Gravity is the physical force of attraction between any two objects in the universe. One of the four fundamental forces (the others are electromagnetism and the strong and weak forces), gravity affects all objects on Earth. From the largest mountains to the smallest grains of sand, gravity pulls everything in a direction toward the center of the planet. As long as material remains on a flat surface, one that is parallel to Earth's surface, gravity will not cause it to move. When material is on a slope and conditions are right, however, gravity will cause it to fall, slide, flow, slump, or creep downward.
That downhill movement of soil, rocks, mud, and other debris can be either slow or fast. Large amounts that move quickly are perhaps the most widespread geologic hazard. Each year in the United States, ground failures of various sorts cause between twenty-five and fifty deaths and roughly $1.5 billion in economic loss. In less-developed nations, where poorly constructed buildings house many people in areas prone to ground failures, the death tolls and amount of property damage are much higher.
Geologists use the term mass wasting to describe the spontaneous movement of Earth material down a slope in response to gravity. This does not include material transported downward by streams, winds, or glaciers. Mass wasting plays an important role in the overall process of erosion, which is the gradual wearing away of Earth surfaces through the action of wind and water. Through mass wasting, material from higher elevations is moved to lower elevations where streams, glaciers, and wind pick it up and move it to even lower elevations. Mass wasting occurs continuously on all slopes. While some mass-wasting processes act very slowly, others occur very suddenly. The general term landslide is used to describe all relatively rapid forms of mass wasting.
Mass wasting may be divided into two broad categories: slope failures and flows. Slope failures occur when debris moves downslope as the result of a sudden failure on a steep slope or cliff. Flows occur when a loose mixture of debris, water, and air move downslope in a fluidlike manner. Each of these categories may be further divided into various types: Slope failures include falls, slides, and slumps; flows include mudflows, debris flows, solifluction (pronounced solih-FLUK-shun), debris avalanches, earthflows, and creep. Flows may be grouped according to the amount of water present in the particular flow. Mudflows, debris flows, and solifluction are labeled slurry flows. These contain between 20 and 40 percent water. Debris avalanches, earthflows, and creep are granular or dry flows, which contain up to 20 percent water.
A fall is a sudden, steep drop of rock fragments or debris. A rockfall commonly occurs on a steep cliff and may involve a single rock or a mass of rocks. As a rock falls down, it may plummet freely through the air or may strike and loosen other rocks in the cliff face. At the base of the cliff, the rock fragments accumulate in a sloping pile known as a talus (pronounced TAY-less). The largest rocks in the pile tend to be located the farthest from the cliff face because of their greater size and momentum. Debris falls differ from rockfalls only in that they involve a mixture of soil, rocks, and vegetation.
In contrast to a fall, material in a slide maintains contact with the slope down which it moves. That material could be a mass of rocks or debris. Piles of talus are common where rock or debris slides end. A rock slide involving tons of material may reach a speed exceeding 100 miles (161 kilometers) per hour.
A slump is the downward movement of a block of material on a curved surface, one shaped like a spoon. Instead of sliding downward parallel to the surface of the slope, a slump block rotates backward toward the slope in a series of curving downward and outward movements, creating a series of steplike depressions. A bulge of material, known as a toe, develops at the base of the slope. At the head of the slump, a scalloped hollow is left in the slope. A slump generally does not travel far, unlike a fall or a slide, normally moving at a pace of 7 feet (2.1 meters) per day or slower.
The most common, the most liquid, and the fastest type of flow is a mudflow. A mixture primarily of the smallest silt and clay particles and water, a mudflow has the consistency of newly mixed concrete. It can travel down a slope 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. Mudflows can travel for great distances over gently sloping terrain. When they reach valley floors, mudflows spread out, depositing a thin layer of mud mixed with boulders. A type of mudflow produced by a volcanic eruption is called a lahar (pronounced LAH-hahr). A mixture of volcanic ash, rocks,
and water from melted snow and glaciers around the volcanic crater, a lahar may be very hot. Traveling down the steep side of a volcano at speeds approaching 200 miles (322 kilometers) per hour, a lahar can flow for great distances, burying everything it encounters.
A mixture of water and clay, silt, sand, and rock fragments (more than half of the particles are larger than sand), a debris flow travels at a slower speed than a mudflow, usually up to about 25 miles (40 kilometers) per hour. At this velocity, a debris flow still has enough energy to pick up and transport large rocks, boulders, trees, and other material in its path. This type of flow generally occurs on steep slopes that have little or no vegetation. Traveling downward, the material in a debris flow tends to mix with more water and even air. Studies have shown that debris flows gain speed because they actually ride on a cushion of air as they flow downslope.
The slowest type of slurry flow is called solifluction. This form of mass wasting occurs in relatively cold regions where short summers thaw the uppermost layers of soil, generally the top 3 feet (1 meter). Below this remains a layer of permanently frozen soil called permafrost through which the water does not drain. Water-saturated and weak, the upper layers flow very slowly downslope at a rate of 0.2 to 5.9 inches (0.5 to 15 centimeters) per year, forming distinct lobes (rounded segments).
The fastest type of granular flow is the debris avalanche. The term avalanche is generally applied to any fast-moving downward flow of any type of material. Similar to an avalanche of snow, a debris avalanche is an extremely rapid downward movement of rocks, soil, mud, and other debris mixed with air and water. Common in areas with steep slopes, debris avalanches usually result from the complete collapse of a mountainous slope, often triggered by earthquakes and volcanic eruptions. They move downward through avalanche chutes (channel-like depressions along which an avalanche has moved), reaching speeds over 300 miles (480 kilometers) per hour. Debris avalanches can travel for considerable distances along relatively gentle slopes.
Another type of granular flow is an earthflow, which usually occurs when clay-rich soil has become saturated by heavy rains. The material in an earthflow is coarser and less fluid than that in a mudflow and finer and more fluid than that in a debris flow. Although it may move at a variety of speeds and over varying distances, an earthflow generally moves slower and travels a shorter distance than a mudflow. Slow earthflows move in starts and stops, covering only several feet per year. This type of flow normally has an hourglass shape. A bowl or depression forms at the head where the unstable material collects and flows downward. It narrows in its central area before widening once again at the base of the slope.
Creep is the extremely slow, almost continuous movement of soil and other material downslope. Most creep movement is less than 0.4 inch (1 centimeter) per year. Occurring on almost all slopes, it is the most widespread and costliest type of mass wasting in terms of total material moved and monetary damage caused. Although creep cannot be witnessed, evidence of the movement can be seen on hillslopes in curved tree trucks and leaning fence posts, telephone poles, and gravestones.
Mass wasting occurs throughout the world, continually sculpting the landscape. The areas at greatest risk for mass wasting events are in mountainous regions with relatively steep slopes. In the United States, those areas are found in the Appalachian Mountains, the Rocky Mountains, and along the Pacific Coast. However, the potential for mass wasting is not determined by slope angle alone. The highest peaks rise in western states, but the largest area at risk from landslides is in the eastern Appalachian states. Water, much more plentiful in the eastern than in the western part of the country (with the exception of the Pacific Northwest), plays a significant role in mass wasting. Other factors play other roles. Earthquakes and other natural disasters, the absence of vegetation, and human activities can also influence the potential for mass wasting.
Playing chief roles in the mass wasting process are weathering, gravity, and water.
Storms or earthquakes may trigger flows of water and sediment down a continental slope, the steeply sloping region of the continental margin (the submerged outer edge of a continent) that extends downward to the ocean basin. As the material begins to move down the slope, it gathers speed and mixes with water to form turbidity currents. Because they are heavier than the surrounding water, the currents are pulled downward by gravity. Flowing at speeds of up to 50 miles (80 kilometers) per hour, the currents gather more sediment by scouring the slope as they travel downward. When they come to the base of the slope, the currents slow and the sediments settle on ocean basin, forming a fanlike deposit. (For more information on turbidity currents, see the Continental margin entry.)
The process by which rocks and minerals are broken down at or near Earth's surface is called weathering. This encompasses all the processes that cause rocks and minerals to fragment, crack, crumble, or decay. There are two types of weathering: mechanical and chemical. Mechanical weathering is the process by which a rock or mineral is broken down into smaller fragments without altering its chemical makeup. Examples of this type of weathering include frost wedging, which takes place when water in a crack freezes and enlarges, forcing apart a rock. Rocks may also be forced apart in places like deserts by drastic temperature changes above freezing. The roots of trees and other plants may wedge into rocks,
|Human Cost of Mass-Wasting Events|
|Debris avalanche||12,000–20,000||Soviet Union (present-day Russia)||1949|
widening cracks. The other type of weathering, chemical weathering, is the process by which chemical reactions alter the chemical makeup of rocks and minerals. It involves the decomposition of rocks and minerals by atmospheric gases and water. Oxygen dissolved in water may oxidize minerals that contain iron. Carbon dioxide dissolved in water forms a weak carbonic acid that can dissolve limestone. Water alone may also dissolve some minerals or combine with others to form new by-products.
Rocks weather at different rates depending on the climate and their mineral composition and texture. Rocks weather rapidly in hot, moist climates, but slowly in cooler, drier climates. In general, weathering tends to produce rounded rocks. Weathering also produces regolith (pronounced REH-gah-lith). This is the layer of loose, uncemented rocks and rock fragments of various size that lies beneath the soil and above the bedrock (general term for the solid rock that underlies the soil). Over time, regolith itself can be further weathered to create soil. It is the movement of regolith downhill under the influence of gravity that defines mass wasting.
The force of gravity acts in two ways on regolith and other material on a slope. As mentioned earlier, gravity is a constant force exerting a pull on everything on Earth's surface in a direction toward the center of the planet. Stated another way, the force of gravity pulls material straight down in a direction perpendicular to the surface. Material on a slope is thus pulled inward in a direction that is perpendicular to the slope. This helps prevent material from sliding downward. However, on a slope, gravity also exerts a force that acts to pull material down a slope, parallel to the surface of the slope. This second force of gravity is known as shear
stress. The amount of shear stress exerted is related directly to the steepness of the slope. Shear stress increases as the slope steepens. As shear stress increases, the perpendicular force of gravity decreases.
When shear stress becomes greater than the perpendicular force of gravity, material on a slope may still not move downward right away. It may be held in place by the frictional contact between the particles making up that material. Contact between the surfaces of the particles creates a certain amount of tension that holds the particles in place at an angle. The steepest angle at which loose material on a slope remains motionless is called the angle of repose. In general, that angle is about 35 degrees. It may vary slightly depending on certain factors, such as the size and shape of the particles. The angle of repose usually increases with increasing particle size. Particles that are irregularly shaped (with angled edges that catch on each other) also tend to have a higher angle of repose than those that have become rounded through weathering and that simply roll over each other.
Water is an important agent in the process of mass wasting. Water will either help hold material together, increasing its angle of repose, or cause it to slide downward like a liquid. In mass wasting, water acts as either a glue or a lubricant. Small amounts of water can strengthen material. Slightly wet particles have a higher angle of repose because the thin film of water that exists between the particles increases the tension holding them together. An example of this action can be seen in sand. Dry sand does not stick together very well. A sand castle made of dry sand will not stand very high. Yet one made with slightly moist sand will. If too much water is added, though, the sand will become waterlogged and the castle will collapse. When material becomes saturated with water, the angle of repose is reduced to a small degree and the material tends to flow like a liquid. This occurs because the excess water completely surrounds the particles in the material, eliminating the frictional contact between the particles that holds them together.
Excess water also increases the mass of material on a slope. Mass is a measurement of how much matter is in an object, while weight is a measurement of how hard gravity is pulling on that object. The force of gravity is proportional to the mass of an object: the greater the mass, the greater the force of gravity. If the mass of the material on a slope increases, so will the force of gravity exerted on it. With low or even nonexistent frictional contact between its particles, waterlogged material is subject to high shear stress, and it will slide or flow down a slope under that force of gravity.
As long as material on a slope stays within its angle of repose, it will remain stationary. Good vegetative cover, a small amount of moisture, and a high amount of binding material such as clay will increase the strength and stability of a slope, preventing mass wasting. Once a slope becomes unstable, mass wasting can occur. In areas where there are alternating periods of freezing and thawing or of wetting and drying, particles of soil and regolith are lifted up and set back down, but not in the same place as before. Gravity always causes the rocks and soil to settle just a little farther downslope than from where they started. This is the slow movement that defines creep, where the slope is unstable all the time and the process is continuous.
But other times, triggering events can arise that cause a sudden instability in a slope. A sudden shock, such as shock waves from an earthquake, can alter the structure of a slope, causing the slipping of surface soil and rock and the collapse of cliffs. Volcanic eruptions produce shocks similar to earthquakes. They can also cause snow and ice to melt, rapidly releasing large amounts of water that can mix with volcanic ash and regolith to produce debris flows, mudflows, and lahars. Sudden heavy rains and floods can saturate the soil and regolith, reducing frictional contact and the angle of repose.
The normal erosive action of streams and waves can undercut stream banks and cliffs along coasts. Since it is no longer at the angle of repose, the bank or cliff becomes unstable and material falls downward.
Human activities may dramatically increase mass-wasting events. Heavy trucks rumbling down a road can send shock waves through nearby unstable slopes. This is especially true in areas that have been altered by grading (leveling-off of an area) for road or building construction. When a portion of a mountainside or hillside is graded, material is cut out of the slope and removed. The slope directly above the graded area is greatly steepened, reducing support for material higher up the slope. Mining is another activity that weakens slopes and promotes mass wasting. The removal of coal, stone, and other natural resources from the area beneath a slope makes the slope unstable and vulnerable to collapse.
Temperature fluctuations, water freezing in cracks, and growing tree roots make rockfalls a common occurrence in Yosemite National Park in California. However, in July 1996, the ground at the park was shaken by a tremendous rockfall. An 80,000-ton (72,560-metric ton) block of granite broke free from a cliff high above Yosemite Valley near Glacier Point. It slid down a steep slope for the first 500 feet (152 meters), then took to the air and fell freely for over 1,700 feet (518 meters). The impact of the rock when it smashed into a rocky slope near the base of the cliff generated winds in excess of 160 miles (257 kilometers) per hour. The blast blew down 10 acres (4 hectares) of trees. The dust created by the impact hung in the air for several hours before settling over an area of about 50 acres (20 hectares). Places near the impact site were covered with up to 2 inches (5 centimeters) of dust.
The removal of vegetation on a slope, such as through forest fire or the cutting down of trees, can also lead to mass wasting. The roots of trees and other plants absorb water from rain or snow and release it slowly into the soil. Roots also act as anchors, holding the soil together. Soil with no vegetative cover erodes quickly. Landslides on deforested slopes, once set in motion, have no natural barriers to slow or stop them.
On April 29, 1903, in a glacier-modified valley near Alberta, Canada, the greatest landslide in recorded North American history took place. A wedge of the eastern slope of Turtle Mountain, measuring approximately 0.5 square mile (1.3 square kilometers) in area, gave way and hurtled 2,300 feet (700 meters) down the mountain. An estimated 100 million tons (91 million metric tons) of rock plowed through a portion of the nearby small coal-mining town of Frank, continued 2.5 miles (3.2 kilometers) across the valley floor, then climbed 400 feet (122 meters) up the opposite side. The fallen rock, which in places along the valley floor reached a height of 100 feet (30.5 meters), dammed the Crowsnest River and created a new lake. Seventy-six people were killed instantly in the rock slide that lasted less than two minutes. Only twelve bodies were ever recovered.
The main cause of the rock slide, which became known as the Frank Slide, was the mountain's unstable structure. Two years prior to the slide, mines had been dug in order to mine the massive deposits of coal beneath the eastern slope of the mountain. Already unstable because of the loss of rock structure underneath, the mountainside was put into further jeopardy when a sudden cold spell caused water from melting snow to freeze in cracks on its surface. As the water turned to ice, it expanded, widening the cracks and initiating the slide.
The Loess Plateau covers an area of about 247,100 square miles (640,000 square kilometers) in the north-central area of China. Loess (pronounced LUSS; a German word meaning "loose") is a deposit of fine, yellowish-gray, silty sediment. The loess covering the plateau was blown in from the Gobi Desert in Mongolia by windstorms over many, many years. Because it is so fine and loosely packed, loess is highly prone to erosion by wind and water.
In 1920, an earthquake struck the Loess Plateau region near Gansu (formerly Kansu) Province. Treeless and covered in loess, the hills and cliffs in the region were highly susceptible to loess flows. The shock of the earthquake caused the sides of 100-foot-high (30-meter-high) cliffs to collapse. Flowing material barricaded the entrances of mountainside caves, in which many peasants made their homes. The flows laid waste to ten cities and numerous villages in the region. An estimated 180,000 people died, more than the number who were killed at Hiroshima, Japan, on August 6, 1945, when the first atomic bomb was dropped.
Mount Huascarán (pronounced wass-ka-RON), the tallest mountain in Peru, is part of the Andes mountain system, the world's longest system on land, which runs for more than 5,000 miles (8,000 kilometers) along the western coast of South America. An extinct volcano, Mount Huascarán rises to a height of 22,205 feet (6,768 meters). Like other peaks in the area, it features many alpine glaciers.
In May 1970, a forty-five-second earthquake struck beneath the mountain, causing a mass of rock and glacial ice measuring 3,000 feet (914 meters) wide and 1 mile (1.6 kilometers) long to break off its west face and slide toward the valley below. Traveling at an average speed of 100 miles (161 kilometers) per hour, the debris avalanche quickly reached the village of Yungay, located 11 miles (18 kilometers) away from the mountain. Estimated to have consisted of almost 80 million cubic yards (61 cubic meters) of rock, ice, water, and other debris, the avalanche completely buried the village and others in its path, killing 18,000 people.
Goodwin, Peter H. Landslides, Slumps, and Creep . New York: Franklin Watts, 1998.
Jennings, Terry. Landslides and Avalanches . North Mankato, MN: Thame-side Press, 1999.
Walker, Jane. Avalanches and Landslides . New York: Gloucester Press, 1992.
Ylvisaker, Anne. Landslides . Bloomington, MN: Capstone Press, 2003.
"Avalanches and Landslides." Nearctica . http://www.nearctica.com/geology/avalan.htm (accessed on August 27, 2003).
"Geologic Hazards: Landslides." U.S. Geological Survey . http://landslides.usgs.gov/ (accessed on August 27, 2003).
"Landslide Images." U.S. Geological Survey . http://landslides.usgs.gov/html_files/landslides/slides/landslideimages.htm (accessed on August 27, 2003).
"Landslide Overview Map of the Conterminous United States." U.S. Geological Survey . http://landslides.usgs.gov/html_files/landslides/nationalmap/national.html (accessed on August 27, 2003).
"Landslides and Mass-Wasting." Department of Geosciences, University of Arizona . http://www.geo.arizona.edu/geo2xx/geo218/UNIT6/lecture18.html (accessed on August 27, 2003).
"Slope Failures." Germantown Elementary School, Illinois . http://www.germantown.k12.il.us/html/slope_failures.html (accessed on August 27, 2003).
"What Causes Landslides?" Ministry of Energy and Mines, Government of British Columbia . http://www.em.gov.bc.ca/mining/geolsurv/surficial/landslid/ls1.htm (accessed on August 27, 2003).