Process by which exposed rocks are broken down by the action of rain, frost, wind, and other elements of the weather. Two types of weathering are recognized: physical and chemical. They usually occur together.
This involves such effects as: frost wedging, in which water trapped in a crack in a rock expands on freezing and splits the rock; sand blasting, in which exposed rock faces are worn away by sand particles blown by the wind; and soil creep, in which soil particles gradually move downhill under the influence of gravity.
Carbon dioxide in the atmosphere combines with rainwater to produce weak carbonic acid, which may then react with certain minerals in the rocks and break them down. Examples are the solution of caverns in limestone terrains, and the breakdown of feldspars in granite to form kaolinite or kaolin, thus loosening the other minerals present—quartz and mica—which are washed away as sand.
Although physical and chemical weathering normally occur together, in some instances it is difficult to determine which type is involved. For example, onion-skin weathering, which produces rounded inselbergs in arid regions, such as Ayers Rock in central Australia, may be caused by the daily physical expansion and contraction of the surface layers of the rock in the heat of the Sun, or by the chemical reaction of the minerals just beneath the surface during the infrequent rains of these areas.