Dictionary Definition

The downward sliding of earth and rock.

Causes

Explanation

There are many different causes of landslides, but most are caused by the weather. All slides involve the failure of Earth materials under stress. A heavy rain can add stress to a slope, weakening it, and make the land slide. Other factors include erosion from streams, glaciers, or waves, and long-shore or tidal currents. So, basically anything that adds stress and takes away strength can cause a landslide, from the weight of rain and snow to the weight of buildings and other structures. They can be triggered by earthquakes, blasting, machinery, traffic, and even thunder. Below is an experiment that shows what happens when water is added to a slope.

Experiment #1: Sliding Land

What You'll Need:

• pencil
• paper cup
• drinking straw
• modeling clay
• cardboard about 1ft. (30 centimeters) square
• dirt
• 1 gallon (4 liter) jar filled with water

Directions:

1. Go outside to do this experiment.
2. Use the pencil to make a hole in the side of the paper cup near the bottom rim.
3. Cut a straw in half and insert one of the pieces in the hole in the cup.
4. Use the clay to form a seal around the hole.
5. Lay the cardboard on the ground and raise one end about two inches (five cm) by putting dirt under that edge of the cardboard.
6. Cover the cardboard with a thin layer of dirt.
7. Place the cup on the raised end of the cardboard, with the straw pointed downhill.
8. Hold your finger over the end of the straw and fill the cup with water.
9. Open the straw and observe the movement of the water.
10. Clean the cardboard and put dirt on it again.
11. Raise the cardboard about six inches (15 cm).
12. Cover the straw and fill the cup again.
13. Let go of the straw and observe the water movement again.

What's Happening?

When the slope is higher, the water moves more quickly and more dirt is washed away by the moving water. The faster the water moves, the more energy it has and the more dirt it can push forward. When it rains a lot on a slope (such as a hill), the water rushes down and brings the mud and houses with it.

Types

There are many different types of movement that the Earth can undergo.

• Mass Wasting: This is the transfer of Earth material which causes slipping, sliding, and creeping. It is influenced by the amount of meteoric water in the soil.
• Creep: This is a slow movement of soil down a gentle slope, and causes poles, fence posts, and trees to lean downhill.
• Earthflow: When there is a lot of water in the soil, an earthflow is triggered. These usually have a spoon-shaped sliding surface, on which some soil breaks away and flows a short distance to leaving a mark where it broke away.
• Mudflow: If there is a lot of water, an earthflow may turn into a mudflow. This slide carries rocks and boulders down the hill, picking up more material along the way to make a swift flood of muddy material.
• Avalanche: Another type of slide can occur on a mountain that is covered by snow. Once triggered, the snow falls down the mountain, picking up snow and building momentum in the same fashion as the mudflow.

Damages

Landslides can be very dangerous and very costly. Landslides have caused many deaths, as well as a huge loss of property. Single large landslides can cost tens of millions of dollars or more. Houses built on the edges of hills for views are especially prone to falling because they are right on the edge. And houses down below, at the base of a hill, may be hit hard by the falling debris from above.