is the displacement of solids (soil
and other particles) by the agents of wind, water or ice, by downward or down-slope movement in response to gravity
or by living organisms (in the case of bioerosion
). Erosion is distinguished from weathering
, which is the breaking down of rock and particles through processes where no movement is involved, although the two processes may be concurrent.
Erosion is a natural process, but in many places it is increased by human land use. Poor land use practices include deforestation, overgrazing, unmanaged construction activity and road or trail building. However, improved land use practices can limit erosion, using techniques like terrace-building and tree planting.
A certain amount of erosion is natural and, in fact, healthy for the ecosystem. For example, gravels continually move downstream in watercourses. Excessive erosion, however, can cause problems, such as receiving water sedimentation, ecosystem damage (including fish kills) and outright loss of soil.
An 18th Century Persian astrolabe used for determining the time at both day and night. The points of the curved spikes on the front rete plate mark the positions of the brightest stars, the name of each star being labeled at the base of each spike. The back plate, or mater, is engraved with projected coordinate lines. From the Whipple Museum of the History of Science collection.