Landslides can occur slowly, hardly noticeable over a period of years. Creep is the slowest movement of land. It is so slow that special instruments must be used to measure the speed of land movement, which is often only a few cm a year. However, sometimes rock and mud move down slopes at tremendous speeds
Mud flows are common forms of landslides. They occur after heavy rains, when the soil is saturated with water so that it cannot seep into the ground. Such a landslide can destroy everything that is in its way, including houses, cars and bridges located at the foot of a slope. Many happen in alpine areas, where mud flows block rivers and lead to the flooding of whole valleys.