Avalanche

An avalanche is a large mass of snow, ice, rocks, soil, or a combination of these elements that moves suddenly and swiftly down a mountain slope, pulled by the force of gravity. It destroys nearly everything in its path. The most common type of avalanche is a snow avalanche. An estimated 100,000 snow avalanches occur in the United States each year. Ice and debris avalanches, while they occur less frequently, are far more dangerous and cause greater damage than snow avalanches.

The Mount Huascarán avalanche of 1962

In the early evening of January 10, 1962, a huge mass of ice measuring about 2.5 million cubic yards (1.9 million cubic meters; the size of a football stadium filled from bottom to top) and weighing approximately 3 million tons (2.7 million metric tons; the weight of six thousand steam locomotives) broke loose from the glacier-capped peak of Mount Huascarán (pronounced wass-ka-RON), the tallest mountain in Peru. As the ice mass hurtled down the cliff face toward the populated valley below, it gained speed and grew in size, picking up rocks and other debris. After traveling nearly 10 miles (16 kilometers) in eight minutes, the mass came to a halt. In its wake, it left a carpet of ice, mud, and rock that covered ten villages and towns, ten thousand livestock, and almost four thousand people.

Mount Huascarán (Nevado Huascarán in Spanish) is part of the Andes, a 5,000-mile-long (8,045-kilometers-long) mountain system along the western coast of South America. The Andes, which run through seven countries—Argentina, Chile, Bolivia, Peru, Ecuador, Colombia, and Venezuela—are very tall. The mountain system contains many peaks that exceed 20,000 feet (6,100 meters) in height—that is thirteen times as tall as the world's tallest building. The only mountain range that exceeds the Andes in average elevation is the Himalayas. Some of the highest peaks in the Andes, including Mount Huascarán, are volcanoes (although most are dormant).

According to the geological theory known as plate tectonics, the Andes began to form millions of years ago when two plates or sections of Earth's crust advanced toward each other. Upon contact, one plate rode up and over the other, causing the land to rise. To this day, the plates continue to move, and the Andes continue to rise. The continual movement of the plates beneath the Andes makes the area geologically unstable, and earthquakes are common.

WORDS TO KNOW

acid rain:

rain that is made more acidic by sulfuric and/or nitric acid in the air, due to the burning of fossil fuels.

Andes:

mountain range extending more than 5,000 miles (8,045 kilometers) along the western coast of South America.

Alps:

mountain system composed of more than fifteen principle mountain ranges that extends in an arc for almost 660 miles (1,060 kilometers) across south-central Europe.

avalanche path:

the course an avalanche takes down a slope, composed of a starting zone, a track, and a runout zone.

avalanche wind:

a cloudlike mixture of snow particles and air pushed ahead of a slab avalanche as it races downward.

clear-cutting:

the logging practice of harvesting all trees from vast forest tracts.

glacier:

slowly flowing masses of ice created by years of snowfall and cold temperatures.

leeward:

the side of a mountain facing the direction toward which the wind is blowing (in the United States, the eastern side). Cold air descends and produces dry conditions on this side.

loose-snow avalanche:

avalanche composed of loosely packed snow that begins at a single point and slides down a slope, fanning out in the shape of an inverted V.

plate tectonics:

the geologic theory that Earth's crust is composed of rigid plates that float toward or away from each other, either directly or indirectly creating the major geologic features on the planet's surface.

Richter scale:

scale that measures the magnitude of an earthquake or size of ground waves generated at the earthquake's source.

slab avalanche:

avalanche that begins when fracture lines develop in a snowpack and a large surface plate breaks away, then crumbles into blocks as it falls down a slope.

windward:

the side of a mountain facing the direction from which the wind is blowing (in the United States, the western side). Warm air ascends, forms clouds, and yields precipitation on this side.

The climate in the Andes varies greatly, depending on both altitude (height above sea level) and latitude (distance north or south of the equator measured in degrees). There are hot regions, alpine meadows, glaciers, and a variety of climate types in between. Glaciers form where the winter snowfall exceeds the summer snowmelt, such as in high mountainous areas or polar regions.

In the Andes, glaciers occupy about 1,900 square miles (4,921 square kilometers). A section in the Peruvian Andes that has a large number of glaciers is called the White Mountains (Cordillera Blanca in Spanish). Named for the ice caps that persist even in the heat of summer, the White Mountains contain dozens of spectacular peaks towering above 19,686 feet (3,000 meters). Mount Huascarán is one of them.

A few years prior to the 1962 disaster, Peruvian geologists had completed a study of the hundreds of glaciers that punctuate the Andes. They had officially labeled the mass of ice atop Mount Huascarán Glacier No. 511. (Since glaciers dot the tops of so many peaks in the White Mountains, geologists assigned them numbers rather than names.)

Glacier no. 511 loomed over a peaceful valley

Glacier No. 511 regularly advanced and retreated with the seasons, creeping forward a few inches each day when fed by winter storms, then retreating slightly during the hot days of summer. Most people living in the valley below the White Mountains simply ignored the glacier—it had always been a part of their landscape. Others relied on the glacier for a source of income. Several Native American families, descendants of the great Inca civilization that had thrived in South America until the Spanish conquest in 1532, regarded the glacier as a type of ice factory. They would scale Mount Huascarán and chip blocks of ice from the glacier, wrap the ice in grass to prevent it from melting, and carry the blocks on their backs into the villages below. There they would sell the ice to restaurants and stores.

West of the White Mountains is a dark and dry section of the Andes known as the Black Mountains (Cordillera Negra in Spanish). Between the White and Black Mountains lies a deep and narrow valley called the Corridor of Greenery (Callejón de Huailas in Spanish). This valley, colored by rich green vegetation, is considered by many to be one of the world's most beautiful places. The Santa River (Rio Santa in Spanish) flows along the valley floor, framed by tall palm trees whose arching green leaves contrast vividly with the icy white glaciers above. Tourist books refer to this area as the "Switzerland of Peru," since it resembles the Swiss Alps, a section of the great European mountain system renowned for its beauty (and its avalanches as well).

The Corridor of Greenery, lying 750 miles (1,207 kilometers) south of the equator, is located in the Southern Hemisphere. Thus, summer begins in January. At only 9,000 feet (2,743 meters) above sea level—an entire 2.5 miles (4.0 kilometers) below the looming glacier—the valley receives the full effect of the warm equatorial sunshine. Valley residents raise sheep for their wool, and from the wool make handwoven blankets and clothing. They also grow fruit, grain, and vegetables in the fertile land along the Santa River.

The formula behind the disaster

Numerous factors tragically combined to send a piece of Glacier No. 511 sliding down the mountain and slamming into the valley below. The glacier had recently grown in thickness due to freak, heavy snowstorms. Several unseasonably hot summer days followed, melting the newly fallen snow. The extreme changes in temperature caused the surface of the glacier to develop cracks, into which flowed melted snow. Increasingly, more surface meltwater flowed downward, creating small streams that seeped to the bottom of the glacier and loosened its hold on the solid rock beneath. The glacier became increasingly unstable.

Geologists do not know for certain which single event forced a massive hunk of the glacier to break off. Some theorize that rocks slid down onto the vulnerable region of the glacier from a rocky peak overhead. Whatever the trigger, at 6:13 pm, as Glacier No. 511 glittered in the setting Sun, an enormous mass of ice broke loose and became the start of a fast-moving, deadly avalanche.

Ripping huge rocks from the cliff face, the falling ice crashed onto a lower section of the glacier 3,000 feet (914 meters) below. The mixture of ice, rock, and snow—preceded by a powdery white cloud—gathered speed as it skidded down the sloped surface of the 2-mile-long (3.2-kilometer-long) glacier. After sliding across the glacier's surface, the speeding ice mass roared into the mouth of the funnel-like valley canyon at more than 65 miles (105 kilometers) per hour.

An eyewitness to the disaster, a man who lived in the nearby city of Yungay, thought he saw a cloud turning golden in the Sun's fading light as he looked at Mount Huascarán. However, he quickly realized, as he told a reporter for National Geographic magazine, that "the cloud was flying downhill."

Slamming against the canyon walls, the avalanche cut away house-sized blocks of granite and carried them along in a 150-foot-high (46-meter-high) wall of ice, rock, and mud. The moving mass also kicked up hurricane-strength gusts of wind along its sides. The avalanche's size and momentum increased as it collected whatever debris lay in its path—topsoil, boulders, even sheep and llamas. Moving swiftly downward, the avalanche created friction along its bottom surface, which in turn melted thousands of tons of ice. The entire mass took on a white, soupy look.

The powerful avalanche scarred the walls of the canyon as it zigzagged downward like a bobsled bouncing against the sides of its track. In a later investigation of the disaster, geologists discovered five separate points of impact where the avalanche had rebounded off the canyon walls. The avalanche gained such force during its descent that it climbed hills as high as 275 feet (84 meters) and even left a 6,000-ton (5,442-metric ton) boulder balanced on top of a ridge.

The thunderous impact of the falling ice was heard and felt by people living in the villages sprinkled throughout the Corridor of Greenery. One person who was at the scene told a National Geographic reporter that the sound of the avalanche was a roar "like that of ten thousand beasts."

The first victims

At 6:15 pm, bloated with debris from the canyon floor and walls, the avalanche struck the first of several villages that lay in its path. Pacucco, Yanamachico, and other nearby mountain villages were quickly engulfed. More than eight hundred people were killed; only eight survived. At the moment of impact, the avalanche was twice its original size and traveling at nearly 100 miles (160 kilometers) per hour. Based on the speed and weight of the ice mass, the victims probably died immediately, even before realizing what was happening. Men returning from tending sheep in the fields, women cooking supper, and children playing outdoors were all instantly crushed when the avalanche poured over them.

Reports from the past: Ancient avalanches

The Alps, a mountain system extending about 660 miles (1,060 kilometers) across south-central Europe, is renowned for its many glaciers and magnificent scenery. Behind its beauty, however, lies the ever-present threat of avalanches, which have destroyed villages and claimed lives in the region for thousands of years.

Although no written records remain, historians believe many men in the army of Carthaginian general Hannibal (247–183 bce) died as a result of avalanches. In 218 bce, during the Second Punic War, Hannibal and his army set out to invade Rome-controlled Italy by crossing the Alps. Historians speculate that during this feat, one of the most remarkable in military history, nearly half of Hannibal's men perished, many smothered by avalanches.

The first written record of avalanches in the Alps appeared nearly two thousand years ago. Greek geographer and historian Strabo (c. 63 bce–after ce 21) wrote in his Geographia that crossing the Alps was dangerous because of ice that fell from the tops of the mountains.

The avalanche then continued moving toward the more populated region of the valley floor. Fortunately, as the valley became less steep, the speed of the avalanche slowed to about 60 miles (97 kilometers) per hour. The avalanche remained a lethal force, however, having grown to five times its original size—a volume approximately equal to seven Empire State Buildings. Although a steep bank of land redirected the avalanche away from the city of Yungay, the avalanche raced toward the town of Ranrahirca, with a population of nearly 3,000.

The avalanche struck as the evening lights came on

At 6:00 pm, Ranrahirca town electrician Ricardo Olivera had arrived at the town's power station to turn on the electricity for the evening. Mayor Alfonso Caballero stopped by the station to watch the lights come on, then continued his evening walk. Within a few minutes, both men heard the thunder of the approaching avalanche. They raced to their homes to warn their families. Meanwhile, panicked crowds of people jammed the streets. Many townspeople pushed their way toward the church, which they believed would be strong enough to withstand the force of the avalanche. Mayor Caballero safely reached his home, but the oncoming avalanche's roar drowned his shouts of warning to his sister inside. As the electrician Olivera reached his home, he came across two little girls from his neighborhood. Grabbing each one, he tried to pull them to safety on a side street.

Eyewitness report: An English avalanche?

In England, a country noted more for rain than for snow, avalanches are rare—but they do occur. The country's most devastating avalanche struck on December 27, 1836, in the town of Lewes in Sussex, an area in southern England. Heavy snow had begun to fall on the town three days earlier. Meanwhile, strong easterly winds blew back and forth over Cliffe Hill on the outskirts of town. The snow and winds combined to create a cornice or projection of snow that hung over the edge of the hill, some 200 feet (61 meters) above a row of houses below.

The next day residents saw the overhanging snow, but considered it more beautiful than threatening. Two days later, after the warmth of the sun had created a crack in the overhang, one man tried to warn the residents of the impending danger, but they did not listen. That afternoon, the snow cornice broke free and an avalanche buried the houses below, killing eight people.

Afterward, to commemorate the event, a tavern named the Snowdrop Inn was built on the site of the mishap.

Just eight minutes after Olivera had turned on the lights, the avalanche reached Ranrahirca. As the avalanche swept over the town, dust filled the air, choking and blinding the residents. The 40-foot-high (12-meter-high) wall of ice and rock knocked away a corner of Caballero's house, but both the mayor and his sister were spared. Olivera, too, was unharmed, but the edge of the avalanche snatched the two young children from his grasp. They, like members of Olivera's family, were buried beneath tons of ice. The people in the church were also buried as an icy sheet more than twice as tall as the church's steeple overtook them. In the span of just a few moments, between 2,400 and 2,700 people were killed; fewer than 100 people were spared. Most of the survivors lived or worked on the outskirts of the town, beyond the avalanche's route.

The avalanche then spread out like a huge fan on the valley floor. The enormous mass of ice and rock spilled into the Santa River, climbing 100 feet (30 meters) up the opposite bank and creating a dam that produced flood waters more than 15 feet (5 meters) deep. There, at 6:20 pm, nearly 10 miles (16 kilometers) from where it had started, the avalanche stopped. In its wake nearly four thousand people lay dead, most of them buried under the massive pile of ice, mud, and rock. Some bodies were later discovered more than 100 miles (160 kilometers) downstream, where the Santa River empties into the Pacific Ocean.

Relief efforts were futile

Because the avalanche had destroyed telephone and other communication lines in the area, word of the disaster was slow in getting out. Hours passed before government helicopters hovered over the huge stretch of icy white debris, dropping off soldiers to provide help. Medical supplies, doctors, and nurses were transported by airplanes to a small airport in an area untouched by the avalanche. Ranrahirca, which had been a thriving community with cobblestone streets and buildings with red-tiled roofs, lay buried beneath 40 to 60 feet (12 to 18 meters) of mud and rock. There were only about twelve injured people to treat; the rest of the avalanche victims had died. Relief teams quickly realized there was little they could do.

In addition to the great human loss, about ten thousand livestock lay beneath the rocky cover of mud. Rescue workers feared that decomposing animals and human bodies would soon contaminate the region's water supply with disease-carrying germs. Temporary medical clinics were quickly set up to administer vaccinations. Survivors were given shots to protect them against typhoid fever, a deadly disease transmitted by contaminated food or water. To prevent the spread of typhus—another deadly disease typically spread by fleas, lice, or mites—insecticides were sprayed on the remaining trees and plants in the valley.

Two days after the avalanche, U.S. president John F. Kennedy sent a telegram to Peruvian president Manuel Prado y Ugarteche offering the sympathies of all Americans. President Kennedy also asked James Loeb, the U.S. ambassador to Peru, to determine what emergency aid the United States could provide. Unfortunately, since there were so few survivors, there was little assistance any individual or country could offer.

The slow recovery

Fearing that a second avalanche could occur, survivors and rescue workers salvaged belongings and began to clear roads. A refugee center was set up in a high school building that was spared by the avalanche; wooden planks were used to create a temporary footbridge across the wide stretch of muddy rock. Bulldozers were used to clear mud and debris. As the ice melted over the next few weeks, some bodies were unearthed from the mud and, when possible, identified and buried. Since most people were torn to pieces by the powerful impact of the avalanche, the thawing mess became a gruesome scene of scattered body parts.

Local legend suggests that the beautiful mountains around the Corridor of Greenery have hurled down deadly avalanches of ice and snow on past occasions. In fact, in the Native American language Quechua, Ranrahirca means "Hill of Many Stones." Months after the disaster, Mayor Caballero issued a proclamation declaring that a new town of Ranrahirca would someday be built. In honor of the lives lost on that tragic day, the new town's main avenue was to be called the Street of January Tenth.

Recent events: The 1998–99 Swiss avalanche

No country in the world has more of an interest in avalanche research than Switzerland. More than 50 percent of Switzerland's population lives in avalanche terrain. During the 1998–99 avalanche season, the Swiss suffered through their worst avalanche season in forty-five years. Hundreds of major avalanches took place in the Swiss Alps, killing thiry-six people and causing more than $100 million in damages.

The worst of these avalanches took place in the resort town of Evolène in southwest Switzerland, where heavy rain and snowfall triggered an avalanche that killed twelve people. The damage was so severe that the mayor and local security chief were later convicted of failing to take appropriate precautions, such as evacuating houses and closing roads.

Because of its population's vulnerability to avalanches, the Swiss government invests significant resources in the study of avalanches. At the Swiss Federal Institute for Snow and Avalanche Research (the SLF), located in Davos, a small town in eastern Switzerland about 92 miles (147 kilometers) from Zurich, scientists oversee a network of electronic monitors that collect meteorological data that help predict when and where avalanches will take place. Based on this data, the SLF sends out avalanche bulletins advising citizens of avalanche conditions and warning of extreme situations as much as seventy-two hours in advance.

Disaster struck once again

A mere eight years after the 1962 disaster, a much larger tragedy befell the Corridor of Greenery. On May 31, 1970, a forty-five-second earthquake with a magnitude of 7.8 on the Richter scale caused a huge amount of rock and glacial ice to break off the west face of Mount Huascarán and plummet down toward the valley. Within three minutes, almost 80 million cubic yards (61 million cubic meters) of ice, rock, water, and debris traveled nearly 11 miles (18 kilometers). Traveling at an average speed of 100 miles (160 kilometers) per hour, the avalanche completely buried the city of Yungay (spared in 1962) and nearly a dozen villages, killing almost 20,000 people in the Corridor of Greenery. (Overall, the earthquake claimed a total of 70,000 lives across an area of about 32,370 square miles [83,000 square kilometers].)

Dangerous science: What causes avalanches?

While any movement of snow, ice, rocks, or mud down the slope of a mountain or hill can be considered an avalanche, the term is most often used to describe the rapid downward movement of a vast quantity of snow. (The movement of rocks and mud is more commonly known as a landslide.) Scientists estimate that as many as one million avalanches take place around the world each year. Of these, most occur in the Alps in Austria, France, Italy, and Switzerland. In the mountainous western region of the United States, approximately 100,000 avalanches tumble down each year (most of them are in the Rocky Mountains). The number of avalanches in the United States is small in comparison to the number in the Alps and the Andes.

Snow avalanches take on many forms but are generally placed into two categories: loose-snow and slab. Slab avalanches are, by far, the more common and more deadly of the two.

Watch this: "Avalanche!"

In 1997, the PBS program NOVA broadcast an episode called "Avalanche!," a look at avalanches and the scientists who study them. According to the show, avalanches are ferocious enough to have earned the nickname "white death." Avalanches are also an increasing problem as skiers, backpackers and snowmobile-riders venture into previously undisturbed back country, where the risk of avalanche is higher.

The show explains the science of avalanches and also follows a team of scientists as they try to learn more about avalanches as they happen. The scientists are so dedicated to understanding and exploring avalanches that at one point in the show an avalanche buries them alive!

Loose-snow avalanches

A loose-snow avalanche (also called a pure avalanche) is, as its name implies, composed of snowflakes or snow crystals that are loosely packed. The crystals behave much like dry sand: the bonds between them are not very strong, and they merely lie upon each other. A loose-snow avalanche usually begins at a single point on a slope when a small portion of snow slips and begins to slide, knocking into other crystals on the surface. As the avalanche runs downward, picking up more snow, it fans out in the shape of an inverted V.

If the snow on the slope is dry and powdery, the loose-snow avalanche can travel at speeds up to 100 miles (160 kilometers) per hour. Conversely, if the snow involved is melting and wet, the avalanche may move at speeds of only 5 to 10 miles (8 to 16 kilometers) per hour. Some loose-snow avalanches travel only 10 to 30 feet (16 to 48 meters) before stopping.

Unstable snow is the most significant factor in the creation of a loose-snow avalanche, as it is in a slab avalanche. Where snow is loosely packed on a slope, any disturbance of the delicate balance existing near the slope's surface will result in a slide. The added weight of new snow dropped by a fierce storm is a leading cause. Additional snow can also be deposited on a slope by winds, which usually blow up one side of a hill or mountain (called the windward side) and down the other (called the leeward side). As winds blow up the windward side, they scrape loose snow from the slope and drop it on the leeward side after passing over the summit. This accumulation of snow stresses the existing snowpack, causing it to slide. Another strain on a snowpack can be brought about by the warmth of the Sun, which melts snow at the surface, making it denser and heavier.

Slab avalanches

A slab avalanche begins when fracture lines develop in the snowpack and a large surface plate breaks away and then crumbles into blocks as it falls down a slope. As with a loose-snow avalanche, many factors combine to produce a slab avalanche—including the condition of the snowpack, temperature, weather, and wind direction. Unlike a loose-snow avalanche, a slab avalanche brings down large amounts of snow all at once, making it much more powerful and dangerous. A slab may be more than 100,000 square feet (9,290 square meters) in area (equal to three 100-unit apartment buildings) and more than 30 feet (9 meters) thick. As it tumbles down the slope at speeds approaching 100 miles (160 kilometers) per hour or greater, it picks up more snow and may grow to one hundred times its original size.

Again, unstable snow is the main trigger behind a slab avalanche. Throughout a winter season, numerous layers of snow build up on a slope. As layers of snow are deposited during storms, the snow crystals making up the existing layers are compacted by the weight of new snow. The older crystals become rounded, generally forming stronger bonds between themselves and making the snow layer more stable. Under optimum conditions, the weather and temperature during and between storms remain the same. Consistently cold temperatures and light snows allow each new layer to bond readily and tightly to the layer just beneath the surface.

Weather and temperature on a mountain slope, however, hardly ever remain the same, even within a single day. Clear, warm spells often abruptly change to stormy ones. These large variations in temperature and snowfall create unstable snow layers. If cold nights follow warm days, then the crystals within a snowpack melt and refreeze, weakening the bonds between them. If warm days follow a snowfall, then the crystals in the upper layer may melt and form a slick surface to which subsequent snowfalls do not easily bond. Rain also creates slick surfaces, not only on the top layer but throughout the lower layers in a snowpack.

Snowfall and wind direction can also contribute to the creation of a slab avalanche. The added weight of a single snowfall measuring 12 inches (30 centimeters) or more can quickly produce an extremely destructive avalanche. Winds blowing up the windward side of a mountain deposit snow unevenly on the leeward side to create unstable conditions.

Earthquakes and even minor earth tremors can also set off a slab avalanche. As the ground beneath a slope moves, fractures may develop in an unstable snowpack and a large section may break loose.

Both slab and loose-snow avalanches can occur on any slope, but they most often take place on slopes that have angles measuring between 30° and 45°. Snow on slopes with angles less than 30° is generally more stable and not affected as much by the pull of gravity. Snow on slopes with angles more than 45° generally does not have a chance to accumulate because it sloughs (pronounced SLUFFS) off in frequent little avalanches.

Aftermath: The effects of avalanches

Loose-snow avalanches are usually not dangerous, but there are exceptions. Large loose-snow avalanches can carry humans and animals over the edge of a cliff or bury them in deep snow. They can also destroy buildings and whole sections of forest. Even worse, on a very unstable slope, a fast-moving loose-snow avalanche can trigger a larger slab avalanche.

Slab avalanches, because of their great size, are almost always dangerous. A large slab avalanche will usually mow down and carry away anything in its path: trees, boulders, animals, humans, and buildings. Slab avalanches composed of powdery snow have an additional destructive aspect—avalanche wind. As an avalanche sweeps down a slope, wind rushes ahead of the sliding snow mass. This wind, a mixture of snow particles and air around the avalanche, is like a dust cloud or a heavy gas and is difficult to breathe. When the avalanche comes to a sudden stop, the wind around it rushes out violently in all directions. The force of this wind is especially destructive if the sliding snow or ice mass has fallen almost vertically to a valley floor. Like a bomb blast, the wind can actually blow down nearby houses and other structures.

Avalanche paths

The course an avalanche takes down a slope is called the avalanche path. Large avalanches traveling repeatedly down the same path leave a lasting scar on Earth's surface. Such scars appear as bare lines on a mountainside otherwise covered with trees and vegetation.

Paths can run through narrow gullies or across open slopes. Although they differ in shape and length, avalanche paths all have three main parts: the starting zone, the track, and the runout zone. The starting zone is where the avalanche begins, typically high up on a slope. At the starting zone, snow collects unevenly; loose surface snow begins to slough, or fracture lines cut slabs from the snowpack. The track is the trail or channel the avalanche takes as it races downward. This middle section of the track is where the rushing snow or ice mass reaches its greatest speed. The runout zone is where the snow and debris finally come to a halt. It may be a level area at the base of a mountain where the avalanche gradually slows down, or a deep gully or ravine where the avalanche stops abruptly. The runout zone, where snow and debris pile the highest, is where victims are most often buried.

Eyewitness report: The Wellington disaster

The worst avalanche disaster in the United States occurred at Wellington, Washington, in 1910. Wellington was a small railroad town consisting of a railroad depot, a few railroad sheds and bunkhouses, and a hotel. It was located at the western end of the Cascade Tunnel, which runs almost 8 miles (13 kilometers) through the Cascade Mountain Range.

Two trains, a mail train and a passenger train of seven cars, came to a halt on the westbound tracks at Wellington in the late evening of February 24, 1910. Heavy snow had been falling for days, and portions of the track ahead of the trains lay buried under snowdrifts and small avalanches. Looming above the trains was the broad, snow-covered slope of Windy Mountain.

For days after, the trains were motionless as railroad workers tried to clear the tracks with rotary plows. Progress was slow. While the plows were clearing packed snow in one area, snow would pile up in another area. One plow soon broke down and another, out of fuel, was stranded between snow piles.

Railroad workers, discovering that the telegraph lines were down, decided to hike 4 miles (6 kilometers) through the snow to the depot of Scenic to send for more plows and men. Shortly thereafter, six male passengers and a few more railroad men joined them. The remaining passengers and railroad laborers remained with the trains, waiting for the weather to change.

During the evening of February 28, the falling snow turned to rain, and a lightning storm followed. At 1:30 am on March 1, a snow mass about 1,350 feet (411 meters) wide slipped loose from the slope of Windy Mountain and dropped 500 feet (152 meters) to the tracks below. The avalanche carried the passenger train, the mail train, a plow, some boxcars and electric engines, and more than 100 people over a ledge and into a canyon 150 feet (46 meters) below.

Railroad workers who had not been on the trains quickly descended into the canyon and began digging for survivors. The digging, slow and by hand, continued for eight days. When it was complete, only 22 of the 118 passengers and workers who had been buried by the avalanche had been found alive.

Avalanches and the paths they create do have certain benefits. Since trees and other large plants have been cleared from these paths, meadows are able to develop in spring and summer. Filled with grasses, wildflowers, and small shrubs, these areas provide necessary food for mountain-dwelling animals such as bear, deer, elk, and moose.

Avalanches have little, if any, benefits for humans. Any interaction between avalanches and humans typically ends in destruction, injury, and death. Roads and towns built near avalanche paths are either partially or completely buried. Each year, thousands of people around the world are killed or injured in avalanches.

The human factor

For thousands of years, humans have settled in valleys at the base of mountains where snow runs down and forms clear streams, and the fertile soil produces abundant vegetation. The natural beauty of such settings is often astounding. As long as humans have inhabited these areas, however, they have had to face the peril of avalanches. For centuries, villages in the Swiss Alps have been buried by avalanches, only to be rebuilt and buried again.

Forests surrounding these valley communities have provided protection against the force of certain avalanches: trees in mature or well-developed forests can slow or stop the rush of a small avalanche. But over time, as these villages grew in size, the inhabitants began to cut down the surrounding trees for fuel and housing. In the process, they destroyed their only protective barrier. In modern times, remaining forest regions in the Alps have withered away because of the effects of acid rain (rain that is made more acidic by sulfuric and/or nitric acid in the air, due to the burning of fossil fuels). In the western United States, the risk of avalanche damage has increased because of the clear-cutting of forests (logging practice of harvesting all trees from vast forest tracts).

Avalanche fatalities have also recently increased because of an upsurge in mountain recreation activities. In the United States and other countries, thousands of people are drawn each year to mountain areas to ski, hike, and take part in other winter sports. To accommodate these recreationists, roads, buildings, and towns have been built in avalanche-prone areas, increasing the risk of avalanche-related deaths.

With increasing numbers of people entering hazardous mountain terrain, more and more avalanches are being triggered. Larger avalanches are usually set off by natural events and do not involve people unless they happen to be in the area. Small- and medium-sized avalanches are responsible for more human deaths overall because humans often set them off. In the United States, snowmobilers, climbers, and backcountry skiers are the parties most responsible for starting avalanches. The simple weight of these people on unstable snow is enough to begin an avalanche that, most times, kills them. Experts predict that as the sport of snowboarding increases in popularity, snowboarders will join the list of victims.

A matter of survival: Living through avalanches

Nearly all avalanche fatalities can be avoided. Two ways to accomplish that are to stop building communities in avalanche-prone mountain valleys and to prohibit recreation on mountain slopes. Both solutions, however, are impractical.

In areas where avalanches frequently threaten communities, numerous steps can be taken to lessen their impact. On the slopes above roads or buildings, structures may be erected to either prevent avalanches from starting or to divert the path of an avalanche. Planting trees close together, for instance, can help prevent the formation of avalanches and stop the approach of ones that do develop. In starting zones, areas higher up on slopes where trees will usually not grow, large fences can be erected to keep the snow from sliding down. Large, slotted barriers called snow rakes can also be used to decrease the amount and speed of the falling snow mass.

Farther down the avalanche path, where roads or railroad tracks pass through, avalanche sheds can be built. A shed, constructed like an overpass with one end built into the slope, diverts the snow over the road or tracks to fall on the other side. Near buildings or other structures, heavy stone or concrete walls can be built to deflect the snow. In the lower reaches of an avalanche path, earth or rock mounds can also be constructed to break up the snow mass and slow its speed.

An interesting design that provides direct protection to buildings is a wedge-shaped wall built in front of the structure with its point facing the slope. Sometimes, the building's wall that faces the slope is itself constructed in the shape of a wedge. Much like a ship's bow that cuts through water, the wedge-shaped wall cuts into the oncoming snow mass, forcing it to travel around the sides of the building.

In areas where roads and railroad tracks follow mountainous terrain for miles, the cost of these protective measures is prohibitive. In such situations, avalanche experts periodically use explosives—shot by cannon or gun, dropped from helicopters, or placed by hand—to dislodge the snow. This creates small avalanches and thus prevents the accumulation of heavier, and possibly more destructive, snowpacks.

Personal safety

For those venturing onto mountains, there are a few steps to avoid becoming a victim of an avalanche. The most critical step is to gather as much information in advance about snowpack conditions and upcoming weather from forest service, national park, or ski patrol personnel. Before going on a mountain, it is also wise to have proper safety equipment, including an avalanche transceiver or beacon (a device that emits a signal indicating one's position) and a portable shovel. In addition, when on a mountainside, it is important to be alert to the surrounding conditions, such as the slope angle, tender or weak spots in the snow, fracture lines and other disruptions on the surface, and wind direction. When traveling on a snowy mountain, it is also safest to be part of a group. If buried by an avalanche, an individual will most likely need the help of others to get out alive.

People caught in an avalanche die in one of two ways: they either suffer a fatal injury when they hit a boulder or tree during the slide, or they suffocate to death shortly after the slide comes to a halt. If caught in an avalanche, one's chances of survival are increased if certain techniques are followed. When the avalanche begins to slide, try to get out of its path or even move to its sides, where the snow will be moving more slowly. Remove any packs, skis, snowshoes, ski poles, or any other baggage that might weigh one down. If possible, grab a tree, large boulder, or something solid before the avalanche picks up speed. During the slide, try to stay near the snow's surface by "swimming" through the snow mass. When the slide begins to

Eyewitness report: The Iceman appears

On September 19, 1991, a German tourist set out walking across a glacier in the Alps between Austria and Italy. That summer and previous winter had both been warm. Warm winds had also been plentiful in the region. As a result, glaciers in the area had been melting; as they did, they revealed bodies of victims of climbing accidents from years past. Already that year Italian authorities had extracted eight bodies from the glaciers.

It wasn't a total surprise, then, when the tourist came upon the head and shoulders of a man frozen in the ice. Seeing a hole in the back of the man's head, the tourist suspected he had been the victim of a murder and so notified the police. When the dead man's body was removed from the ice, however, it was evident that he had been dead for an extremely long time. While the bodies of most victims trapped in a glacier are white and waxy, the body of this victim was brown and dried out.

Scientists were called in to determine the age of the dead man. After performing radiocarbon dating tests (a method of measuring the amount of carbon 14 left in organic matter), the scientists concluded the man in the ice was at least 5,200 years old. Dubbed the Iceman, he was the oldest human being ever discovered whose body was virtually intact.

Scientists speculated that the Iceman had been a shepherd who was caught in a storm and froze to death. Either icy winds or foehns dried out his body before it was covered by heavy snowfall or, more likely, an avalanche. Over time, the heavy snow compacted into ice and the Iceman's body was preserved by the cold temperature (about 21°F [−6°C]). The Iceman remained hidden for fifty-three centuries before the warm seasons and the dry winds uncovered him.

slow down, move around as much as possible. It is important to create a large breathing space rapidly, for within seconds after the snow stops moving it will harden.

Once trapped, a person may not know in what direction the surface lies. An easy way to find out is to spit or drool. The surface will be the opposite direction that the saliva flows. If a person is near the surface, he or she may be able to dig through the snow and stick a hand out to be visible to rescuers. If this is not possible, a person should remain calm—it is very important not to waste energy or remaining air by struggling. If one's avalanche beacon is in the transmit mode—and it should have been from the time he or she stepped onto the mountain—then searchers will have a better chance for a successful rescue.

Finding survivors or victims

Survivors of avalanches are most often found within the first thirty minutes after an avalanche comes to a stop. Group members who have not been buried are the ones who make the most rescues. They do so by honing in on the buried person's beacon signal with their own transceiver; by finding a glove, hat, or other sign that the person might be near the surface; by using metal probes; or by remembering where that person was last seen and then checking downhill from there.

The longer it takes to find a buried person, the less chance that person will survive. By the time most rescue teams arrive, the chance of finding any survivors is very slim.

Reports from the past: Avalanche casualties in World War I

World War I (1914–18) is known as the Great War because it was the largest war up to that time. In addition to the men killed by weapons, many died from natural occurrences such as disease and avalanches. It is believed that between forty thousand and eighty thousand men were victims of avalanches during the conflict.

Experts estimate that in the Dolomites, a section of the Alps in northern Italy, more men died in avalanches than from bullets, shells, and other weapons of war. During the early winter of 1916, the region received more snow than it had in fifty years. A warm period in December thawed the snow, and on December 13, more than one hundred avalanches plunged down the valleys in the Dolomites. Almost ten thousand Austrian and Italian troops were killed on that single day. Their bodies were still being recovered over thirty-five years later.

Professional rescuers use a combination of equipment (such as sonar, radar, and infrared detectors) and trained dogs to find buried people. An avalanche dog, relying on its keen sense of smell, can search an area of more than 1,000 square feet (93 square meters) in less than thirty minutes. A team of twenty people searching the same area would need four hours. By then, anyone who was buried would almost certainly be dead.

Technology connection: Measuring and predicting avalanches

Unfortunately, scientists cannot accurately predict exactly when and where an avalanche will take place. They can determine, however, when conditions exist that are favorable for an avalanche to occur.

Avalanche experts first look at the layers within a snowpack to decide if the snow in that area might slide. They start by digging what is called a snow pit. Dug deep into a snowpack, the snow pit reveals the composition of the layers of that snowpack. After digging, the avalanche testers use shovels to probe the various layers and determine if they differ in hardness. The crystals in the layers are examined to see if a layer of powdery, loosely packed snow is lying underneath a layer of wet, denser snow. The depth of the snowpack and the angle of the slope on which it lies is also checked.

Having gathered this information, avalanche professionals monitor the weather forecast. Is a low-pressure system moving into the area, bringing with it colder temperatures and snowstorms? Or is a warm front forecasted, which may cause surface melting? Finally, they check previous data to see if a particular area is prone to avalanches and if so, what time of year they typically happen. (Most avalanche fatalities in the United States occur in February.)

Around the world, many avalanche-prone regions have prediction services. The Swiss Federal Snow Institute for Snow and Avalanche Research, founded in 1936 in Switzerland, has about seventy observation stations located in the Alps at altitudes of 3,280 to 5,905 feet (1,000 to 1,800 meters). Observers in these stations record information about snow conditions, then transmit it to the institute. There the information is processed and, if necessary, avalanche warnings are issued to newspapers and radio and television stations. In the United States avalanche danger is monitored mainly by the U.S. Forest Service, since most ski areas are located within national forests. In addition, various western states prone to avalanches hire avalanche professionals to provide prediction services.

[See AlsoEarthquake; Landslide ]

For More Information

BOOKS

Drohan, Michele Ingber. Avalanches. New York: Rosen Publishing Group, 1998.

Ferguson, Sue, and Edward R. LaChapelle. The ABCs of Avalanche Safety. 3rd ed., Seattle, WA: Mountaineers Books, 2003.

Hamilton, Richard. Avalanches: Nature's Fury. Minneapolis, MN: Abdo and Daughters, 2005.

Merrick, Patrick. Avalanches. Plymouth, MN: Child's World, 1998.

WEB SITES

American Avalanche Association. 〈http://www.americanavalancheassociation.org/〉 (accessed August 17, 2006).

"Avalanche!" NOVA Online. 〈http://www.pbs.org/wgbh/nova/avalanche/〉 (accessed August 17, 2006).

Avalanche Awareness. National Snow and Ice Data Center. 〈http://nsidc.org/snow/avalanche/〉 (accessed August 17, 2006).

National Avalanche Center. U.S. Forest Service. 〈http://www.avalanche.org/%7enac/〉 (accessed August 17, 2006).

Avalanche

A sudden slide of snow and ice, usually in mountainous areas where there is heavy snow accumulation on moderate to steep slopes. Snow avalanches flow at an average speed of 80 mph (130 km/hr), and their length can range from less than 300 ft (100 m) to 2 mi (3.2 km) or more. Generally the term "avalanche" refers to sudden slides of snow and ice, but it can also be used to describe catastrophic debris slides consisting of mud and loose rock. Debris avalanches are especially associated with volcanic activity in which melted snow, earthquakes, and clouds of flowing ash can trigger movement of rock and mud. Snow avalanches generally consist either of loose, fresh snow or of slabs of accumulated snow and ice that move in large blocks. Snow avalanches occur most often where the snow surface has melted under the sun and then refreezes, forming a smooth surface of snow. Later snow falling on this smooth surface tends to adhere poorly, and it may slide off the slick plane of recrystallized snow when it is shaken by any form of vibration—including sound waves, earthquakes, or the movement of skiers.

Several factors contribute to snow avalanches, including snow accumulation, hill slope angle, slope shape (profile), and weather. Avalanches are most common where there is heavy snow accumulation on slopes of 25–65°, and they occur most often on slopes between 30° and 45°. On slopes steeper than 65° snow tends to sluff off rather than accumulate. On shallow slopes avalanches are likely to occur only in wet (melting) conditions, when accumulated snow may be heavy, and when snow melt collecting along a hardened old snow surface within the snowpack can loosen upper layers, allowing them to release easily. Slab avalanches may be more likely to start on convex slopes, where snow masses can be fractured into loose blocks, but they rarely begin on tree-covered slopes. However, loose snow avalanches often start among trees, gathering speed and snow as they cross open slopes. Weather can influence avalanche probability by changing the stability and cohesiveness of the snow pack. Many avalanches occur during storms when snow accumulates rapidly, or during sustained periods of cold weather when new snow remains loose. Like snow melt, rainfall, can increase chances of avalanche by lubricating the surface of hardened layers within the snow pack. Sustained winds increase snow accumulation on the leeward side of slopes, producing snow masses susceptible to slab movement. When conditions are favorable, an avalanche can be triggered by the weight of a person or by loud noises, earthquakes, or other sources of vibration. Avalanches tend to be most common in mid-winter, when snow accumulation is high, and in late spring, when melting causes instability in the snow pack.

Avalanches play an ecological role by keeping slopes clear of trees, thus maintaining openings vegetated by grasses, forbs, and low brush. They are also a geomorphologic force, since they maintain bare rock surfaces, which are susceptible to erosion .

Most research into the dynamics and causes of avalanches has occurred in populous mountain regions such as the Alps, the Cascades, and the Rocky Mountains, where avalanches cause damage and fatalities by crushing buildings and vehicles. Avalanches are very powerful: they can crush buildings, remove full-grown trees from hillsides, and even sweep railroad trains from their tracks. One of the greatest avalanche disasters on record occurred in 1910 in the Cascades near Seattle, Washington, when a passenger train, trapped in a narrow valley in a snow storm for several days, was caught in an avalanche and swept to the bottom of the valley. Ninety-six passengers died as the cars were crushed with snow. Although avalanches are among the more dangerous natural hazards, they have caused fewer than 200 recorded mortalities in North America, and most avalanche victims in North America are caught in slides they triggered themselves by walking or skiing across open slopes with accumulated snow.

[Mary Ann Cunningham Ph.D. ]

RESOURCES

BOOKS

Brugnot, G., ed. Snow and Avalanches. Conference proceedings, Chamonix, Switzerland, 1992. Zurich: Cemagret, 1995.

Salm, B., and H. Gubler. Avalanche Formation, Movement, and Effects. Wallingford, Oxfordshire, UK: IAHS Press, 1987.

United States Forest Service. "Snow Avalanche: General rules for Avoiding and Surviving Snow Avalanches." Portland, OR: USDA Forest Service, 1982.

Avalanche

An avalanche is a rapid downslope movement of some combination of rock , regolith , snow, slush, and ice . The movement can occur by any combination of sliding, falling, and rolling of pieces within the avalanche mass, but is generally very rapid. Avalanche velocities can reach tens to hundreds of kilometers per hour.

The term avalanche is generally associated with snow and ice. In its most general form, however, it can refer to the cascading of sand grains down the leeward face of a dune or the rapid downslope movement of largely disaggregated rock without snow or ice. Rock avalanches, for example, are very rapid and catastrophic mass movements of bedrock that has been broken into innumerable pieces either before or during movement.

Snow avalanches, hereafter referred to simply as avalanches, are classified according to whether they move across existing snow layers (surface avalanche) or the ground (ground avalanche), whether they are dry or wet, whether they move through the air or over ground and snow, and whether they consist of loose snow or intact slabs. Like landslides, avalanches begin when the weight of snow above some potential sliding surface exceeds the shear strength along that surface. In many cases, sliding occurs along a former snow surface that is quickly buried by new snow during a storm. The physics of slip surface formation, however, are more complicated for avalanches than most landslides because the snow and ice in an avalanche prone slope are near their melting points. Thus, phase transitions and metamorphosis of snow and ice crystals can alter the strength of snow and ice slopes in a way that does not occur in soil or rock slopes. Melting can also trigger avalanches. Although it is not proven that loud noises such as shouting can trigger avalanches, the vibrations caused by explosives can do so—and explosives are often used to deliberately trigger avalanches under controlled conditions as a safety measure.

The aftermath of an avalanche is an avalanche track or chute, which is commonly marked by bent or broken trees and significant amounts of erosion . The track can be either a channel-like or open feature. The rock and debris carried by an avalanche can be deposited as an avalanche cone when the avalanche comes to rest, and the rock debris deposited at the base of a cliff or other steep slope by an avalanche is known as avalanche talus.

See also Catastrophic mass movements; Freezing and melting; Ice; Landslide; Mass movement; Mass wasting; Phase state changes

av·a·lanche / ˈavəˌlanch/ • n. a mass of snow, ice, and rocks falling rapidly down a mountainside. ∎  a large mass of any material moving rapidly downhill: an avalanche of mud. ∎ fig. a sudden arrival or occurrence of something in overwhelming quantities: we have had an avalanche of applications. ∎  Physics a cumulative process in which a fast-moving ion or electron generates further ions and electrons by collision.• v. [intr.] (of a mass of snow, ice, and rocks) descend rapidly down a mountainside. ∎  [tr.] (usu. be avalanched) engulf or carry off by such a mass of material: the climbers were avalanched down the south face of the mountain. ∎  [intr.] Physics undergo a rapid increase in conductivity due to an avalanche process.

avalanche wind A blast of often very destructive air ahead of a descending avalanche (see mass-wasting).