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Slopes and landslides

{alt}Landslide—a mudflow—near Nara, Japan, 10th August 2004{more}Click to play video Slopes are unstable because gravity causes material such as soil, mud, rock fragments and rock itself to creep, slide, flow or fall down them. The presence of water is an important factor. Saturation after heavy rainfall both reduces the material's cohesion (the way it is held together) and adds to its weight, making slopes less stable. Other causes of downslope movement are undercutting by waves or rivers, earthquakes and volcanic eruptions. Human activity can be significant, too. Removing the vegetation that binds the soil, construction projects that affect the composition or shape of a slope and vibrations from nearby traffic are all ways in which a slope may become destabilized. 

Creep, slides, flows and falls

Different types of landslideLandslides, the downslope movement of material, include a wide range of movements, such as soil creep, earthflows, mudflows, rock slides, slumping and rock falls. Creep is slow, almost imperceptible movement, made in small steps. Slides are faster, and can remove entire hillsides. Flows are when the material, most commonly silt and sand, breaks up and tumbles downwards, not holding any shape. Falls are free movement of rock through the air—typical on very steep slopes or cliffs. 

Large lahars hundreds of metres wide can flow at several tens of metres per second—much too fast for people to outrun. They can also flow distances of more than 300 km (200 miles).

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