Weathering

I

Introduction

Weathering, breakdown of rocks by physical and chemical processes at the surface, or in the top few metres, of the Earth’s crust. It is the initial stage in the denudation (lowering of the land surface) of landscape, which produces debris known as regolith. This can subsequently be transported away by such agents of erosion as running water, ice, the wind, or the sea.

Two main types of weathering can be identified. Physical or mechanical weathering breaks rocks into smaller fragments without changing their chemical composition. Chemical weathering decomposes the rock mass to form new minerals and substances. However, both chemical and physical weathering can also be brought about by the activity of plants and animals, in which case it is referred to as biological weathering.

II

Physical or Mechanical Weathering

This type of weathering attacks exposed surfaces of rocks, and can penetrate into the interior of rocks along cracks, joints, or bedding planes. Changes of temperature and the action of water are the most important agents of physical weathering of which several main types can be identified.

A

Freeze-Thaw

Freeze-thaw is important where temperatures fluctuate above and below freezing (0^o C/32^o F), and water is present. Water enters cracks in rocks and freezes if temperatures fall below 0^o C. As it crystallizes, it expands by about 9 per cent, creating pressures calculated to be around 2,100 kg/sq cm, which force the cracks open. Repeated cycles of freeze-thaw cause fragments to break away from the parent rock to form block fields, scree, or talus. The process is not effective where conditions remain below freezing for long periods, such as in polar regions, but it is important in climates such as that of northern Britain or of low-latitude, high-altitude areas such as the Himalaya.

B

Salt Crystallization

Salt crystallization is important in desert environments, where temperature ranges are extreme and daytime rock surface temperatures may reach temperatures as high as 80^o C, falling to 0^o C at night. Evaporation is high and humidity levels low, so salts are drawn up through the soil by capillary action to the surface where, because of the arid conditions, they remain. If some moisture is present, a saline solution may form which can enter cracks or crevices in rocks, evaporate, and form salt crystals. The growth of crystals exerts a pressure as in the freeze-thaw process, forcing cracks open and breaking up the rock.