Erosion is a continual process, and it can be caused naturally or through artificial means. While many people associate erosion with negative things like land slides and poor soil conditions, it is an important part of the geologic processes which make the Earth what it is. As soil and rock are slowly worn down, they cycle through the Earth's crust and the general environment, causing a constantly shifting and varied surface.
One of the most powerful causes of erosion is water. Water is sometimes called the universal solvent, because it is so effective at dissolving and changing things. Rain and runoff contribute to erosion, as do glaciers, snow, and ice. Ice can be particularly insidious, because it will literally rip rock and soil apart as it expands and contracts. Many seashores distribute spectacular examples of water erosion, in the form of huge terraces of rocks slowly worn away by the ocean.
Tectonic movement can also contribute to erosion, as can the wind. Wind transports materials from one place to another, and in extremely windy locations it can contribute a powerful scouring force to the process of erosion. Materials also naturally tend to slide down a slope, in a process called mass wasting. This downward pull is what causes mountains to slowly melt into hills and plains, and it is constantly happening, although not always in the spectacular form of a landslide.
Humans can also bring about erosion, usually through poor land management. Overgrazing, for example, is a serious cause of erosion. Cows in particular are known for causing serious problems, especially along river banks. Areas which have been heavily grazed for centuries show clear signs of erosion and soil exhaustion due to the demands made on the soil. Deforestation can also cause erosion, since it strips the protective surface plants and trees from the soil. The soil is no longer held in by roots and plant matter, so it slides away in rainy periods.
Once of the most famous instances of erosion is the Dust Bowl. The Dust Bowl was caused partially through human means, like farming the soil to exhaustion, and also through weather patterns. The end result, however, was a loss of an extremely valuable layer of top soil in many parts of North America. Modern farming techniques are remarkably similar to the techniques which increased the severity of the Dust Bowl in the 1930s, and this is an issue of some concern to environmentalists.