Weathering is the breakdown and alteration of rocks in their original position (in situ) without being moved. It's one of the key processes that shapes our landscapes and creates the raw materials for soil formation. Unlike erosion, weathering doesn't involve the movement of rock material from one place to another.
Key Definitions:
Weathering is the first step in the rock cycle and soil formation. Without weathering, we wouldn't have the fertile soils that support agriculture, the varied landscapes we see around us, or many of the natural resources we depend on. Understanding weathering helps us comprehend how landscapes form and change over time.
There are three main types of weathering processes that break down rocks in different ways. Let's explore each one in detail.
Mechanical weathering breaks rocks into smaller pieces without changing their chemical composition. Think of it as breaking a biscuit into crumbs - the pieces are still made of the same stuff, just smaller.
Water seeps into cracks in rocks. When it freezes, it expands by about 9%, creating pressure. Repeated freezing and thawing widens cracks until pieces break off.
Example: Common in mountainous regions like the Alps and Scottish Highlands.
Rocks heat up during the day and expand, then cool and contract at night. This repeated stress causes the outer layers to crack and flake off.
Example: Particularly effective in hot deserts with large daily temperature ranges.
Salt water enters rock cracks and evaporates, leaving salt crystals that grow and exert pressure, breaking the rock apart.
Example: Common in coastal areas and deserts with high evaporation rates.
When overlying rock is removed (by erosion or human activity), the pressure on deeper rocks decreases. This causes the rock to expand upward and develop cracks parallel to the surface, creating an 'onion skin' effect where layers peel off.
Example: Half Dome in Yosemite National Park, USA shows dramatic exfoliation features.
When rock particles carried by wind, water, or ice scrape against rock surfaces, wearing them down. This is technically a form of erosion but is often included in discussions of physical weathering.
Example: Wind-blown sand creating mushroom rocks in desert environments.
Chemical weathering changes the mineral composition of rocks through chemical reactions. Unlike mechanical weathering, the rock doesn't just break into smaller pieces - its chemical makeup actually changes, often making it weaker or more soluble.
Carbon dioxide from the air dissolves in rainwater to form weak carbonic acid. This reacts with minerals in rocks (especially limestone) to form soluble compounds that are washed away.
Equation: H₂O + CO₂ → H₂CO₃ (carbonic acid)
Example: Limestone pavements in the Yorkshire Dales, UK.
Water molecules react with minerals, breaking them down into new compounds. Common in feldspar minerals in granite, which break down to form clay minerals.
Example: Granite weathering to form clay-rich soils in Cornwall, UK.
Oxygen in the air or water reacts with minerals containing iron, causing them to 'rust' and weaken. This often gives rocks a reddish-brown colour.
Example: The red colour of many soils and rocks in tropical regions.
Some minerals dissolve directly in water. For example, rock salt (halite) dissolves easily, while limestone dissolves slowly in acidic water. This process creates caves, sinkholes and other karst features in limestone areas.
Example: The extensive cave systems in the Peak District, UK.
Air pollution adds sulphur dioxide and nitrogen oxides to the atmosphere, which combine with water to form stronger acids than natural carbonic acid. These accelerate chemical weathering, particularly of limestone buildings and statues.
Example: Damage to historic buildings in industrial cities.
Biological weathering occurs when living organisms contribute to the breakdown of rocks. This can involve both mechanical and chemical processes.
Plant roots grow into cracks in rocks and expand, widening the cracks (mechanical). Roots also release organic acids that can dissolve minerals (chemical). When plants die and decompose, they release humic acids that further weather rocks.
Example: Tree roots breaking up pavements or growing through rock crevices.
Burrowing animals like worms, rabbits and moles break up rock and soil particles. Larger animals can dislodge rocks when walking or digging. Even tiny organisms like lichens and bacteria can break down rock surfaces by secreting acids.
Example: Lichens growing on rock surfaces, gradually breaking down the outer layer.
The speed and intensity of weathering processes depend on several key factors:
Temperature and rainfall strongly influence weathering rates. Warm, wet climates generally promote rapid chemical weathering, while areas with freezing temperatures experience more freeze-thaw weathering.
Different rocks weather at different rates. Limestone is vulnerable to chemical weathering, while granite is more resistant but susceptible to physical weathering along joints.
Weathering is generally a slow process that occurs over hundreds or thousands of years, though some processes can happen more quickly in the right conditions.
Hot Desert (Sahara): Physical weathering dominates due to large daily temperature ranges causing thermal expansion. Salt weathering is also significant where there is occasional moisture. Chemical weathering is limited by lack of water.
Tropical Rainforest (Amazon): Chemical and biological weathering dominate due to high temperatures and abundant rainfall. Thick vegetation contributes to biological weathering. Weathering can extend deep below the surface, creating thick soils.
Polar Region (Antarctica): Freeze-thaw weathering is common where temperatures fluctuate around freezing point. Chemical weathering is slow due to low temperatures and limited liquid water.
Temperate Maritime (UK): Moderate rates of both chemical and physical weathering. Freeze-thaw occurs in winter, while chemical processes continue year-round due to regular rainfall.
Weathering works together with erosion, transportation and deposition to shape landscapes. The weathered material (regolith) may remain in place to form soil or be carried away by erosion to be deposited elsewhere.
Different weathering processes create distinctive landscape features:
The granite tors of Dartmoor in southwest England are excellent examples of differential weathering. The granite was originally jointed (cracked) due to cooling and pressure release. Weathering attacked these joints, gradually rounding the blocks between them. Over millions of years, the weathered material was eroded away, leaving behind the more resistant core stones as the distinctive tors we see today.
Famous examples include Haytor and Hound Tor, which rise dramatically from the surrounding moorland.
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