As soon as any rock is exposed at the surface of the earth it is subjected to various forces of ero­sion, which reduce the rock to fragments and carry the resulting debris to areas of deposition.

The weather is the most significant agent of this erosion and can act in one of two ways. It can pro­duce physical changes in which the rocks are bro­ken down by the force of rain, wind or frost; or it can produce chemical changes in which the minerals of the rocks are altered and the new sub­stances formed dissolve in water or crumble away from the main rock mass.

The different processes involved do not act independently of each other, the resulting erosion is caused by a combination of physical and chemical effects, although in some areas one erosive force tends to predominate.

Rain Erosion

The effects of rain erosion of the landscape are best seen in areas of loose topsoil. Rock or soil that is already loose is easily dislodged and washed away in heavy rainstorms. The most spectacular examples of this type of rain erosion occur in volcanic areas, where the soil consists of deep layers of volcanic ash deposited by recent eruptions. Streams of rainwater running down the slopes carry away fragments of the exposed volcanic top-soil, and the force of these moving fragments dis­lodges other fragments. As a result, the slopes become scarred with converging gullies and small gorges that form where the erosion is greatest. In some places, the lower slopes are worn away so rapidly that the higher ground is undercut, result­ing in a landslip. The rain-erosion features of vol­canic terrain are also found on mining spoil heaps which, like the surface layers in volcanic areas, are composed of small particles.

In regions that have a deep topsoil, small areas may be protected from rain erosion by the pres­ence of large rocks on the surface. The soil around these rocks may be worn away, leaving the rocks supported on pedestals of undisturbed material.

Rain falling on grassy slopes may cause Soil Creep, or Downhill Creep. The soil tends to be washed down the slope, but the interlocking roots of the grass prevent it from moving far, leading to the formation of a ser­ies of steps in the hillside where bands of turf have moved slowly downwards. (Soil creeps in bands because the force of gravity overcomes the roots' cohesion in the downwards direction whereas the root network remains strong in the sideways direction.)

The chemical effect of rain depends on the fact that carbon dioxide in the atmosphere dissolves in the rain, forming weak carbonic acid. The acid reacts with the calcite (a crystalline form of cal­cium carbonate, the substance responsible for "hardness" in water) in limestone and with certain other minerals, thereby dissolving them. This ero­sive effect may give rise to any of several geolog­ical features, such as grikes, which are widened cracks in the exposed rock, and swallow holes, where streams disappear underground - features that are particularly common in limestone areas, such as north-western Yugoslavia and the county of Yorkshire in the UK.

Temperature Change and Exfoliation

Uluru or Ayers RockTemperature changes are an important part of the weathering process, particularly in arid areas where the air is so dry that its insulating effect is negligible (normally, moisture in the air acts as a blanket, reducing heat loss from the ground); the lack of insulation results in a large daily range of temperature.

Repeated heating and cooling of the surface of a rock while the interior remains at a constant temperature weakens the rock's outer layers.

When this effect is combined with the chemical action that takes place after the infrequent desert downpours, the outer layers of the rock peel off - a process called exfoliation.

Exfoliation may occur on only a small scale, affecting individual rocks, or it may affect whole mountainsides, espe­cially those in which the bedding planes of the rock are parallel to the surface.

Exfoliation of entire mountains typically produces prominent, rounded hills called inselbergs, a well-known example of which is Uluru (Ayers Rock) in central Aus­tralia.

Wind Erosion

As with heat, the weathering effects of the wind are also greatest in arid regions, because the soil particles are not stuck together or weighed down with water and are therefore light and easily dis­lodged.

Coarser soil particles blown by the wind bounce along close to the ground (a mode of travel called saltation), rarely rising more than about lm above ground level. These moving par­ticles can be highly abrasive and, where the top of an exposed rock is above the zone of attrition, can erode the rock into a pedestal shape.

Stones and small boulders on the ground may be worn smooth on the side facing the prevailing wind, eventually becoming so eroded on one side that they overbalance and present a new face to the wind. This process then repeats itself, resulting in the formation of dreikanters - stones with three or more sides that have been worn smooth.

The effect of the various abrasive processes is cumulative: particles that have been abraded from the surfaces of exposed rocks and stones further abrade the landscape features (thereby increasing the rate of erosion), eventually giving rise to a typical desert landscape.

People and Erosion

A natural landscape is a balance between the for­ces of uplift, which produce new topographical features, and erosion, which gradually wears away exposed surface features. People's activities, espe­cially farming, may alter this balance - sometimes with far-reaching effects.

The removal of natural vegetation may weaken the topsoil, and when the soil particles are no longer held together by exten­sive root systems they can be washed away easily by the rain. This process may result in a "bad­lands" topography initially, fields of deep, fertile soil are cut with gullies then, as erosion continues, the soil is gradually broken down into small par­ticles that are eventually washed away by rain or blown away as dust.