Gravity is the primary factor in a landslide. Soil on a flat surface does not move. On a slope, gravity alone also may not trigger a landslide. But when another disrupting factor -- like rain -- comes into play, gravity will pull sediment downhill.

Marcus Williams, HowStuffWorks

Causes of Landslides

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, too much water causes the sediment to flow, which is why many landslides occur after rainstorms.
  • 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.
  • Wildfires: Plants help to stabilize the soil 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 a fertile starting point for especially destructive landslides. On the next page, you'll learn just how powerful these volcanic landslides can be.