What causes landslides?
If you looked at a mountain or hill in the distance, what would you notice? Both of these features have sides that are sloped. If you stood at the top of a mountain and dropped a stone, it would roll down the mountain until it reaches the bottom. What keeps the side of a mountain together?
Usually, the ground on a slope is strong enough to withstand the force of gravity. Over time, natural processes like weathering and erosion can make the ground on a slope weak. Landslides happen when this ground no longer stable. When the ground loses stability, gravity will pull it down the side of the slope.
Weathering and Landslides
If water enters the cracks in a rock and the weather is too cold, it will freeze. Water expands when frozen, so this force can crack the rock even more! If this happens several times, the rock can fall apart and lead to a weakened slope.
Erosion and Landslides
Erosion is when a surface process moves material from one location to another. Wind and water erosion are two examples. Imagine that the mountain in our example was next to a river. Over time, the river may remove enough of the ground under the slope to make it unstable. This could lead to a landslide!
A landslide must be triggered before it can begin. Sometimes, this process is very slow. The slow weathering of rocks may itself be a trigger. In other cases, an abrupt tectonic event will trigger a landslide. For example, an earthquake could shake any loose material on a slope and start a landslide.
What are the different types of landslides?
Landslides come in four varieties: flows, falls, topples, and slides. Landslides are classified based on how the material in a landslide moves.
For example, a flow occurs when material moves down a slope in a way that resembles a liquid. One major subtype of flow landslides is called a mudflow. Mudflows are caused when large amounts of water make the ground too heavy. This water usually comes from a heavy rainfall or melting snow.
Some mudflows are slow, but many move quickly. That is because the water makes the ground flow like a liquid. Mudflows are extremely dangerous. They can travel as fast as a car on the freeway!
A fall happens when some soil or rock detaches from the side of a steep slope. Once it hits the side of a slope, it makes its way down by rolling or bouncing.
A topple happens when a mass of soil or rock falls forward out of the slope. As an example, imagine yourself standing flat on the floor with your back against the wall. Imagine that the wall is the slope of a mountain and you are the ground covering it. If you slowly lean forward away from the wall, you would fall flat on your face! That is exactly what happens in a topple landslide.
Imagine that you put down a carpet on the side of a very slippery slope. What would happen when you let go? The carpet would slide down to the bottom. The carpet is still fully intact – it just moved positions. This is what happens in slide landslides. The top surface of a slope detaches and slides down. When this happens, the ground that slid down remains intact.
Other Great Resources
Landslides 101 – USGS: https://landslides.usgs.gov/learn/ls101.php
Landslides – National Geographic:
Types of Landslides – eSchoolToday: http://eschooltoday.com/natural-disasters/landslides/types-of-landslides.html