Landslides are downward flows of rock, dirt, and other materials. Landslides can cause a great deal of damage, especially when they are large, and they are a geological hazard of concern around the world, as they can happen anywhere and at any time. A number of steps can be taken to prevent landslides and reduce their severity, and these steps are often used in areas where landslides are common, such as the infamous Devil's Slide in California.
In a landslide, the materials are dry, and can include rocks, dirt, trees, and structures which happen to be pulled along with the falling material. Landslides can be caused by earthquakes, destabilization as a result of water flowing underground, the collapse of overhanging rocks, and the simple battle with gravity. Human activities such as heavy construction, undermining of the earth, and stripping groundcover like plants and trees can also trigger landslides. When a landslide involves mud and wet material, it is known as a mudslide; mudslides can follow storms and floods.
The land involved in a landslide is typically unstable. One of the most common reasons for instability is poor groundcover. Trees and plants, especially when they root deeply, can hold soil in place so that it cannot slide, even on very steep grades. If the groundcover is sparse or poorly rooted, rocks and loose materials can start to slip. Slides also occur on very steep grades, especially if the grades include overhangs.
Landslide disasters occur on a regular basis. A landslide can destroy homes, chew apart a highway, or trigger more catastrophic events such as flooding if it happens in the wrong place at the wrong time. Human settlements around the world are built near landslide-prone cliffs and mountains, which can result in tragic consequences. Geologic history has a number of examples of landslides which were large enough to move entire mountains, and in the modern era, several landslide disasters such as the Monte Toc landslide in Italy, the 1991 Pubjabi landslide in India, and the Khait landslide in Russia claimed thousands of casualties and dealt out considerable damage.
Techniques to manage landslides can include the establishment of retaining walls which trap landslides before they can cause damage, the growth of groundcovers to hold topsoil in place, and building codes which prohibit construction near landslide-prone areas so that structures will not be damaged when falls and slides occur. Human activities which can increase the risk of landslides may also be restricted, just as activities which contribute to avalanches are restricted in regions where avalanches are common.