Weathering

Weathering is the breakdown of rocks in situ.

There are three types of weathering; physical, chemical and biological.

The rate that weathering occurs at will depend on the rock type, the climate and the relief.

Physical weathering

Physical weathering is also known as mechanical weathering. This type of weathering occurs when rocks are broken down by physical factors in the environment, such as wind, water, and temperature change. Importantly physical weathering does not change the chemical composition of the rock.

1. Frost wedging or freeze-thaw weathering primarily occurs in temperate humid climates. Water gets into cracks in the rock and at night, when temperatures fall the water can freeze. When water freezes it expands, by on average 9%, this puts pressure on the surrounding rock. This cycle will eventually put enough pressure on the rock to causes pieces to fall off.
2. Salt crystallisation occurs in semi-arid environments, where the evaporation of water from rock surfaces leads to the crystallisation of salts. Crystallisation leads to a dramatic increase in volume which exerts pressure on the surrounding rock, and can eventually fracture the rock.
3. Thermal expansion or insolation usually occurs in arid regions. During the day temperatures rise and heat the rock. As the rock heats up it expands. Then at night, when temperatures fall, the rock also cools and contracts. This cycle leads to exfoliation where the top layers of rock peel away.

Chemical Weathering

Chemical weathering is the breakdown of rocks because of the interaction of air, water or acid with the chemical composition of the rock.

• Oxidation occurs when oxygen reacts with minerals such as calcium and magnesium to form iron oxide. Iron oxide is reddish brown in colour and causes the decomposition of rock.
• Hydration occurs when water is absorbed by minerals in the rock, creating crystals which exert pressure on the surrounding rock, eventually causing it to crumble.
• Hydrolysis occurs whenwater reacts with minerals in the rock causing the chemical composition of the rock to change and become more unstable. This reaction makes the rock more susceptible to decomposition.
• Carbonation occurs when carbon dioxide from moisture in the air reacts with carbonate minerals found in rock. This creates carbonic acid which breaks down rock.
• Solution occurs because many minerals are soluble and are removed when they come into contact with water.

Biological Weathering

Biological weathering is the breakdown of rock by plants, animals and micro-organisms. For example plant roots can enlarge cracks in a rock as they grow and expand.