The most common natural causes of landslides include volcanic eruptions, seismic vibrations from earthquakes, erosion beneath banks or cliffs and increases in pore water pressure. Human causes include deforestation, alterations to natural paths of drainage, pipe leaks and other activities placing significant pressure on landforms.
While all of these factors have caused landslides, in most cases one event sends the land into motion. When an area receives an unexpectedly high amount and concentration of rain, the saturation of the land leads to a higher pore water pressure, causing shifts in the land.
When volcanoes erupt and earthquakes occur, the changes far beneath the Earth's surface cause shifts that send the land on top down a slope. Some of the most visually arresting examples of landslides happen when houses on the side of a cliff or overlooking a lake collapse because erosion has taken away much of the soil and rock beneath them.
When human activities involve changes to landforms, policy planners must pay attention to the effects of those activities. Changing draining passages, putting too much load on a particular slope, quarrying or mining without considering the effects on nearby slopes, or even vibrations from heavy traffic patterns all have sent land sliding.