Think you know what a landslide is? Did you know there are different kinds? Here you will learn how landslides differ, why they happen, and where they are most likely to occur.
What Is a Landslide?
If you are somewhere very flat with no hills as far as you can see and it's not raining, then you're probably safe from landslides. If, on the other hand, it's pouring rain and you're on a bare cliff overlooking the ocean in California, you might want to back away. That's because there are a few landslide hazards in your midst!
A landslide happens when rocks, debris, and soil move down a slope. They're also known as landslips. Things can go downhill suddenly, or they can slide at the snail's pace of just a couple centimeters a year. They can come from natural causes or from the activities of people. The one thing they all have in common is gravity. What is up sometimes slides down.
What Causes Landslides?
You can think of the different types of landslide material like the different types of ice cream. Some is smooth and soft, some is smooth but thicker and doesn't flow as well, and some has chunks like nutty boulders and fruity vegetation. As you might imagine, all these consistencies would move differently.
The cause of a landslide that has to do with the structural combination of rock, soil and vegetation is the morphology. If a hillside has lost vegetation because of a fire for instance, then the addition of water can cause erosion, upping the chances for a landslide. It's not only rain that can cause erosion but flowing rivers, moving glaciers, and crashing ocean waves. The geology or strength of the earth material itself can also be a factor in landslides. Stronger material is less likely to break apart and slide down the slope. And finally, there are human activities that can expose slopes to erosion and lead to landslides. Clearing land for agriculture or construction are some of the human causes of landslides.
Types of Landslides
As you might be gathering, not all landslides are the same. There are landslides that are described as falls and topples, which sum up what happens when a boulder comes down the slope. Translational slides happen when a less stable soil type on the surface slides off a stronger layer below.
And despite most landslides needing a slope to move down, there is one type of landslide that can move sideways. That's a lateral spread or flow landslide, and it can happen when an earthquake moves the soil material as though it were a liquid.
One very dramatic type of landslide is a lahar. They happen when a volcano erupts, quickly melting snow and releasing a slide of ash and other material down the volcano's steep slopes. One the largest lahars in recent history is the one that developed in 1980 when Mount Saint Helens erupted.
Landslides: Not an Equal Opportunity Disaster
There are certain places in the world that are much more prone to landslides than others. In the U.S., areas with mountains, slopes, or cliffs are more likely to have landslides. The coastal mountains that run through Washington, Oregon, and California have an increased risk of landslides - so do the mountains of the Intermountain West and the mountain chains of the East Coast - same with Alaska and Hawaii. In a typical year, about 25 to 50 people die in landslides in the U.S. alone, and they cause billions of dollars of damage to homes, businesses, roads, and railways.
A landslide happens when soil, debris, and rock move down a slope, which can be either fast or very slow. The morphology, or structure of the land itself, along with erosion, can cause the land to slip. This is especially likely when vegetation is not there to stabilize the slope. Landslides that have to do with geology usually involve the strength of the soil or rock itself. Human activity, such as construction or clearing natural vegetation for agriculture, is also a cause of landslides.
The type of landslide that occurs when a volcano erupts and quickly melts snow, sending a landslide down the volcano's slopes, is called a lahar. There are fall and topple landslides that occur when a boulder falls downslope, translational slides that happen when a less sturdy type of soil slides off a more solid layer below, and a lateral spread or flow landslide that happens when an earthquake moves soil sideways.
The parts of the U.S. that are more likely to have landslides are the mountainous areas of the West Coast, East Coast, Alaska, and Hawaii.