compressed snow. When the avalanche is triggered, the weak layer breaks off, pulling all the layers on top of it down the slope. These layers tumble and fall in a giant block, or slab.
Once a slab avalanche starts, the slab shatters into many separate blocks. These snow blocks break up into ever-smaller pieces. Some of the pieces rise into the air as a moving cloud of icy particles. The cloud races downhill at very high speeds.
The thickness and speed of slab avalanches make them a threat to skiers, snowboarders, mountaineers, and hikers. In the mountains of the western United States, there are about 100,000 avalanches each year. Avalanches kill more than 150 people worldwide each year. Most are snowmobilers, skiers, and snowboarders.
Avalanche Control
Storminess, temperature, wind, the steepness of the slope, terrain, vegetation, and general
snowpack
conditions are all factors that influence whether an
avalanche
happens and what type occurs.
Snow
avalanches
are most likely to occur after a fresh
snowfall
adds a new layer to a
snowpack
. If new
snow
piles up during a storm, the
snowpack
may become overloaded, setting off a slide.
Earthquakes can set off
avalanches
, but much smaller
vibrations
can trigger them as well. A single skier can cause enough
vibrations
to set off a slide. In fact, 90 percent of
avalanche
incidents involving people are triggered by the victim or someone in the victim’s party.
Currently, scientists are not able to predict with certainty when and where
avalanches
will happen. However, they can estimate hazard levels by checking on the
snowpack
,
temperature
, and wind conditions.
Many ski areas employ
avalanche
control
teams to lessen the danger by starting slides before skiers head for the slopes. At some ski areas, patrols use explosives to set off
avalanches
. Or they may blast hazardous slopes with a cannon to shake loose any large, new accumulations of
snow
.
In the high
mountains
of Canada and Switzerland, special military troops are in charge of
avalanche
control
. Many Swiss
mountain
villages protect homes from
snowslides
by building large, sturdy structures to anchor
snowpacks
.
Dangers of an Avalanche
An
avalanche
is one of the most powerful events in nature. A fractured mass of
snow
may flow down a slope or become airborne. As a large
avalanche
speeds down a
mountainside
, it may
compress
the air below it, producing a powerful wind that can blow a house apart, breaking windows, splintering doors, and tearing off the roof.
Avalanches
strike suddenly and can be deadly. In 1970, a massive
avalanche
of
rocks
and
ice
destroyed the town of Yungay, Peru, killing 18,000 people.
If you are caught in an
avalanche
, the first thing to do is try to get off the slab. Skiers and
snowboarders
can head straight downhill to gather speed, and then veer sideways out of the slide path.
Snowmobilers
can punch the throttle to power out of harm’s way. If this is not possible, reach for a tree. As a last resort, try to “swim” up out of the
snow
. The human body is three times denser than
avalanche
debris and will sink quickly. This makes finding and rescuing
avalanche
victims much more difficult.
If buried in an
avalanche
, try to clear some space in front of you to breathe, then punch a hand skyward. Once the
avalanche
stops, it settles like concrete. Bodily movement is nearly impossible. Most
avalanche
victims are rescued, but those who aren’t die of suffocation as the
snow
hardens and buries them.
Avalanche beacons are the most common tools to help rescuers find
avalanche
victims.
Avalanche
beacons
are “beepers” that emit consistent noise when activated. Beacons can help rescuers locate a buried victim more than 80 meters (262 feet) away.
