2B.6A: Weathering
Weathering (mechanical, chemical, biological) is important in sediment production and influences rates of recession.
Weathering is the breakdown of rock in situ at or near the surface of the Earth.
Physical (mechanical) weathering
Mechanical weathering is the application of force to physically fragment rock into smaller pieces called clasts. It breaks down rocks by the exertion of a physical force, and does not involve any chemical change.
Some examples of it are:
Chemical Weathering
Where chemical reactions attack individual minerals in the rock, breaking bonds and producing new chemical compounds.
Three examples are:
Biological weathering
Biological weathering is the break down of rock in situ by living or once-living organisms. It often speeds up mechanical or chemical weathering through the actions of plants, bacteria or animals.
Examples are:
Weathering is the breakdown of rock in situ at or near the surface of the Earth.
- Weathering and mass movement are subaerial processes.
- Weathering attacks the backshore and foreshore parts of the littoral zone.
- Weathering creates rock fragments that form sediment.
- It's most active in the source zone of the sediment cell.
Physical (mechanical) weathering
Mechanical weathering is the application of force to physically fragment rock into smaller pieces called clasts. It breaks down rocks by the exertion of a physical force, and does not involve any chemical change.
Some examples of it are:
- Freeze-thaw weathering
- Water seeps into cracks in rocks
- When the water freezes, it expands in volume by 9%, exerting a tensional force that widens the rock
- Thawing allows more water to enter the crack, and the process repeats until cracks are forced open large, angular pebble, cobble, or boulder-sized fragments are loosened off.
- With porous rocks, water in the pores may freeze, prising off individual rock grains and producing sand sized fragments.
- Any rocks with cracks and fissures are vulnerable to it, especially high on cliffs away from sea spray.
- Freezing is relatively uncommon on UK coasts.
- Salt Crystal Growth
- This is common at coasts because the sea is salty.
- The breaking force is less than that of freeze-thaw weathering
- Porous and fractured rocks (e.g. sandstone) are vulnerable to it.
- The effect is greater in hot and dry climates, promoting the evaporation and precipitation of salt crystalls.
- It attacks the foreshore zone and backshore zone that's reached by destructive wave spray.
- Seawater penetrates small cracks or pores in rock at high tide, and evaporates at low tide, leaving precipitated salt crystals.
- Repeated tidal cycles lead to growth of salt crystals until they begin to press against the sides of cracks or pores, exerting tensional pressure.
- Eventually angular fragments of rock are broken off or there is granular disintegration (where individual grains break away)
- Wetting and Drying
- Rocks containing clay minerals, such as clays and shales
- At high tide minerals on the rock surface are soaked with sea water and expand in volume.
- At low tide, minerals dry and shrink.
- Repeated cycles of expansion and contraction eventually cause the rock to fragment and crumble.
- Rocks containing clay minerals, such as clays and shales
Chemical Weathering
Where chemical reactions attack individual minerals in the rock, breaking bonds and producing new chemical compounds.
Three examples are:
- Carbonation
- This attacks calcium carbonate in limestones, other carbonate rocks and sedimentary rocks with calcite sediment.
- Rainwater mixed with carbon dioxide from the air to form weak carbonic acid (pH 5.6).
- The acidic rain mixes with calcium carbonate to form soluble calcium bicarbonate solution.
- 'Rock disappears' as new minerals dissolve into the solution.
- Only sediment left from limestone is clay particles that had formed impurities in the original rock.
- Where calcite sediment is weathered, previously cemented clasts are released to form sediment.
- This attacks calcium carbonate in limestones, other carbonate rocks and sedimentary rocks with calcite sediment.
- Hydrolysis
- The breakdown of minerals to form new clay minerals, plus materials in solution, due to the effect of water and dissolved carbon dioxide.
- Rocks vulnerable to it are: igneous and metamorphic rocks containing feldspar and other silicate minerals
- This attacks the feldspar (pinky) minerals in granite
- H+ ions in water attack the feldspar minerals forming kaolin clay cement
- Bonds between feldspar and quartz minerals broken, releasing quartz grains as sand sediment.
- Oxidation
- The addition of oxygen to minerals, especially iron compounds, which produces iron oxides and increases volume, contributing to mechanical breakdown.
- This attacks iron minerals in haematite cements, e.g. Devonian sandstone
- Wetting of haematite cement with seawater leads to a loss of an electron by iron, converting iron (II) oxide to red-coloured iron (III) oxide
- Change in iron compound breaks cement bonds releasing previously cemented clasts as sediment
- Rocks that are vulnerable to it are: sandstones, siltstones and shales that often contain iron compounds which can be oxidised
- It's much more effective in seawater or water with impurities than in pure water.
Biological weathering
Biological weathering is the break down of rock in situ by living or once-living organisms. It often speeds up mechanical or chemical weathering through the actions of plants, bacteria or animals.
Examples are:
- Tree root weathing
- seeds falling into cracks in rocks can germinate, nourished by rainwater and nutrients from wind-blown sediment.
- as the plant grows its roots expand and thicken
- tree roots exert sufficient tensional force to widen the crack
- eventually angular fragments of rock break away as cobble or boulder-sized sediment
- it occurs in the backshore zone, away from the reach of the spray from destructive waves
- it's an important process on vegetated cliff tops which can contribute to rock falls
2. Rock boring (there are many species of clams and mollusc that bore into rock, and may also secrete chemicals that dissolve rocks.
3. Seaweed acid
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Back to Weathering
- Weathering increases rate of recession
- It weakens rocks, making them more vulnerable to mass movement and cliff retreat
- Rocks weathered in the foreshore zone (between the high and low tide mark) are more rapidly eroded by marine erosion or hydraulic action
- For example, in some more vulnerable strataa wave-cut notch forms and deepens more rapidly in weathered rock, leading to faster recession through undercutting and mass movement collapse
- It weakens rocks, making them more vulnerable to mass movement and cliff retreat
- Rates of weathering are very slow
- Even in a hot, wet climate, basalt (igneous rock) weathers at a rate of 1-2 mm every 1000 years.
- A hot, wet climate encourages chemical and biological weathering
- Carbonation increases in winter because calcium bicarbonate is more soluble in cold conditions.