Paper 1 Physical Core Geography

Cambridge International A and AS Level Geography © Hodder Education 20111

3.2 Weathering and rocks

Key termsBiological weathering is the form of weathering caused by the activities of living organisms. There are two types: chemical and mechanical (physical). Chemical weathering occurs with the release of organic acids as vegetation decays.

Carboniferous limestone (karst) is formed of the remains of organic matter, mostly plants and shells. Limestone scenery is unique because it:

● is very hard but also permeable, and ● dissolves very slowly in acid water.

Chemical weathering is the form of weathering brought about by chemical attack of rocks, usually in the presence of water. Chemical weathering involves the ‘rotting’ or breakdown of the original minerals within a rock to produce new minerals. Some chemicals are dissolved and carried away from the weathering source.

Freeze–thaw is the form of physical weathering, common in mountains and glacial environments, caused by the expansion of water as it freezes. Water in a crack freezes and expands in volume by 9–10 per cent as it turns to ice. This expansion exerts great pressure on the rock, causing the crack to enlarge. After many cycles of freeze–thaw, rock fragments may break off to form scree slopes.

Granite is coarse-grained, intrusive, igneous rock, typically consisting of the minerals quartz, feldspar and mica.

Joint is a vertical crack in a rock, often formed by compression; it is usually several metres in length. The weathering of joints in rocks such as limestone and granite is responsible for the formation of features such as swallow holes and tors.

Karst is the characteristic scenery of a carboniferous limestone region with underground streams, caverns and potholes forming a drainage system.

Mechanical (physical weathering) includes such processes as freeze–thaw and exfoliation or onion-skin weathering. Mechanical weathering leads to the breakdown of rocks but does not change the composition of the rocks.

Scree is angular sediment that collects at the foot of a mountain range or cliff. The rock fragments that form scree are usually broken off by the action of freeze–thaw weathering.

Solution occurs on rocks such as chalk and limestone that contain calcium carbonate. When dissolved in rain, carbon dioxide forms weak carbonic acid. This acid changes the limestone to calcium bicarbonate, which is soluble, and can be carried away by fl owing water.

Tor is an isolated mass of rock, often granite, left upstanding on a hilltop after the surrounding rock has been broken down and removed. Weathering takes place along the joints in the rock, reducing the outcrop into a mass of unweathered stones surrounded by weathered material.

Weathering is the process by which exposed rocks are broken down on the spot 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 recognised: physical (or mechanical) and chemical. They usually occur together.

Topic summary ● Weathering is the decomposition and disintegration of rocks

in situ. ● Decomposition refers to chemical weathering and creates altered rock substances.

● Disintegration or mechanical weathering produces smaller, angular fragments of the same rock, such as scree.

● There are four main types of mechanical weathering: freeze–thaw (ice crystal growth), salt crystal growth, disintegration and pressure release.

● There are four main types of chemical weathering: carbonation-solution, hydrolysis, hydration and oxidation.

● The type and rate of weathering varies with climate. ● Rock type and rock structure influence the rate and type of weathering in many ways due to chemical composition, the nature of cements in sedimentary rock and joints and bedding planes.

● Limestone scenery is unique on account of its permeability and solubility in rain and groundwater.

● The main processes to affect limestone are carbonation-solution, freeze–thaw, and river erosion underground.

● Limestone features include clints and grikes, limestone pavements and caves, stalagmites and stalactites.

● Granite is an igneous, crystalline rock. It has great physical strength and is very resistant to erosion.

● Tors, a characteristic feature of granite, may be the result of equifinality.

Additional work1 Outline the importance of weathering.2 ‘Climate is the most important factor affecting weathering.’

Discuss.3 Examine the influence of rock type on weathering.4 To what extent do geomorphic processes vary with climatic

regions?

Paper 1 Physical Core Geography

Cambridge International A and AS Level Geography © Hodder Education 20112

Suggested websites ● Fundamentals of Physical Geography:

www.physicalgeography.net/fundamentals/10r.html ● How you can use gravestones to find out about weathering:

www.es.ucl.ac.uk/schools/Pancras/intro_weathering.htm ● Visit National Geographic for some stunning images of erosion and weathering: http://science.nationalgeographic.com/science/photos/weathering-erosion-gallery/#/nevada-snake-range_309_600x450.jpg

● A cartoon of weathering and erosion:www.youtube.com/watch?v=lyysL02ZvQ8