Weathering: the disintegration and decomposition of rocks in situ

Types of weathering

Physical (mechanical) weathering is the disintegration and decomposition of rocks due to changes in temperature, moisture or pressure.

Chemical weathering is the disintegration and decomposition of rocks due to chemical reactions with substances in the rocks.

Biological weathering is the disintegration and decomposition of rocks by living organisms. Biological weathering can be biophysical (eg. roots exerting pressure on rock) or biochemical (eg. secreting chemicals that wear away the rocks)

Weathering processes

Freeze-thaw action

Potholes in Heol y graig, Cwmgwrach
Potholes in Heol y graig, Cwmgwrach (Jaggery) / CC BY-Sa 2.0)

Freeze-thaw action (frost shattering) occurs mainly in alpine regions (cold climate) where diurnal temperature variation is high. During the day, as temperatures are above 0°C, water runs into the cracks of a cliff. As the temperature falls below 0°C, water freezes (becomes ice) and expands by around 10%, exerting a pressure of up to 2100 kg/cm² on the rock wall. Rocks can withstand only pressures of up to 500kg/cm² so they are likely to break, especially, as the alternating process of freezing and cooling exerts varying levels of pressure, thus straining the rock. Eventually, cracks will be widened and parts of the cliff will break off, possibly accumulating as scree that might form a talus cone.

Exfoliation

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Exfoliation dome at Yosemite National Park

Exfoliation occurs in areas with a warm climate and high diurnal temperature variation, where a bare rock surface can be heated and cooled. During daytime, the outer layers expand more rapidly than the inner layers due to heat energy from the sun. At night, the outer layers contract more rapidly than the inner layers, so this exerts pressure on the rock wall, and eventually the outer layers peel off like an onion, leaving behind exfoliation domes.

Carbonation

Carbonation is most common in areas with soluble rock eg. limestone. Rainwater reacts with carbon dioxide to produce carbonic acid, which dissolves the limestone (as calcium carbonate reacts with acidic water). Limestone is washed away in the solution. Movement of water through joints and bedding planes speeds up the process.

Oxidation

Oxidation is when minerals such as iron react with oxygen in the atmosphere and produce a brownish crust (iron oxide), decomposing the entire rock.

Biological weathering

Biological weathering can occur when seeds fall into cracks in rocks and a plant begins to germinate. Its roots exert stress on the cracks and eventually break the rock apart, aided by organic acids that react with the rock.

Rate of weathering

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Weathering is more rapid in humid, tropical regions than in temperate regions. This is because tropical areas receive more rainfall, which encourages chemical weathering such as carbonation. Also, tropical areas are warmer, and the rate of chemical weathering doubles with every rise of 10°C. Besides, tropical areas have more plant growth, and consequentially more biological weathering, for instance by tree roots in cracks, and by the more rapid release of CO2 and humic acids from decaying plant matter.

Weathering is also faster if the rock is soluble and if it has many lines of weakness. Furthermore, mineral composition is important: rocks from limestone are susceptible to carbonation, rocks from iron are prone to oxidation, but quartz does not weather chemically.

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