Landslides are not individual events, they occur in conjunction with other factors Landslides occur when the hill or mountain side is unstable. Factors that can allow gravity to overcome the resistance of earth material are:

Above landslide in Oregon caused by massive flooding (Photo courtesy of www.lcd.state.or.us/ coast/hazards.html)

Photo credit: U.S.G.S., Menlo Park, CA

Click on the above picture to see a short informational movie clip about the Mount St. Helens landslides!

The Human Effect on Landslides

Overdevelopment - Human activities such as construction, building, transportation, building dams and canals, and mining can disturb large volumes of earth materials. In fact, landslide damage is increasing every year as our population expands further into hilly regions. The San Francisco Bay region is turbulent enough on its own, but as more people overdevelop the land becomes less and less stable. The picture to the right displays the devastating effects of building too close to a hill with landslide potential. (photo credit: R.L. Schuster, U.S.G.S)

Deforestation - As the human population grows the demand for clear land for crops and housing grows too. So forests, with trees that hold the soil in place, are logged, burned, and developed. And while this alone will usually not cause a landslide, the land becomes much more susceptible to heavy rains and floods and landslides can occur with much less rain than if a forest was still there. Wildfires, either natural or manmade, also have the same effect. (Socioeconomic and environmental...)

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