How Landslides Work

Mass Movements

landslide Washington — 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.

Mount St. Helens

Most people are familiar with the enormous volcanic eruption of Mount St. Helens in 1980, but fewer people are aware that the landslide it triggered is the largest in recorded history. Moving at speeds of up to 150 mph (241 kph), the historic landslide destroyed 27 bridges, many miles of roads and roughly 200 homes. The debris covered an area of 23 square miles (37 square kilometers) and 3.7 billion cubic yards (3.38 billion cubic meters) [source: Brantley]. That's nothing compared to some prehistoric mass movements. Twenty-one million years ago, one Utah landslide sent a huge mass of volcanic debris across 2,000 square miles (5,180 square kilometers) of terrain [source: Biek, David and Rowley].