The Earth’s external forces

Denudation, weathering, mass wasting and erosion Weathering is the breakdown of rocks. Mass wasting is movement of weathered material down a slope. Erosion is removal of material, for example by rivers or the sea. Eroded material is transported by rivers or the sea, and deposited in a new location. Erosion, transport and deposition make up the process of denudation.

Denudation, weathering, mass wasting and erosion Landforms are altered by weathering, mass movement, erosion, transport and deposition. This makes up the process of denudation. Erosion may be by running water, the sea, wind, or moving ice (glaciers). Constructional landforms such as volcanoes are not the result of denudation.

Weathering and the regolith Weathering breaks down rocks at or near the surface. The layer of weathered and broken rock fragments at the surface is the regolith. At the base of the regolith is the weathering front. Beneath the weathering front is unweathered rock.

Types of weathering Physical weathering Chemical weathering Frost action breaks off angular fragments when water freezes. Pressure release creates joints and cracks when the weight of overlying rocks is removed. Solution: minerals such as rock salt are dissolved. Hydrolysis: minerals such as feldspar react with water. Oxidation: minerals such as iron react with oxygen. Carbonation: limestone is weathered by rainwater.

Types of weathering Biotic weathering Carbonation Tree roots or burrowing animals break up rocks. Dead plants and animals produce acids which aid chemical erosion. Algae and molluscs break up coastal limestone at the water line. Carbonation is an important form of chemical weathering. Rainwater and CO2 produce a weak carbonic acid. This reacts with the calcium carbonate in limestone. The reaction produces calcium bicarbonate, which is soluble and easily removed by water.

Factors which influence weathering Climate Rock type Frost shattering operates in cold climates. Most chemical weathering processes are faster in a region with a warm, moist climate such as the Caribbean. Biotic weathering is effective in warm, moist climates where plants grow rapidly. Chemical and physical weathering are faster in rocks with joints and fissures. Limestone contains calcium carbonate, which is weathered by carbonation. Rocks containing feldspar are weathered by hydrolysis. Rocks with minerals such as quartz resist erosion.

What weathering processes are at work here?

Igneous, sedimentary and metamorphic rocks

Mass wasting: soil creep Soil or unconsolidated rock moves downslope. Slow movement: around 1 mm. a year Most effective when: Rainfall is high. Slopes are steep. The regolith is thick.

Mass wasting: landslides Can be slow or very rapid. Landslides are most likely when there is: a steep slope a thick layer of sand, clay or volcanic ash wet, slippery rock such as clay or shale undercutting at the base of a slope by a river, wave erosion, or a road cutting.