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Summary 3.4 Weathering and Mass movement AQA Physical Geography
Key notes on Unit 3.4 Weathering and Mass Movement in AQA Physical Geography A level. Includes key definitions etc and provided a final A* grade.
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3.4 – Weathering and Mass MovementSub-aerial weathering: the gradual break down of rock by agents such as ice, salt, plants
roots and acid.
Weathering: the breakdown or disintegration of rock in situ, at or close to the ground
surface.
3 types of weathering:
1. Mechanical (physical) – breakdown of rock without any chemical changes
Freeze-thaw: water enters crack or joint in rock when it rains then freezes when
temperature drops. Expansion exerts pressure on rock forcing crack to widen.
Freeze-thaw repeated eventually breaking rock (water expands by 10% when it
freezes).
Salt crystallisation: when saline water evaporates it leaves salt crystals behind. These
grow over time and exert pressure on rock.
Wetting and drying: some rocks contain clay when clay gets wet it expands and
the pressure caused breaks fragments off.
2. Biological – breakdown of rock by organic activity
Plant roots grow into small cracks in cliff. Cracks widen as roots grow, breaks rock.
Water running through decaying vegetation becomes acidic chemical weathering
Birds and marine organisms dig burrows in cliffs.
3. Chemical – breakdown of rock by changing its chemical composition
Carbonation: rainwater absorbs C02 from air to form weak carbonic acid, reacts with
calcium carbonate in rocks to form calcium bicarbonate easily dissolved
Oxidation: reaction of rock materials with oxygen (like iron) forming rusty red
powder leaving rocks more vulnerable to weathering.
Solution: the dissolving of rock materials.
- If the rate of debris removal exceeds the rate of weathering and mass movement then a
positive feedback may operate, as the rate of weathering and mass movement could
increase.
- If debris removal is slow and ineffective, this will lead to a build-up of talus (scree) that
reduces the exposure of cliff face as it extends up the cliff face. Negative feedback as
weathering and mass movement rates will decrease.
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