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 most 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 what triggers it. The force of landslides can cave houses, dam rivers and annihilate entire towns. Worldwide, landslides were responsible for 32,322 deaths between 2004 and 2010 [source: Petley]. They inflict damage that costs the United States alone between $3 and $4 billion each year [source: American Geosciences Institute].

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 kinds of mass movements categorized by the type of material involved, the way they moved and how fast they move. 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

Mass Movements
This aerial survey shows the upper parts of a northwest Washington state landslide that occurred on March 22, 2014. It's a good example of how large segments sediment can break off in one piece. Jonathan Godt, U.S. Geological Survey. Public domain

As we discussed previously, a landslide is only one of the many kinds of mass movements. Gravity is also responsible for several different forms of sediment relocation. Other major categories of mass movement include flows, creep and slump.

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

Creep is a type of gradual flow, one that can take several years to unfold. With creep, sediment moves slowly down an incline. 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 [source: United States Geological Survey].

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.

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 next.

Weathering and Erosion

Weathering and Erosion
This 2005 landslide La Conchita, California occurred after the culmination of an extremely wet two-week period. Ten people were killed. Mark Reid, USGS. Public domain

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 subtler 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 [source: NPS Park Geology Tour].

  • 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 continually reshaping 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, waves, running water, glaciers and gravity. As we discussed 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

Causes of Landslides
Montecito, California was all but destroyed in March 2018 after massive mudslides ripped through town. The mudslides were triggered by flash floods in the Santa Ynez Mountains, which were charred by earlier wildfires. Justin Sullivan/Getty Images

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, the sediment gets heavier as more water is added and that can cause it to flow downhill. This is why many landslides occur after rainstorms [source: Washington Geological Survey].
  • 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. Seismic activity can also make it easier for water to seep into the soil, further destabilizing the slope.
  • Wildfires: Plants help keep the soil stable 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 fertile starting points for especially destructive landslides. On the next page, you'll learn just how powerful these volcanic landslides can be.

Volcanic Landslides

Volcanic Landslides
In March 2007, a massive lahar (mudflow) made its way along the Whangaehu River in the National Park, New Zealand after breaking the crater wall on Mount Ruapehu stratovolcano. Phil Walter/Getty Images

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 them more likely to be carried away.

Lahars are powerful landslides that originate on the slopes of volcanoes. They're set off by water and don't necessarily occur during eruptions. The water could be delivered in the form of a rainstorm, melted snow and ice, or a glacier melted by lava flow. Along with the contributing water, lahars contain mud and rock. Sixty to 90 percent of a lahar's weight may come from rock debris, lending it grave force. Lahars can reach speeds of 20 mph to 40 mph (32 kph to 64 kph) and travel more than 50 miles (80.4 kilometers) ripping up any trees and homes in their path. This makes them an especially dangerous kind of volcanic hazard [source: Landslide Hazards, USGS].

Sometimes, lahars are generated when ice melts at the hands of a pyroclastic flow. Pyroclastic flows are amalgamations of ash, lava, rocks, and gas that come barreling down volcanic mountains either during an eruption or when a volcano's dome collapses. Setting off landslides is just one of the threats they pose to human life. These high-speed flows have been clocked at up to 450 mph (724 kph) and they can reach temperatures of 1,500 degrees Fahrenheit (815.5 degrees Celsius). Pyroclastic flows that surge with hurricane-force haste are capable of tearing up and burning anything in their path [source: Landslide Hazards, USGS].

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

People and Landslides
This slope in the Cayaguas watershed in eastern Puerto Rico is an example of extreme deforestation and would be very susceptible to landslides. Robert Stallard, USGS. Public domain

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 of a landslide. For example, landslides are much more likely to occur in mountainous areas that have been clear-cut. We may have seen this unfold in 2014, when a landslide hit the Indian village of Malin, killing at least 151 people. Previously, thousands of trees in the neighboring hills had been chopped down to make room for new farms, construction projects and mining operations [source: Sarvade et al.].

By the way, you've probably seen signs warning of rock falls if you've ever driven through the mountains. The drop-offs with loosened soil on both sides of manmade roads 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.

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 [source: Kansas Geological Survey].

If you find yourself in an area where landslides are likely, be ready with an emergency evacuation plan. 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.

Last editorial update on Jun 6, 2018 01:13:27 pm.

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