A landslide is the movement of a lot of earth, debris, mud, or rocks down a slope.  Some landslides occur very rapidly, and others move slowly and randomly down a slope, taking years to travel a few feet. The basic reason that all landslides happen is that the force that pulls the slope downwards, gravity, becomes greater than the strength of the earth that makes up the slope.  In order for the land to be pulled down, something has to cause the slope to be vulnerable to gravity.  Some of the factors that contribute to the start of a landslide are:

• rivers, ocean waves, glaciers or rain fall cause part of a slope to erode and make it even steeper, letting gravity pull more easily
• the rocks and soil of a slope become very wet and slippery from heavy rain, or melting snow, which lets the land slip down very easily
• earthquakes that have a magnitude of 4 or higher can cause a landslide to start
  
• loose ash, lava, and debris from a volcanic eruptions can trigger a landslide
  
• man made structures, or other objects being deposited on a slope, can add too much weight to it and stress the soil
  

Landslides usually occur in several different types of areas.  Canyon bottoms and stream channels are places where landslides commonly occur because many landslides start high up in the canyon, and then flow down into the canyon bottom, or stream channels, where they flow for long distances because the channel walls don't let it spread out and stop, they guide the earth through the channel.

There are several kinds of landslides, including debris flows (mudflows), lahars, and submarine landslides.  Submarine landslides are landslides that occur underwater.  These landslides can cause tidal waves, which in turn can inflict a lot of damage on a place near the shore. A debris flow, which is the same thing as a mudflow, is a flow of mud and rocks that usually moves very quickly.  Although they can slide downward at speeds around two hundred miles per hour, most of the time they only move between thirty and fifty miles per hour.  The speed at which they travel depends on how steep the slope is, how much water is in the slide, and the type of earth and debris that are in the slide. Debris flows are usually started by rain that is coming down steadily, or by rain that is coming down very hard and quick for a short period of time.  Areas that have recently been burned by forest fires are very vulnerable to debris flows because of the condition of the slope and soil.

"Lahar" is an Indonesian word describing hot or cold mixtures of water and rock. Another name for lahars are lava slides. In looks, it they are very much like wet concrete but they have the power to move boulders more than 10 feet in diameter. There are many different sizes and speeds of lahars. Small lahars can be only a few meters wide and deep and flow a few meters per second. Large lahars are deadly, moving at up to 30 meters per second; no human can out run them. To make matters worse they can be hundreds of meters wide and 20 meters deep. Lahars do decrease in size as they get farther away from their source. Lahars are often caused by volcanic eruptions though a moderate rain can cause them. Lahars do have to start on an elevated slope to develop their momentum and speed. They can often cause serious damage, both economic and environmental. Lahars can level almost anything in their path.They can also trap people by leaving deposits that are too deep, soft, or hot to cross.

The biggest landslide in the world that has ever been recorded was when Mt. St. Helens (a volcano in the Cascade Mountain Range, Washington, USA) erupted and caused a lahar.  It is estimated that in the United States, between twenty five and fifty people are killed each year because of landslides, and that between one and two billion dollars are spent repairing damage that was inflicted by landslides.

The first picture is of a fresh mudflow, courtesy of earthsci.terc.edu/.../es1204/ es1204page07.cfm The second picture is of an ash mudflow, courtesy of www.sheppeyfossils.com/pages/ coastal_erosion_3.ht The last picture is of a lahar, courtesy of earth.esa.int/.../ndis/volcano/ etna/ip_12060.gif