Weathering is the geological process by which rock is broken down into smaller and smaller pieces. It is commonly separated into two basic types: chemical weathering and physical weathering. The two processes generally work together. Weathering can reduce rock entirely to dissolved chemical elements and tiny bits called sediments. Common examples of sediments are clay, gravel, silt, and sand. Weathering is an important process on Earth’s surface for many reasons. Without sediments produced by weathering, there would be no soil for growing plants. Weathering also releases from rock nutrients such as potassium and phosphorus that living things need to survive. Sediments produced by weathering can cement together to form a major type of rock called sedimentary rock. Weathering also produces the sand and gravel we use to make concrete.
Most rocks at and near Earth’s surface formed at higher pressures and temperatures than they are exposed to today. Many of the minerals that make up these rocks are not stable at Earth’s surface. They are thus easily changed or dissolved by chemicals on the surface, including water. Chemical weathering occurs over time as exposure to carbon dioxide, oxygen, and water changes the original minerals and dissolves some elements. Chemical weathering may break down rock directly or may weaken rock, helping to break it apart physically.
Physical weathering usually occurs as rock breaks down along cracks in the rock or along boundaries between different minerals. Water can work its way into such spaces. The water may then freeze and thaw with the weather. As it does so, it expands and contracts, slowly prying the rock apart. Warming and cooling from day-night temperature cycles and from fires can expand and contract the rock itself, speeding physical weathering.
The two basic types of weathering work together. Chemical weathering weakens rock, making it easier to weather physically. Physical weathering, in turn, can expose fresh rock surfaces to chemical weathering.
Certain planets and many moons and other rocky objects in space lack air and water, two dominant factors in weathering on Earth. These bodies undergo different weathering processes. On the moon, for example, weathering results primarily from bombardment by meteorites. These impacts shatter rock and stir loose material on the moon’s surface. Radiation from the sun may also contribute to weathering on some bodies in space.
