Earth's surface is never perfectly flat but instead consists of slopes.Some are steep and precipitous; others are moderate or gentle. Some are long and gradual; others are short and abrupt Some slopes are mantled with soil and covered by vegetation; others consist of barren rock and rubble. Their form and variety are great. Although most slopes appear to be stable and unchanging, they are not static features because the force of gravity causes material to move. At one extreme the movement may be gradual and practically imperceptible. At the other extreme it may consist of a roaring debris flow or a thundering rock avalanche. Landslides are a worldwide natural hazard(Figure 4.23). When these natural processes lead to loss of life and property, they become natural disasters