Landslide

Alandslide is the movement of large amounts of soil, rocks, mud, and other debris downward along a slope. The movement is caused by the pull of gravity and occurs when a mountainside or hillside weakens and is unable to support its own weight. The amount of material that falls in a landslide can be as small as the size of a refrigerator or as large as an entire mountainside. The falling material can move slowly or quickly, and may travel a few feet (meters) or several miles (kilometers) before it stops. Some landslides move only a little each year. Such movement may be irregular, or it may happen at the same time each year, such as in the spring, when the snow melts.

The most common cause of landslides is excessive moisture in the ground, because of heavy rains or melting snow. When the ground becomes so full of water it cannot hold any more, the soil loses its ability to stick together. It also becomes heavier, which hastens its movement down the slope. Not all landslides are caused by water. Earthquakes, the sudden movements of earth which release huge amounts of energy, trigger the largest and most devastating landslides. Other events that cause landslides include volcanoes and vibrations from explosions or heavy traffic. Human activities, such as mountainside development and mining, also contribute to instability on slopes. Forest fires are indirect causes of landslides, since they remove the vegetation and roots that hold the soil in place.

While rapid landslides cause the greatest loss of life and property, even slow landslides can cause structural damage to buildings and rupture underground power lines and water mains. Each year in the United States, landslides cause between twenty-five and fifty deaths and up to $2 billion in damage. In less developed nations, where there are often less strict zoning laws (allowing construction in landslide-prone areas), higher population densities, and a lack of protective structures, the death tolls and amount of property damage are much higher.

WORDS TO KNOW

debris avalanche:

a downward slide of loose, earthen material (soil, mud, and small rocks) that begins suddenly and travels at great speeds; similar to a snow avalanche. It builds into a fearsome mass of mud, trees, and rocks that can cause much damage.

debris slide:

a slide of small rocks and shallow layers of loose soil that commonly follows volcanic eruptions.

deforestation:

the removal of all or most of the trees from a region.

earthflow:

a landslide that consists of material that is moist and full of clay, yet drier than the material in mudflows.

earthquake:

a sudden shifting of masses of rock beneath Earth's surface, which releases enormous amounts of energy and sends out shock waves that cause the ground to shake.

erosion:

the removal of soil by water or wind. This is especially harmful when the uppermost layer of soil, called the topsoil, is stripped away, because this is the layer where plants grow.

fall:

the downward motion of rock or soil through the air or along the surface of a steep slope.

lahar:

a mudflow composed of volcanic ash and water that occurs in the wake of a volcanic eruption.

landslide:

the movement of large amounts of soil, rocks, mud, and other debris downward and outward along a slope.

mudflow:

a landslide consisting of soil mixed with water. It is wetter than the material in an earthflow.

rock slide:

a cascade of rocks (of any size) down a steep slope at high speeds.

saturated:

containing the maximum amount of water a material can hold.

slump:

the slow downhill movement of large portions (called blocks) of a slope. Each block rotates backward toward the slope in a series of curving movements.

solifluction:

the most rapid type of earthflow, occurring when snow or ice thaws or when earthquakes produce shocks that turn the soil into a fluid-like mass.

volcano:

an opening in Earth's surface through which gases, hot rocks, and ash are ejected from the heated inner portion of the planet.

The Frank slide

In the early morning hours of April 29, 1903, a block of limestone approximately 0.5 square miles (0.8 square kilometers) in area cascaded onto the coal mining village of Frank in south-central Alberta, Canada. The limestone, which weighed 50 million to 90 million tons (45 million to 82 million metric tons) and was 500 feet (152 meters) thick, came hurtling down from a height of 3,100 feet (945 meters) between two peaks of Turtle Mountain. Seventy-six people were killed instantly in the landslide, although some records place the number as high as ninety. In addition to burying part of the town, the fallen rock dammed the Crowsnest River and created a new lake.

Native inhabitants feared the mountain

The native people, who were the original inhabitants of the region, recognized the dangers of Turtle Mountain. The mountain had been named for its shape, which resembled a turtle's shell and had a slab of limestone sticking out like a turtle's head. To the Indians, it was "the mountain that walked." Fearing a rock slide, the native people shunned the idea of settling, or even so much as camping, at the base of the mountain.

Turtle Mountain was indeed geologically unstable, meaning it had little resistance to sliding or collapsing. The twin-peaked mountain was composed of weather-worn limestone near its peak and soft stone laced with veins of coal at lower altitudes. The angle of the mountain was especially steep on the eastern slope, and the face had developed cracks. Along the cracks, the rock was fragmented. An earthquake two years prior to the rock slide had made the rock fragmentation worse. Further weakening of the rock foundation was brought about by the tunneling of coal mining shafts, which were dug about 1 mile (1.3 kilometers) into the mountain.

Frank established as a mining town

The town of Frank was incorporated on September 10, 1901, just eleven months after a coal deposit had been discovered at the base of the mountain. It was named after H. L. Frank, a banker from Montana who had funded a mining operation to extract the coal. He persuaded adventurous frontiersmen, many of them recent arrivals from Europe, to travel to Turtle Mountain and work in the mine. He also paid the Canadian Pacific Railway to run railroad tracks from the main line to the mine entrance. By the time the town of Frank was established, miners were extracting hundreds of tons of coal each day.

The town of Frank looked just like a Hollywood set for a Western movie. Among the company buildings—miners' cabins, office buildings, and a boarding house—sprang up hotels, saloons, and casinos. The miners spent a significant portion of their earnings on drinking and gambling. By the spring of 1903, Frank had a population of approximately six hundred people.

The mountain tumbles down

In the months prior to the landslide, miners had reported numerous sounds that should have been taken as warnings of the mountain's instability. The wooden supports in the tunnels groaned loudly and the tunnel walls shook. The vibrations had become so violent that coal literally dropped from the ceiling, forming piles that the miners merely had to shovel into cars. The night before the landslide, the mountain emitted a rumble that could be heard throughout the valley. Area residents ignored the noise and went to sleep.

Disaster struck at 4:10 am the next morning. According to Sid Choquette, a brakeman on the railroad who had been walking alongside a slow-moving train, the mountain began to creak and groan loudly, then emitted an eerie whistling roar. Next Choquette heard a sound like cannon fire and saw a huge slab of rock racing down the slope, right toward the town. Choquette jumped aboard the train and the engineer set the controls on full-throttle. The train raced for the bridge that spanned the Crowsnest River and made it across just moments before the avalanche of rocks wiped out the bridge. The railroad crew watched, stunned, as millions of tons of limestone plowed through the south side of Frank, continued 2.5 miles (3.2 kilometers) across the valley floor, and then climbed 400 feet (122 meters) up the opposite slope.

In a little more than a minute the rock slide, plus a wall of cold air that forcefully thrust ahead of it, leveled much of the town of Frank, killing many of its residents. Three-quarters of the houses were destroyed, as were the electric power plant, the livery stable, the shoe store, the miners' temporary quarters, the construction camp, the cemetery, and more than 1 mile (1.6 kilometers) of the Canadian Pacific Railroad. The debris tumbled into the river, blocking the flow of water, and portions of the railroad tracks were covered by rocks 100 feet (30 meters) deep. Structures were ripped apart, flung across the valley, and then buried forever by the massive limestone boulders. Only twelve bodies were recovered, and the exact number of dead was never determined. Because there were no accurate records of the number of inhabitants at the time, it was impossible to compile a list of the missing.

Trapped miners dig themselves out

When the rocks fell, seventeen workers were inside the mine and three others were taking a lunch break outside the entrance. The rock slide fell on the entrance, killing the three men outside and trapping the others inside. The miner most familiar with the mine's layout informed his trapped co-workers that they were at least 295 feet (90 meters) from the edge of the mountain. The miners knew they had to act quickly if they were going to get out alive. They realized the airshafts were probably blocked, meaning that breathable air would be in short supply. They were also concerned that explosive and lethal gases might be escaping from cracks in the rock walls.

The miners first tried to tunnel outward, but made little progress. Every time they dug into the wall, more rock tumbled down to fill in the space. After a few hours they tried a new strategy: digging upward, along a vein of coal. The coal was softer and easier to remove than the limestone. Three men dug at a time. They worked in shifts, making slow but steady progress. About thirteen hours after the rock slide, the miners reached the surface. They climbed out onto the mountainside and looked in horror upon the devastation below.

Rescue efforts

People in Frank not directly in the rock slide's path were shaken from their slumber by the loud roar. Stunned, they staggered into the streets to see what had happened. Seeing fires and hearing screams for help, some people grabbed their lanterns and rushed toward the wreckage. The avalanche, however, spared few of those it touched. Rescuers only found twenty-three people alive in the landslide's path.

At one ruined home, merchant Alex Leitch and his wife and four sons were found dead. At another home, Sam Ennis had dug himself out of the rubble and worked with rescuers to free his wife, Lucy, from a beam that had fallen on her. Even while trapped, Lucy Ennis had managed to save their baby daughter, Gladys, who was choking on a clod of dirt. Sam Ennis' brother-in-law, James Warrington, was also buried beneath a pile of rocks. Warrington warned rescuers to dig gingerly, since he felt something soft beneath him. It turned out that Warrington had landed on top of his next-door neighbor, Mrs. Watkins, who had been thrown from her own house.

On the railroad tracks, brakeman Sid Choquette, still shaken from his brush with death, turned his attention to the passenger train that was due to arrive in an hour. If it wasn't warned, the train would crash into the rock-covered tracks. He made his way through the cloud of dust that hung in the air and across rocks the size of small buildings until he arrived at the tracks on the east side of the rockpile. With only minutes to spare, he was able to flag down the train and prevent a second disaster.

The aftermath of the slide

It was later determined that the slide had been set in motion when a sudden cold spell caused water from melting snow to freeze within the cracks. The freezing water had expanded as it turned to ice and had forced the cracks to widen, sending the rocks tumbling down the slopes.

The mine reopened briefly after the landslide, only to be shut down by a fire in 1905. H. L. Frank suffered a nervous breakdown and was placed in a mental institution; he died there in 1908. The mine reopened one more time, but was permanently closed in 1918. Today, about two hundred people live in the town of Frank.

Recent devastating landslides

On February 17, 2006, a series of mud slides caused widespread damage and loss of life in the Philippine province of Southern Leyte. Over 80 inches (200 cm) of rain had fallen in ten days, loosening the soil. Deforestation, the removal of all or most of the trees, of mountain slopes may have contributed to the unstable conditions. The actual landslide was triggered by small earthquake of magnitude 2.6 on the Richter scale.

Devil's Slide is an area of California State Highway 1 that suffers from frequent blockage and damage due to slides and slumps, which are slower downhill movements. The most recent closure was in April of 2006, when longitudinal cracks appeared in the roadway. The highway was reopened for limited use in August 2006. Efforts to build a bypass around Devil's Slide that would have bisected a California state park were opposed by environmentalists concerned with loss of valuable and ecologically sensitive coastal habitat. Instead, California is constructing a tunnel, with completion set for 2011.

Mudflows strike southern Italy

Two days of heavy rains set off deadly mudflows in the Campania region of southern Italy on May 5, 1998. The rivers of mud that rushed down from the mountains flowed into several towns across a 35-mile (60-kilometer) stretch from Sarno to Naples. One week after the disaster the death toll was 139, with an estimated 146 people still missing. The majority of the fatalities occurred in Sarno.

The mudflows were so forceful that they inundated buildings, destroyed homes, and buried people. At least fifteen hundred people were left homeless. After the disaster, the streets of the affected towns were wastelands of mud, wood, boulders, and other debris. Telephone, electricity, and water service were cut off. The mudflows were among Italy's worst natural disasters of all time; they were nicknamed by the Italian media the "Pompeii of the Year 2000," after the famous volcanic eruption that took thousands of lives in 79 ce.

Several thousand soldiers, firefighters, and military personnel from a nearby U.S. Navy base searched for survivors. They freed several people trapped in homes or cars. Searchers found a man who, for three days, had been buried up to his neck in mud in his cellar. Rescue workers used bulldozers and shovels to clear streets of hardened mud, which in some places was 10 feet (3 meters) high.

The damage from the mud slide was blamed on lax construction codes, which allowed the building of houses in areas known to be vulnerable to mud slides. Another factor was the illegal building of houses in areas where construction had been off-limits. The intensity of the mud slides was due, in part, to recent forest fires that had stripped the slopes of vegetation.

The eruption of Mount St. Helens on May 18, 1980, produced a series of lahars, or volcanic mudflows. The lahars (pronounced LAH-hahrs) flowed many miles down the Toutle River and Cowlitz River, destroying bridges and lumber camps. A total of 3.9 million cubic yards (3.0 million cubic meters) of material was transported by the lahars.

Dangerous science: How landslides happen

Landslides are classified into three groups: slides, falls, and flows, depending upon the type of material involved, the amount of air and water they contain, and the speed with which they fall. Another group, called, slumps, are sometimes included in the definition of a landslide.

Experiment: What triggers a landslide?

You can trigger a landslide right in your own home or classroom. Here is what you will need:

• newspaper or plastic sheeting—this is a messy experiment!
• a cardboard milk carton with one side cut away
• "land" materials, like pebbles, sand, dirt, and clay
• small watering can or a soda bottle
• graduated cylinder or measuring cup

Remember, there are different variables—such as slope, type of earth, amount of water—that contribute to landslides. This experiment helps determine how landslides occur given different variables. To set it up, fill your milk carton half way with a mixture of your "land" material. Record what type of mixture you use. Next, prop one end of your carton up with a couple of books. You may have to place another book at the other end (under the plastic) to keep the whole thing from sliding. Pour a measured amount of water on the higher end of the soil mixture in the carton until it is soaked but not sliding. You will need to do this gently, so use your watering can and pour slowly. Once the material is soaked, you are ready to trigger the landslide. You can choose to do this in a couple of ways: make the angle of the slope steeper gradually, an inch or two at a time, until the land material slides or begin slowly pouring a measured amount of water over the land material until a slide begins. Record exactly the angle of the slope or the amount of water used to trigger the slide.

You can repeat this experiment with different variables and compare the results. For example, try using different mixtures of land materials and see how results differ. Try setting similar land mixtures at different slope angles and see how results differ. What type of land material is most prone to landslides?

Slides

A slide consists of rock, mud, soil, water, or debris—or any combination of those materials—that goes tumbling down a steep slope at high speeds. A slide typically destroys everything in its path before finally coming to rest on a plateau or in the valley below. Rock slides are common in the European Alps, the U.S. Appalachians, and the Canadian Rockies, especially on slopes with few trees. They are frequently triggered by heavy rains, but can occur at any time without an obvious cause. Debris slides, made up of small rocks and shallow layers of loose soil, commonly follow volcanic eruptions.

A debris avalanche, a particularly dangerous variety of slide, begins suddenly and travels at speeds as fast as hundreds of miles (kilometers) per hour. Debris avalanches, which resemble their snowy cousins, occur most frequently on mountains in humid climates. They begin when soil at the top of a slope becomes saturated with water. That material begins to slide downward, building into a fearsome mass of mud, trees, rocks, and anything else in its path. A debris avalanche that occurred in 1977 in the Peruvian Andes, for example, contained between 20 million and 45 million cubic yards (15 million and 34 million cubic meters) of material traveling at about 100 miles (160 kilometers) per hour. The debris fell upon a city in the foothills, killing some nineteen thousand people.

Falls

A fall involves rock or soil dropping from an overhanging cliff or a steep slope. The most dangerous type of fall is a rockfall. Huge boulders may fall freely through the air or race down a mountainside, fragmenting into small pieces as they descend and becoming a raging current of debris. Rockfalls typically occur where cliffs have become steepened by erosion, the removal of soil, by rivers, glaciers, or waves. The rocks may be pried loose from the cliff or mountain by the freezing and thawing of water in the slope's cracks. Evidence of rockfalls can be seen in the piles of rock and debris at the base of steep slopes.

Large rockfalls can cause terrific damage. In 1970, for instance, a rockfall from the peak of the Huascarán volcano in Peru, triggered by an earthquake, created a debris avalanche that buried villages at the base of the volcano and killed almost twenty thousand people.

A geologist who witnessed the 1970 rockfall reported:

I heard a great roar coming from Huascarán. Looking up, I saw what appeared to be a great cloud of dust and it looked as though a large mass of rock and ice was breaking loose from the north peak…. The crest of the wave [of rock and ice] had a curl, like a huge breaker coming in from the ocean. I estimated the wave to be at least 80 meters [260 feet] high. I observed hundreds of people in Yungay running in all directions and many of them towards Cemetery Hill. All the while, there was a continuous loud roar and rumble. I reached the upper level of the cemetery near the top just as the debris flow struck the base of the hill and I was probably only ten seconds ahead of it…. It was the most horrible thing I have ever experienced and I will never forget it.

Flows

A flow is a landslide of wet material, which may contain rock, soil, and debris, combined with water. Mudflows are the most common, most liquid, and fastest type of flow. They contain water and soil, with a consistency somewhere between soup and freshly poured concrete.

Mudflows frequently occur in dry or semidry mountains and on steep-sided volcanoes that receive sudden, heavy rainfall. Loose, weathered rock and steep slopes with little or no vegetation are prone to mudflows. Mudflows can travel as fast as 55 miles (88 kilometers) per hour and have enough force to pick up and carry along debris the size of boulders, cars, trees, and houses. They typically spread out across great distances on valley floors, depositing a thin layer of mud mixed with boulders. In the United States, mudflows do millions of dollars of damage every year. One region vulnerable to mudflow damage is Southern California, especially the hilly suburban communities.

Another type of flow, called an earthflow, consists of material that is moist and full of clay, yet drier than the material in mudflows. Earthflows are most often set in motion by heavy rains and move at a variety of speeds and distances, yet are generally slower and travel shorter distances than mudflows. Slow earthflows creep along, stopping and starting, moving sometimes just several feet per year. They are common on hillsides on the California coast where the soil has a high clay content.

A type of earthflow that occurs on slopes at high elevations and in polar regions is solifluction (pronounced so-lih-FLUK-shun; also called soil fluction); this action is not included in narrow definitions of landslide. Solifluction involves sensitive layers of silt and clay that underlie level terraces. It takes place, at speeds ranging from very slow to very fast, when snow or ice thaws or when earthquakes produce shocks that turn the soil into a fluid mass. This flow of watery sediment is common in Scandinavia and on the slopes above the St. Lawrence River valley in Quebec, Canada. In 1971, a solifluction earthflow at St.-Jean Vianney in the St. Lawrence River valley swept away thirty-eight homes and took thirty-one lives.

There are also earthflows of dry material that move very quickly, sometimes for great distances, over gentle slopes. Dry-material earthflows can be triggered by earthquakes or the falling of rock from steep slopes above. An earthquake in 1920 in the Gansu (formerly Kansu) province in China set into motion the massive flow of dry loess (pronounced LOW-ess; wind-deposited silt), which resulted in the deaths of tens of thousands of people.

Slumps

A slump, which is included in broad definitions of landslide, is the slow, downhill movement—7 feet (2 meters) per day or slower—of portions of a slope. Slumps take place on slopes where there is a strong surface layer of rock or sediment and a weaker layer of material underneath. When the lower layer is no longer able to support the surface material, both layers slip downward together. A slump may range in size from a few square yards (square meters) to thousands of square yards (square meters). As the ground moves, it tilts, or rotates, backward toward the slope in a series of curving downward and outward movements. On a slope on which slump is occurring, steplike depressions are created, and a bulge of earth forms at the base of the slope. A curved scar is left in the area where the material existed before the slump.

Slumps often occur on sea cliffs, the bases of which have been cut away by currents or waves. They can also be seen on slopes that have been eroded by a stream or glacier, as well as those that have been made steeper by construction, such as along roads and highways. They are usually triggered by heavy, prolonged rains or earthquakes.

Landslides prompted by heavy rains

One cause of landslides is the saturation of soil on steep slopes, caused by prolonged or heavy rainfall (such as from severe storms) or the melting of large quantities of snow or ice. Once the soil on the surface becomes saturated, the water makes its way down to lower layers. Those layers become slippery at the same time that the surface material is made heavier by the water. At some point the soil, pulled downward by gravity and lubricated by the water underneath, slides away from the slope. On January 11-13, 1966, for instance, heavy rains gave way to landslides on the mountainsides above Rio de Janeiro, Brazil. The cascading mud and debris killed some 550 people and brought transportation and communication systems to a halt.

Garbage landslide in the Philippines

One of the strangest and most tragic landslides happened on July 10, 2000, in Manila, capital of the Philippines. A mountain of garbage in the metropolitan dump, saturated (filled with water to the absolute maximum) from monsoon rains, collapsed and swept away hundreds of shacks and huts used by locals who scavenged the dump for usable materials and food. The shanty town was nicknamed "Promised Land" because of the opportunity it offered to make a little money. Estimates of the number of lives lost range from two hundred to more than eight hundred.

Similarly, on April 26, 1974, driving rains caused a landslide on a mountain above the Mantaro River in Huancavelica Province, Peru. The falling debris landed on twelve small villages, causing the deaths of two hundred to three hundred people. It also blocked the river, backing up the water into a natural reservoir 8 miles (13 kilometers) long, 200 yards (183 meters) wide, and 10 to 20 yards (9 to 18 meters) deep. The landslide caused about $5 million in damages. The greatest cost was for the repair of the Huancayo-Ayacucho Highway. In 1990 a rain-induced landslide again occurred in Peru, this time in the village of San Miguel de Río Mayo, some 500 miles (800 kilometers) north of the capital city of Lima. About two hundred people were unaccounted for and presumed dead after mud flooded the town.

In the low, rolling mountains capped with red sandstone above the Gros Ventre River in northwestern Wyoming (just south of Grand Teton National Park), several days of heavy rain, coupled with melting snow, caused the largest landslide in U.S. history on June 23, 1925. Some 50 million cubic yards (38 million cubic meters) of rock and debris fell into the Gros Ventre River, creating a natural dam 350 feet (107 meters) in height. The 5-mile-long (8-kilometer-long) and 225-foot-deep (68-meter-deep) body of water created behind the dam was named Slide Lake by local residents. On May 18, 1927, almost two years after the landslide, melting snow flooded the river and the dam broke loose. Luckily there had been plenty of advance warning, and most of those living in the area evacuated before floodwaters destroyed their downriver settlement.

Intense rainfall from hurricanes can cause multiple landslides. In August 1969, for instance, Hurricane Camille dumped 27 inches (68 centimeters) of rain on the Appalachian Mountains in central Virginia over an eight-hour period. The soil at the top of steep slopes became saturated and set in motion dozens of debris avalanches. About 150 people were killed by the flowing material. Throughout the region, houses were destroyed and roads and bridges were buried or washed out.

Earthquake-generated landslides

Earthquakes that occur in areas with steep slopes can cause the slipping of surface soil and rock, and the collapse of cliffs. The shock waves produced by earthquakes send material hurtling downward in violent landslides. Earthquake-induced landslides happen in mountainous regions such as China, parts of Southern California, Alaska, Turkey, and Iran.

One landslide produced by a severe earthquake occurred in Montana, west of Yellowstone Park, on August 17, 1959. In that case, 40 million cubic yards (30 million cubic meters) of rock tore off the wall of the Madison River Canyon and slid into the river below at a speed of about 100 miles (160 kilometers) per hour. The rocks killed twenty-six people and blocked the river. A lake 6 miles (9.7 kilometers) long and 180 feet (55 meters) deep was formed in the process. After the landslide, a spillway (a passageway near the top of a dam through which water from the reservoir travels when the water level becomes high) was carved out of the natural dam and lined with large blocks of rock, creating sufficient drainage to handle heavy rains and keep the dam from collapsing.

In Kashmir (a region in southwest Asia adjacent to India and Pakistan) in 1840, an earthquake sparked a landslide that dammed the Indus River, forming a lake about 40 miles (64 kilometers) long and 1,000 feet (300 meters) deep. In 1949 in Tajikistan (then part of the Soviet Union), earthquakes in the Pamir Mountains set off landslides that buried the town of Khait, killing all of its twelve thousand residents. Today, Russian

seismologists (scientists who study earthquakes) operate a laboratory for earthquake prediction research near Khait.

Landslides triggered by volcanic eruptions

Volcanic eruptions often produce a type of mudflow called a lahar. The material in a lahar is created when volcanic ash mixes with water; the water comes from the melting of snow and glaciers around the volcanic crater. The lahar may be very hot and can travel down the steep sides of a volcano at speeds of 100 miles (160 kilometers) per hour. It can flow for great distances, sweeping up houses and cars, uprooting trees, and burying entire communities.

Sinkholes

The movement of Earth's surface sometimes takes the form of a vertical drop, in which case a sinkhole is formed. A sinkhole is a large depression in the ground, often shaped like a bowl or a funnel. Sinkholes vary greatly in diameter from several feet to several miles, and may attain depths of 100 feet (30 meters) or more. Some sinkholes become clogged with clay and then collect water, forming small lakes.

One cause of sinkholes is that the underlying layer of water-soluble rock, such as limestone, marble, or dolomite, is dissolved by groundwater. As the rock dissolves, underground spaces form and the support for the ground is reduced. When the spaces grow large or numerous and the remaining rock can no longer hold the land above it, the surface collapses.

Large sinkholes form when cave ceilings weaken, become unable to support the weight of the ground above them, and collapse. Another cause of sinkholes is the depletion of aquifers, which are underground layers of spongy rock, gravel, or sand in which water collects. Aquifers become depleted when the underground water is pumped out of the ground faster than it can be replenished by rainwater. A sinkhole may also be formed when pockets of underground gas escape, such as during an earthquake.

In the United States, sinkholes have caused the greatest damage in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. The country's largest sinkhole on record, called the "December Giant," formed in the woods near Montevallo, Alabama, in Shelby County, on December 2, 1972. The sinkhole, discovered by hunters, was 425 feet (130 meters) long, 350 feet (107 meters) wide, and 150 feet (45 meters) deep. Two days prior to the discovery, a resident in the vicinity had reported that his house shook, trees broke, and he heard a roaring noise.

In 1981 a large sinkhole formed in Winter Park, Florida. As the ground gave way, a three-bedroom house and three cars were swallowed up. The formation of sinkholes is a growing problem in urban areas of central Florida, as the population grows and underground aquifers become depleted at an alarming rate.

In May 1980, with the eruption of Mount St. Helens in southern Washington State, came one of the largest landslides in U.S. history. Part of the north face of the mountain blew out and the volcanic ash combined with water from lakes and rivers to form colossal mudflows. The mud blocked rivers and ruined bridges and roads. (Fortunately, the area had been evacuated in advance.) Mud even stopped up the Columbia River, a main thoroughfare to the Pacific Ocean, trapping thirty ocean-going ships downstream.

Lahars resulting from the 1985 eruption of the Nevado del Ruiz volcano in the Colombian Andes struck an area that had not been evacuated, despite warnings from geologists (scientists who study the origin, history, and structure of the Earth). The largest of the mudflows overtook the city of Armero, where it claimed at least twenty-three thousand lives and left another twenty thousand people homeless. In some places, the mud was 12 feet (3.7 meters) deep. About 15 square miles (39 square kilometers) of land, including rich farmlands where coffee and rice were grown, were covered by the lahar.

Where landslides occur

Landslides occur throughout the world. They are serious problems in several nations, however, particularly in Italy and Japan. More than one thousand urban areas in Italy are in danger zones

for landslides. In Japan, thousands of homes are lost and more than one hundred people are killed by landslides each year.

In the United States, landslides take place in all fifty states. Across the country, they kill between twenty-five and fifty people and cause up to $2 billion in damage each year. Landslides are the most prevalent in the Appalachians and Rocky Mountains, as well as along the Pacific Coast. It is estimated that more than two million landslides have occurred in the Appalachians, and evidence of past landslides exists on more than 30 percent of the area of West Virginia. In Colorado that figure is 8 percent. More than six hundred landslides have been distinguished in Utah. Coastal slides are a constant menace in California; the San Gabriel Mountains frequently unleash debris flows that ruin homes in the northern Los Angeles area.

Consequences of landslides

Each year, landslides take hundreds of lives and cause billions of dollars in damage throughout the world. Landslides engulf villages and kill people and animals. Falling and sliding rock, soil, and debris flatten houses and cars and uproot trees. Material that spills onto a roadway or railroad tracks halts traffic and causes accidents and sometimes fatalities. When rocks fall into a lake from high above, they create waves that threaten coastal settlements.

When a large quantity of material falls, it forces a wall of air ahead of it. That wind may be strong enough to knock down trees and houses. When the material strikes the ground it sends up a cloud of dust that may darken the sky and spread over a large area. Landslides also knock down utility poles and wires, and the region loses power and communication with the outside world. Landslides also scar the face of a hill or mountain, stripping it of soil, trees, and other vegetation.

Rock or soil that flows into a valley often blocks the flow of rivers, thus disrupting ecosystems and shipping routes and sometimes contaminating drinking water. The natural dam may later give way, causing floods.

The human factor

Most of the destructive impacts of landslides are due, in some part, to human activity. Many landslides occur on slopes that have been altered by grading (leveling off) for road or building construction. When a portion of a mountainside is graded, material is cut out of the slope and removed. The slope directly above the graded area is made much steeper, reducing support for earth and rock higher up the slope. If the excavated material is deposited beneath the graded area, it may overload the lower portion of the slope and cause a landslide.

The Yosemite rockfall

On July 10, 1996, a tremendous rockfall shook the ground in Yosemite (pronounced yo-SEM-it-ee) National Park in California. Two slabs of an enormous boulder, balanced 2,600 feet (79 meters) above Yosemite Valley on a granite arch, suddenly broke loose. The rocks, weighing 68,000 tons (61,676 metric tons), slid down a steep slope for the first 500 feet (152 meters) and then took to the air and fell freely for the next 1,700 feet (518 meters) until smashing into a rocky slope near the base of the cliff. When the fragmented rocks struck the ground, they were traveling faster than 160 miles (257 kilometers) per hour.

On impact, the rockfall released a wind blast that knocked over hundreds of pine and oak trees and destroyed a nature center and snack bar nearby. The falling trees killed one park visitor and injured several others. The dust from the rockfall and blast of wind blotted out the sky and hung in the air for several hours before settling over an area of about 50 acres (20 hectares).

Construction is especially dangerous on slopes that, due to their geologic composition, are unstable (prone to landslides) in their natural state. For example, mountains or cliffs that have a layer of sandstone on the surface and a layer of shale beneath are geologically unstable. Water can seep into pores and cracks in the sandstone and collect on the shale. The shale surface then becomes slippery, allowing the sandstone layer to slide off.

Mining is another activity that weakens slopes and promotes landslides. The removal of coal, stone, or other natural resources from the ground makes the remaining slope unstable and vulnerable to collapse.

Landslides in Southern California

The mountains of Southern California are prone to mudflows, placing communities on the slopes above and in the valleys below in the danger zone. The slopes are steep and unstable. Much of the soil is barren, since frequent wildfires remove vegetation. Some landslides are triggered by intense storms, which saturate the soil and start it moving downhill. Frequent earthquakes also set landslides in motion.

The deadliest landslide in the region's history occurred in February 1969. During the winter of 1968–69, about 44 inches (112 centimeters) of rain fell on the Los Angeles area over a forty-two-day period. As the record-setting rain (which came after forest fires had cleared the slopes of vegetation the previous summer) fell on the San Gabriel Mountains, it soaked layers of soil and gravel. That material eventually began traveling down the mountainsides, gathering debris as it went. The muddy torrent drowned or buried one hundred people and caused about $1 billion in damage. One of the hardest-hit places was the fashionable suburb of Glendora, where the mud slide damaged 160 homes and destroyed 5 others. Mud piled up on the major highway leading into Los Angeles and destroyed citrus groves as far north as the Ventura, Santa Barbara, and San Luis Obispo counties.

Another part of the Los Angeles area that has suffered from slides is Portuguese Bend on the Palos Verdes Peninsula. Portuguese Bend is an expensive clifftop housing development overlooking the Pacific Ocean. It was built despite warnings from geologists about the region's instability. The material on the top of the cliffs, on which the houses sit, is sandstone mixed with a clay material made of hardened volcanic ash—materials that readily absorb water. The layer of rock below is shale, which becomes slippery when wet. The bases of the cliffs are subject to erosion by waves.

In 1956, heavy rains caused the first movement of the cliff tops since the homes went up. From the 1960s through the 1980s the gradual slide was made worse by a series of earthquakes, plus the increased weight from additional homes. In 1969, houses on nearby Point Fermin went sliding into the ocean. By the end of the 1980s, the slide of the cliff top in Portuguese Bend had damaged or destroyed 150 homes for a total cost of $10 million. Thereafter, new development in Portuguese Bend was halted and a drainage system was put in place to keep the slide from slipping further and to protect the remaining houses.

Another contribution people make to landslides is cutting down trees on slopes, for use as fuel and lumber or to clear the land for farming. Trees protect slopes by trapping rain on their leaves and reducing the erosive impact of wind and water on the soil. The roots of trees and other forms of vegetation absorb rainwater like a sponge and release it slowly into the soil. Roots also act as anchors, holding the soil together. Soil with no vegetative cover erodes quickly. It glides more easily over the rocky subsurface than does compact, cohesive soil. Landslides on deforested slopes, once set in motion, have no natural barriers to slow or stop them. Foot traffic on mountains, from sightseers or hikers, also tramples vegetation and increases the slopes' vulnerability to landslides.

Technology connection

Technology is of limited usefulness in predicting and preventing landslides. Most large slides occur without warning and are more powerful than any barrier that can be erected. A California engineer, at a conference in 1980, likened landslide prevention to "trying to hold back the storm tides of the ocean." Nonetheless, numerous measures are used to lessen the impact of landslides, as well as to predict certain kinds of landslides.

Limiting landscape damage

Many methods are used to protect populated areas from material that may fall or slide down a slope. For instance, water drainage systems are employed to keep water from saturating ground that is vulnerable to landslides. Wells are pumped in the potential slide area to keep the rain from overflowing aquifers and soaking the ground. (An aquifer is an underground layer of spongy rock, gravel, or sand in which water collects.)

World's deadliest landslide: Gansu, China

On December 16, 1920, the deadliest landslide in recorded history struck Gansu (formerly Kansu), China, resulting in 180,000 deaths. The cause of the landslide was an earthquake centered near the border with Tibet. The hills and cliffs in the region were treeless and covered with a layer of loess (pronounced LOW-ess or LUSS), a soft, loosely packed material formed from the yellowish dust of the Gobi Desert. The combination of the bare slopes and the fine dust made the soil highly susceptible to slides.

The shock of the earthquake caused the sides of 100-foot (30-meter) cliffs to collapse. Falling material barricaded the entrances of mountainside caves, in which many peasants made their homes. The landslide laid waste to ten cities and numerous villages in the valleys.

In one village, the only survivors were a farmer and his two sons. Their plot of land had broken loose from a mountaintop and slid down the slope intact, atop a stream of flowing debris. The day of the Gansu landslides is known in China as Shan Tso-liao, or "When the mountains walked."

Trees, shrubs, or grasses are planted on bare slopes to hold the soil in place and to stop material that begins to slide. Terraces (broad, steplike cuts) are constructed on steep slopes, so that falling material or water is only able to travel short distances before landing on a plateau and losing its energy. Loose material is removed from high elevations before it begins rolling down a slope. The bases of slopes on which bulges of material, called "toes," have formed due to slump are immobilized with walls of rock, concrete, or soil. Strong wire-mesh fences are secured to some cliff faces above roadways to prevent rocks from falling. Railroads can be protected by electric fences that detect rock falls and communicate the need to halt trains in that section of track.

Another measure undertaken to prevent landslides is the filling-in of cracks that develop in the faces of mountains or hills. An option for protecting structures in landslide-prone areas is to build retaining walls or earth buttresses along slope bottoms. In some valleys, basins are constructed to trap landslide material. In Southern California, for instance, a series of 120 football-stadium-sized basins have been excavated along the base of the San Gabriel Mountains to catch rocks and debris. They must be emptied frequently in order to continue catching the falling material.

In Japan, which is home to 10 percent of the world's active volcanoes, walls of steel and concrete—as well as drainage systems—have been constructed on mountainsides to protect valley-dwellers from lahars. Television cameras are employed on some slopes to detect the start of landslides and provide advance warning to people in the path of danger.

Predicting landslides

While most landslides occur without warning, certain types can be predicted. The U.S. Geological Survey, together with the National Weather Service, provides advance warning for landslides that are caused by heavy rains. Both organizations monitor rainfall data

and forecasts in areas prone to landslides and issue warnings when the ground is becoming saturated. Landslides that come in the wake of volcanic eruptions can also be predicted. Volcano early warning systems detect rumblings that precede eruptions and, quite likely, volcanic

Housing development collapses in the Philippines

On August 3, 1999, following four days of heavy rains that caused widespread flooding throughout eastern Asia, a hillside collapsed in Antipolo City, a suburb of Manila. Thirty-one people were killed as the wall of earth crashed into a housing development; as many as forty others were missing and presumed dead. Twenty-five houses were buried by the landslide, and 378 others were damaged. During that same week in the central Philippines, sixty-one other people died in the flooding.

Filipino officials placed the blame for the landslide on a nearby rock and gravel quarry. They claimed that too much rock had been excavated from the slope too close to the houses, thereby allowing groundwater to reach the ground and soften it. The heavy rains proved to be the final straw that caused the earth to give way and collapse on the houses. Officials also noted that they had issued evacuation warnings when cracks appeared in houses and streets earlier that day, but most residents had ignored the warnings.

mudflows. People living in the vicinity of a volcano are given plenty of advance warning of the need to evacuate.

Assessing the danger of building

When construction is proposed in hilly areas, an assessment is made of the hazards posed by landslides. To determine the stability of the slope and the suitability of the region for construction, researchers conduct geologic explorations to determine soil and rock properties and look at the history of landslides in the area. From that information they can predict the frequency with which landslides will occur, as well as the destructive potential of those landslides. Areas where landslides are likely to occur would then be placed off-limits to construction and possibly designated for parkland or other limited use.

[See AlsoAvalanche; Drought; Earthquake; El Niño; Flood; Tsunami; Volcano; Weather: An Introduction ]

For More Information

BOOKS

Bishop, Amanda, and Vanessa Walker. Avalanche and Landslide Alert (Disaster Alert!). St. Catherines, Ontario: Crabtree Publishing Company, 2005.

Committee on the Review of the National Landslide Hazards Mitigation Strategy, Board on Earth Sciences and Resources, National Research Council. Partnerships for Reducing Landslide Risk: Assessment of the National Landslide Hazards Mitigation Strategy. Washington, DC: The National Academies Press, 2004.

Hyndman, Donald, and David Hyndman. Natural Hazards and Disasters. New York: Brooks Cole, 2006.

National Geographic Society and Ralph M. Feather Jr. Earth Science. New York: Glencoe/McGraw-Hill, 2002.

Robinson, Andrew. Earth Shock: Hurricanes, Volcanoes, Earthquakes, Tornadoes and Other Forces of Nature. New York: W. W. Norton, 2002.

PERIODICALS

Simpson, Sarah. "Raging Rivers of Rock." Scientific American. (July 2000): pp. 24-25.

WEB SITES

"Landslide and Debris Flow (Mudslide)." Federal Emergency Management Agency: Disaster Information. 〈http://www.fema.gov/hazard/landslide/index.shtm〉 (accessed March 22, 2007).

"Landslide Hazard Program." United States Geological Survey. 〈http://landslides.usgs.gov〉 (accessed March 22, 2007).

"Landslides in British Columbia." British Columbia Geological Survey. 〈http://www.em.gov.bc.ca/Mining/Geolsurv/Surficial/landslide〉 (accessed March 22, 2007).

"Slide!" Open Learning Agency, British Columbia Knowledge Network. 〈http://www.knowledgenetwork.ca/slide/splash.html〉 (accessed March 22, 2007).

Landslides

Introduction

A landslide or debris flow is a downhill movement of a significant amount of rock and soil. Mudslides are a type of landslide that occurs when the loose material is mixed with a large amount of water. A mudslide can move quickly and travel miles along a valley or other channel, causing many fatalities: A single mudslide in Venezuela in 1999 killed approximately 20,000 people. In the United States, landslides kill 5-50 people every year and do more than $2 billion of damage. As of 2008, the greatest loss of life in a single landslide in U.S. history was in Puerto Rico in 1985, when a landslide killed 129 people. A single 1983 landslide in Thistle, Utah, was the most expensive single landslide in U.S. history, causing over $400 million in direct and indirect costs.

Historical Background and Scientific Foundations

Landslides occur on Earth and on many other bodies in the solar system, including Mars, Jupiter’s moon Io, and various asteroids. They occur whenever a steeply sloping mass of loose material—sand, rock, soil, or any combination of these, often (on Earth) mixed with water—is set into motion. This often occurs when intense rainfall increases the density and mobility of the mass and it begins to flow. Motion may also be triggered by an earthquake, volcanic eruption, or artificial explosion. A single heavy storm or strong earthquake can trigger thousands of landslides over a large area. Landscapes are an important contributor to the evolution and erosion of mountainous landscapes.

Landslides also occur underwater along the edges of continental shelves and along the steep slopes of some islands. A large submarine landslide, as an underwater landslide is called, may trigger a landslide tsunami. Although not covering as large an area of coast as a typical large earthquake tsunami, a landslide tsunami can be deadly: In July, 1998, a 50-ft (15-m) high, 6-mi (20-km) long landslide tsunami struck the coast of Papua New Guinea, a large island in the western Pacific, killing over 2,000 people.

Once set in motion, a landslide or debris flow can move as slowly as a few inches/centimeters a year or accelerate to speeds of up to 35 mph (56 km/h) and travel great distances along valleys or over level ground. The areas at greatest danger from landslides are the bottoms of canyons, stream channels, and areas where such features empty out. Most often, a landslide begins at the top of a small gully (small, eroded cut in the land); once a small amount of material begins to move, it entrains other material, and the landslide grows until all the material that is free to move is doing so. Slopes that have been altered for roads or building construction or that have been deforested or burned over by wildfires are more likely to give way than undisturbed slopes or slopes that are stabilized by vegetation. In the United States, many landslides in the western part of the country occur in the fall and winter, when rains come after the summer fire season.

According to the U.S. Geological Survey, all 50 U.S. states experience landslides, with 36 states having moderate to highly severe landslide hazards. Most landslide damage occurs in the Pacific Coast region (including Alaska and Hawaii), the Rocky Mountains, the Appalachian Mountains, and Puerto Rico. Seismically active areas are most at risk, because earthquakes are effective triggers for landslides. For example, several thousand landslides were triggered in central California by the 1980 Mammoth Lakes earthquake sequence.

WORDS TO KNOW

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

EROSION: The wearing away of the soil or rock over time.

Impacts and Issues

Globally, landslides are one of the most common natural disasters, causing thousands of deaths and billions of dollars worth of damage every year. Direct impacts include property destruction (no structure can withstand a large landslide) and death; landslides cause death more often than injury, because if a person is caught up in or buried under a debris flow at all, their chances of survival are usually small.

Landslides also cause indirect impacts by destroying facilities that people depend on for well-being, what the World Health Organization calls “lifeline systems.” These include roads, communication lines, pipelines for fuel and water, electrical transmission lines, and healthcare facilities. By destroying livestock and crops, landslides in Asia and South America have also increased the vulnerability of rural sustenance farmers in developing countries to food insecurity, also making them more vul-

nerable to other disasters. Indirect costs include rescue, cleanup, tourism losses, and lost productivity.

Life and property losses from landslides are increasing around the world, including the United States, for two reasons. First, more people are in harm’s way, as increasing populations settle locations vulnerable to landslide. Second, increasingly frequent intense-rainfall events in many parts of the world, possibly a result of global climate change, are increasing the frequency of landslides, especially in deforested parts of Asia, Latin America, Australia, and New Zealand.

Predicting landslides would save many lives and greatly reduce property damage. Despite strong motivation for the development of predictive techniques, the science of landslide prediction remains crude. Site-specific monitoring systems similar to those used to detect the early-warning signs of volcanic eruptions can be installed to measure water pressure, subsurface slippage, and subtle movement of the surfaces. However, such systems are expensive. Regional-scale landslide prediction relies on satellite data to monitor soil moisture in landslide-prone areas. Based on such information, landslide warnings may be issued. A landslide warning system was under development by the U.S. National Aeronautics and Space Administration (NASA) in the early 2000s, but had not yet been activated as of mid-2008.

See Also Earthquakes; Floods

BIBLIOGRAPHY

Books

Cornforth, Derek. Landslides in Practice: Investigation, Analysis, and Remedial/Preventative Options in Soils. New York: Wiley, 2005.

Periodicals

Keefer, David K., and Matthew C. Larsen. “Assessing Landslide Hazards.” Science 316 (2007): 1136–1138.

Stone, Richard. “Too Late, Earth Scans Reveal the Power of a Killer Landslide.” Science 311 (2006): 1844–1845.

Web Sites

Centers for Disease Control and Prevention (CDC). “Landslides and Mudslides.” http://www.bt.cdc.gov/disasters/landslides.asp (accessed May 10, 2008)

Federal Emergency Management Administration (FEMA). “Landslide and Debris Flow.” http://www.fema.gov/hazard/landslide/index.shtm (accessed May 10, 2008).

National Geographic. “Satellites Can Warn of Floods, Landslides Worldwide, Scientists Say.” May 25, 2006. http://news.nationalgeographic.com/news/2006/05/060525-flood-warn.html (accessed May 10, 2008).

U.S. Geological Survey. “Landslides Hazard Program.” http://landslides.usgs.gov/ (accessed May 10, 2008).

Landslides

Landslides are natural hazards that cause millions of dollars of damage each year and also cause many deaths. They are defined as downslope movements of soil and rock under the influence of gravity. They are triggered primarily by water, but sometimes earthquakes can lead to some spectacular landslides. The water comes mainly from high-rainfall storms, but also can come from rapid snowmelt.

Factors

The stability of a slope can be described as two forces working against one another. Driving forces work to cause slope materials to move downslope, whereas resisting forces act to keep the materials on the slope. When the ratio of resisting forces to driving forces (called the factor of safety) is greater than 1, the slope is stable. When it is less than 1, the slope usually fails.

Water and Vegetation.

Water can increase the driving forces and reduce the resisting forces. Saturation from rainfall can increase the slope mass, thereby increasing the driving forces. Filling the slope soil pores with water also reduces soil cohesion by allowing particles to pass by one another, thereby reducing the internal resistance of the soil to sliding. To reduce landslide danger on a slope, the first thing done is to remove the water.

Vegetation also is important to slope stability because it increases resisting forces through its roots, especially tree roots, that bind the soil. Trees also act as natural pumps that remove water from the soils through evapotranspiration , thereby increasing slope stability.

Slope and Materials.

As the slope angle increases, the driving forces also increase. Few landslides occur on slopes less than 15 degrees. Cutting a road into a slope will create an oversteepened slope prone to landslides if a wall is not built. Certain slope materials also have weak strengths and low resisting forces to landslides. For example, clay, shale, serpentine, and uncompacted fill are prone to failure.

If the slope bedrock is inclined and is somewhat parallel to the slope, it is called a dip slope. Landslides are prone along failure planes (clay beds and old soils) on these dip slopes. Examples of dip-slope failure are Italy's Vaiont Dam disaster in 1963 that killed 2,600 people, and the Gros Ventre landslide that dammed Wyoming's Gros Ventre River in 1925.

To stabilize a slope or prevent landslides, one needs to lower the slope angle, drain the slope of water, build retaining walls, plant vegetation, and avoid building on old landslides. If the slope has moved once, it has a high chance of moving again.

Classification

Names are given to different landslides depending upon the process that brings the soil and rock down the slope. Falls are the free fall of detached materials, usually rocks, which descend down a steep slope. Translational slides occur along a failure surface in the bedrock and move parallel to the surface. If the sliding mass occurs along a curved plane, it usually is called a slump.

Flows.

Flows occur when material moves downslope as a viscous (thick) fluid. Most of these flows are saturated with water. Fast-moving ones move as a slurry that can be as much as 70 percent water and 30 percent sediment, and these are called debris flows (see figure). If the flow contains mostly fine-grained particles like sand, silt and clay, it is called a mudflow. They frequently follow stream canyons and pose significant hazard to life and property. Velocities can reach 55 kilometers (34 miles) per hour. Earthflows are slower flows that generally originate on hillslopes as large tongues that break away from scarps (arc-shaped steep slopes cut into a hill).

Debris Avalanches.

The fastest and largest landslides are called debris avalanches, because they travel at speeds up to 300 kilometers (186 miles) per hour, travel long distances sometimes in excess of 30 kilometers (18.6 miles), and are very large. The world's largest historical landslide occurred in Washington state when Mount St. Helens erupted in 1980.^* In Yungay, Peru, about 22,000 people were killed in 1970 when a debris avalanche descended from the volcano Nevado Huascaran, traveling a distance of 14 kilometers (8.7 miles) in only a few minutes.

Creep.

Some movement on slopes can be imperceptible, called creep. Creep is caused by gravity, and is assisted by freeze–thaw or shrink–swell processes operating on the soil sediments. Rates range from 0.1 millimeter (0.004 inch) per year to 10 centimeters (about 4 inches) per year. Evidence of movement is found in cracked walls, leaning telephone poles, and leaning trees that adjust (straighten) their direction of growth, creating so-called "trees with knees," also called pistol-butt trees.

Recognizing Landslide Terrains

Scientists "read" the landscape for signs that tell of past processes. Several diagnostic landslide features tell of past movement on the terrain. First, scarps are arcuate, steep slopes cut into the hill where the landslide has torn away. Second, hummocky topography is "bumpy" ground that has been produced by the landslide mass-weathering over time. A hummocky slope with a steep, half-moon slope at the top usually means an old landslide occurred there.

Younger landslides generally will have vegetation on the slide mass that is different than vegetation on the surrounding slopes. In coniferous forests, landslides typically are covered with deciduous trees for the first 100 years after the failure.

If one can see the actual sediments making up the landform, landslide debris and colluvium are unsorted, with all particles mixed together in a random fashion. Colluvium can be confused with glacial till, because both are unsorted.

see also Forest Hydrology; Groundwater; Volcanoes and Water.

Scott F. Burns

Bibliography

Ritter, D. F., Kochel, R. C., and Miller, J. R. Process Geomorphology, 3rd ed. Dubuque, IA: Wm. C. Brown Publishers, 1995.

Turner, A. K., and R. L. Schuster, eds. Landslides: Investigation and Mitigation. Washington, D.C.: Transportation Research Board, National Research Council, Special Report 247 (1996).

^* See "Volcanoes and Water" for a photograph of the Mount St. Helens eruption.

Landslide

Landslide is a general term used to describe a variety of geologic processes involving the movement of fine-grained earth, coarse-grained debris, or rock down a slope under the influence of gravity . This broad definition of downslope movements includes falling, toppling, sliding, spreading, and flowing. Landslides can also occur under water (submarine landslides) and trigger tsunamis.

Commonly used landslide classification systems rely on separate terms to describe the type of movement (falling, toppling, sliding, spreading, or flowing) and the type of material involved. Unlithified material is classified as debris if it is predominantly coarse-grained (sand , gravel, cobbles, or boulders) and earth if it is primarily fine-grained (silt or clay ). Thus, a debris flow would involve the down slope flow of predominantly coarse-grained material, whereas a debris slide would involve the sliding of the same kind of material along a well-defined slip surface. Additional detail can be included by specifying the rate of movement (which can range from several millimeters per year to tens of meters per second) and the water content of the moving mass (which can range from dry to very wet). Landslides moving at velocities faster than a few meters per minute, particularly when they are large, have the potential to cause catastrophic damage and loss of life. The volume of material involved in a landslide, however, is irrelevant to its classification and can range from a few cubic centimeters to several cubic kilometers. Landslides can also change modes as they move. For example, a debris slide may mobilize into a debris flow as the debris begins to move down slope.

The term mudslide, which is often used in news reports, does not exist within the classification systems used

by most geologists and engineers. It is an imprecise term that is best avoided.

Landslides occur when the forces tending to keep a soil or rock mass in place (resisting forces) are exceeded by those promoting movement (driving forces). Resisting forces most commonly arise due to the shear strength of the material acting over an area , such as the slip surface beneath a landslide, or as a consequence of engineered works such as retaining walls. The primary driving force—the component of the weight of the earth, debris, or rock mass acting parallel to the slope—and the force occurring when the potential landslide mass is accelerated during an earthquake can also trigger landsliding. Changing the geometry of a slope, for example by excavating some areas and placing fill in others during construction, can alter the balance of resisting and driving forces enough to trigger a landslide.

It is a widely held misconception that landslides occur because slopes are lubricated by water. Water does not act as a lubricant in landslides, but instead decreases the shear strength of the earth, debris, or rock by decreasing the normal force acting across a potential slip surface. It is well known from basic physics that the sliding of a block down an inclined plane is resisted by the product of the normal force acting on the plane and a coefficient of friction. Similarly, a decrease in the normal force acting across a potential slip surface will decrease the resistance to sliding. Sources of water leading to landsliding can include infiltrating rain and melted snow, leaking water pipes, and irrigation.

It is difficult to estimate the monetary costs of landslides because they can include both direct and indirect costs. Direct costs include damage to structures and roads, whereas indirect costs include items such as decreased property values, lost productivity, and the expense of driving longer distances when roads are blocked. Difficulties aside, in 1985 the National Research Council estimated that landslides cost between $1 billion and $2 billion per year in the United States alone. Estimates for other countries range from tens of millions to billions of dollars per year.

See also Catastrophic mass movements; Debris flow; Lahar; Mass movement; Mass wasting; Mud flow; Rockfall; Slump; Talus pile or talus slope

Landslide

A general term for the discrete downslope movement of rock and soil masses under gravitational influence along a failure zone. The term "landslide" can refer to the resulting land form, as well as to the process of movement. Many types of landslides occur, and they are classified by several schemes, according to a variety of criteria. Landslides are categorized most commonly on basis of geometric form, but also by size, shape, rate of movement, and water content or fluidity. Translational, or planar, failures, such as debris avalanches and earth flows, slide along a fairly straight failure surface which runs approximately parallel to the ground surface. Rotational failures, such as rotational slumps, slide along a spoon shaped failure surface, leaving a hummocky appearance on the landscape. Rotational slumps commonly transform into earthflows as they continue down slope. Landslides are usually triggered by heavy rain or melting snow, but major earthquakes can also cause landslides.