weathering

The physical weathering and erosion of a limestone landscape. The freezing and thawing of rain and its mild acidic properties cause cracks and joints to enlarge, forming limestone pavements, potholes, caves, and caverns.

A weathered gravestone at Cassington, Oxfordshire, England. Gravestones are excellent laboratories for geographers. The date on the grave gives us a good idea how long the rock has been exposed to the forces of nature. The gravestone has been subjected to biological weathering (the growth of lichens, ivy and other plants). In the UK, west-facing stones are usually more weathered than east-facing stones, because winds from the west are warmer and moister, whereas east winds are typically colder and drier. Rock type is also important: igneous and metamorphic rocks, such as granite and marble, weather more slowly than sedimentary rocks, such as limestone.

Behind the wire mesh is bare sedimentary rock, exposed by weathering. Many cliff faces remain steep due to rapid breakdown. Weathering of the rock followed by rock falls (a type of mass movement) help to preserve a steep incline. In this picture, layers of sediment in the rock are clearly visible.

This rock face in the Dolomite mountains of northern Italy has been subjected to extensive weathering. Unlike mountains in other parts of the Alps, the various peaks are not linked by smooth and winding crests, but instead stand apart and isolated.

Process by which exposed rocks are broken down on the spot (in situ) by the action of rain, frost, wind, and other elements of the weather. It differs from erosion in that no movement or transportion of the broken-down material takes place. Two types of weathering are recognized: physical (or mechanical) weathering and chemical weathering. They usually occur together, and are an important part of the development of landforms.

Physical weathering 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.

Chemical weathering 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.