How Landslides Work

Introduction to How Landslides Work Introduction to How Landslides Work
Two homes in the Hollywood Hills of Los Angeles sit perilously on the edge of a cliff in January 2017 after a mudslide caused by heavy rains washed away the land beneath. ROBYN BECK/AFP/Getty Images

When it comes to natural disasters, the tornadoes and tsunamis of the world tend to get all of the attention. Rarely do landslides seize as many headlines as the volcanoes and earthquakes that can cause them. But when the ground literally rips downhill, the effect is often more damaging than the trigger. The force of landslides can cave houses, dam rivers and annihilate entire towns. Worldwide, landslides were responsible for more than 30,000 deaths in 2005 [source: Petley]. They inflict damage that costs the United States alone at least $1 billion to $2 billion each year, making them more damaging than all other natural disasters combined [source: United States Search and Rescue Task Force].

Landslides are a form of mass movement, a term used to describe any sort of gravity-induced movement of sediment down a slope. Mass movements can occur slowly over a period of years, or they can happen in a matter of minutes. A mass movement can be as small as some rocks and debris you kick down a small incline or as big as the 1980 landslide set off by the eruption of Mount St. Helens.

­There are many different kinds of mass movements categorized by the type of material involved, the way it is moved and how fast it moves­. However, with any mass movement, a soil layer is separated to some degree from the underlying bedrock. Soil is the relatively loose mixture of worn-down rock, minerals, air, water and decayed organic matter that covers the ground. Bedrock is the more stable, solid layer of rock underneath.

Although the word landslide often is used (incorrectly) to encompass many types of mass movements, a landslide is actually something more specific. A slide refers to a mass movement where rocks and sediment are loosened from the stable, underlying bedrock along a distinct zone of weakness. The rocks and sediment separate and move down the slope rapidly. You could think of it as a poster fastened to a wall with tape. The poster will remain on the wall barring any outside force acting on it. But if extra weight is attached to the poster, or if the tape is moistened, the connection will be weakened and the poster will fall.

In this article you'll learn what happens if a landslide happens underwater, why deforestation and water don't mix and just how powerful (and hot!) volcanic landslides can be.

Mass Movements

Pistol butt trees show evidence of creep.
Pistol butt trees show evidence of creep.

As you learned on the previous page, a landslide is only one of the many types of mass movements. Gravity is also responsible for several other forms of sediment relocation. The other major types of mass movements are creep, slump and flow.

Creep is the slow movement of sediment down an incline -- so slow that it sometimes takes place over several years. With creep, the adherence of the sediment to the bedrock is hampered, but not completely destroyed. Creep often occurs in areas that have experienced repeated freezing and thawing, which changes the structure and composition of the soil. Creep can even occur on gentle slopes. Have you ever seen trees or telephone poles bent at strange angles in relation to the ground? Creep is the culprit.

In slump, a large segment of sediment breaks off in one piece rather than in lots of segments. It occurs when the base can no longer support the weight of what's on top. The sediment involved in slump is usually wet or claylike, and water is often the factor that causes it to fall. Water either adds mass to the top layer of sediment or wears the soil away at the base, weakening the connection between the top and bottom layers. You've probably observed slump if you've ever gone to the lake or the beach and stepped on some wet sand overhanging the shore: The whole chunk simply drops in one piece.

Flows are the result of water mixing with sediment to form a soupy mass of rock, water, soil and other materials. The resulting mixture easily and swiftly slips downhill. Mudflows and avalanches are examples of this type of mass movement and can be especially destructive.

To better understand landslides and other mass movements it helps to learn a bit more about the two contributing processes of weathering and erosion, which we'll do on the next page.

Weathering and Erosion

Depiction of erosion, weathering and deposition. Soil is first mechanically weathered by temperature changes. Then water is the agent of erosion that carries the sediment. Finally, the deposition of eroded material.
Depiction of erosion, weathering and deposition. Soil is first mechanically weathered by temperature changes. Then water is the agent of erosion that carries the sediment. Finally, the deposition of eroded material.
Lynn Betts/Tim McCabe/USDA Natural Resource Conservation Service

A landslid­e can strike in an instant, true. But in reality, forces have quietly been at work on that section of land for a long time. Weathering is one of these more subtle forces behind landslides.

Weathering is the natural wearing down of objects by elements in the environment. Although the term is sometimes mistakenly used interchangeably with erosion, the two processes are distinct. Erosion is the process of transporting weathered material, and weathering is the actual wearing down of that material. Weathering is an important factor in landslides; heavily weathered landscapes are much more likely to be eroded. Heat, cold, water and oxygen are all common forces of weathering. This wearing down of objects can be either mechanical or chemical.

  • Mechanical weathering is the physical breakdown of an object into smaller components without changing its chemical composition. Changes in temperature, the freezing and thawing of water and plant growth are forces of mechanical weathering.
  • Chemical weathering refers to the breakdown of an object into particles with a different mineral composition than the original object. Water is perhaps the most powerful agent of chemical weathering: Over time, it can dissolve many kinds of rocks into a solution that has a different chemical makeup than the original substance. Other types of chemical weathering involve more complicated chemical reactions with oxygen, carbon dioxide, water or other compounds.

Erosion, the transfer of weathered sediment, always involves deposition, or the deposit of the weathered sediment in a new location. Along with weathering, erosion and deposition are responsible for the continual reshaping of the Earth's surface. Sediment is constantly being worn down by some type of weathering, carried away by an agent of erosion and deposited in a different place. Landslides are actually a very extreme, fast-acting method of erosion: They transfer sediment down a slope and deposit it at the end of their path. The sediment a landslide deposits is known as its talus.

The five agents of erosion are wind, water, glaciers, waves and gravity. As you learned earlier, gravity is the underlying agent of all types of mass movement. Without the force of gravity to pull sediment down an incline, a landslide would not occur. But any of the other four agents can also play a part. Read on to learn more about what causes landslides.

Causes of Landslides

Gravity is the primary factor in a landslide. Soil on a flat surface does not move. On a slope, gravity alone also may not trigger a landslide. But when another disrupting factor -- like rain -- comes into play, gravity will pull sediment downhill.
Gravity is the primary factor in a landslide. Soil on a flat surface does not move. On a slope, gravity alone also may not trigger a landslide. But when another disrupting factor -- like rain -- comes into play, gravity will pull sediment downhill.
Marcus Williams, HowStuffWorks

We know gravity is the ultimate force behind any landslide and that weathering plays a part. But what pulls the trigger to set a slide in motion?

Land surfaces are held together by multiple forces. The most important of these is friction. Some soil particles, like clay, cling to each other tightly, while others, like sand, are only loosely joined. All landscapes are held together by friction between the sediment cover and the underlying bedrock, some more tightly than others. If something is introduced to disrupt the friction on an incline, a landslide slips into action. Landslides occur when gravity overcomes the force of friction.

Several common causes of landslides are:

  • Water: Perhaps the most common trigger of a landslide, water reduces the friction between the bedrock and the overlying sediment, and gravity sends the debris sliding downhill. In sand and clay soils, a small amount of water may increase stability. You've likely seen this when building a sand castle or working with clay. However, too much water causes the sediment to flow, which is why many landslides occur after rainstorms.
  • Earthquakes: If the Earth's crust vibrates enough to disrupt the force of friction holding sediments in place on an incline, a landslide can strike.
  • Wildfires: Plants help to stabilize the soil by holding it together like glue with their roots. When this glue is removed, the soil loosens, and gravity acts upon it much more easily. The loss of vegetation after a fire makes the razed land susceptible to slides.
  • Volcanoes: Several characteristics of volcanoes make them a fertile starting point for especially destructive landslides. On the next page, you'll learn just how powerful these volcanic landslides can be.

Volcanic Landslides

Lava flow near the shore line at Pupapau Point, Kalapana, on Nov. 27, 1989 during the Kilauea east rift eruption.
Lava flow near the shore line at Pupapau Point, Kalapana, on Nov. 27, 1989 during the Kilauea east rift eruption.
J.D. Griggs/Hawaiian Volcano Observatory/U.S. Geological Survey

Volcanoes have unstable surfaces, so landslides are likely even when the volcano is dormant. In addition to the fact that the surface is composed mostly of loose rock, the volcanic gases create acidic groundwater. This contributes to the rocks' breakdown, making the them more likely to be carried away.

Landslides can also occur during volcanic activity. There are two types of volcanic landslides.

Pyroclastic flows occur during an eruption or after the collapse of a volcano's dome. These high-speed flows, composed of ash, lava, rocks and gas, can reach temperatures of 1,500 degrees Fahrenheit (815.5 degrees Celsius) and have been clocked at up to 450 mph (724 kph) [source: Landslide Hazards, USGS]. Pyroclastic flows that surge with hurricane-force haste are capable of tearing up and burning anything in their path.

Lahars, which don't necessarily occur during an eruption, are set off by water. The water could come from a rainstorm, melted snow and ice or a glacier melted by lava flow. Along with the contributing water, lahars comprise mud and rock. Sixty to 90 percent of a lahar's weight may come from rock debris, lending it grave force. Generally considered the most deadly type of volcanic hazard, lahars can reach speeds of 20 mph to 40 mph (32 kph to 64 kph) and travel over 50 miles (80.4 kilometers) [source: Landslide Hazards, USGS], ripping up any trees and homes in their path.

U.S. Geological Survey

Not all causes of landslides are as obvious as the ones listed so far. Humans certainly play a part in causing equally devastating landslides. Read on to find out what mistakes we make and how we can prevent and prepare for landslides.

People and Landslides

Clear cuts in Prince of Wales Forest, Alaska
Clear cuts in Prince of Wales Forest, Alaska
Chip Porter/Getty Images

Humans make landslides more likely through activities like deforestation, overgrazing, mining and road-building. Remember when we explained that vegetation acts like glue, holding the soil in place? These activities rob that glue from the land, increasing the probability for a landslide. For example, landslides are much more likely to occur in mountainous areas that have been clear-cut for roads. You've probably seen signs warning of rock falls if you've ever driven through the mountains. The drop-offs with loosened soil you see on both sides of the road require much less water to set off a landslide than natural drop-offs.

Although landslides cannot be avoided entirely, people can do several things to discourage them. Drainage pipes installed into slopes can carry away excess water, and impermeable membranes like plastic sheeting can prevent water from building up and dislodging the soil. In addition, setting up retaining walls at intervals will catch loose debris and keep it in place, while removing excess mass from the top of a slope could prevent the bottom from giving way. Reforestation is a good deterrent to landslides as well. When an area is clear-cut for harvesting timber, road-building or mining operations, restoring it to its natural condition stabilizes the land.

Plant roots help hold the soil in place.
Gary Vestal/Getty Images

Perhaps the most important thing people can do to avoid the dangers of landslides is to avoid building in hazardous zones. Ideally, buildings shouldn't be placed on steep slopes or in drainage areas. But if people do build in areas like these that are susceptible to landslides, they should use protective measures. For instance, construction sites should use barriers to mitigate runoff and erosion.

If you find yourself in an area where landslides are likely, be ready with an emergency evacuation plan if there is any danger. Be aware of any sudden rises or falls in water flow and listen for sounds that may signal moving debris or falling rocks. If you suspect landslide activity, you should evacuate immediately if it is safe to do so. Remember to be especially vigilant when driving, as roadside embankments are also hazardous.

While you certainly wouldn't want to be caught in the path of a landslide, it is exciting to view their awesome strength and force on video. Don't miss the close-up footage of landslides in action posted on the following page, as well as the related HowStuffWorks articles and other interesting Web sites.

Related Articles


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