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Summary Surface forces (geomorphology) - Grade 9
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Surface forces (geomorphology) for grade 9. There are spaces (blocks) for drawing diagrams.
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Surface forces that shape Earth- Earth’s surface = worn away by natural forces (wind, rain, frost, waves)
o shapes all of natural landforms.
- Humans also contribute to the shaping of Earth’s surface (building, mining, farming)
Weathering
- Weathering --> the wearing away of rocks and soil by the action of the weather.
- Three types:
o Physical weathering – frost and temperature
o Chemical weathering – chemicals weaken rock.
o Biological weathering – plants/animals
Physical weathering
- Involves physical processes (ice, water, and temperature change)
o Freeze thaw action
o Exfoliation
- Freeze thaw action
o Water collects in cracks in rocks.
o The water freezes from the top of the crack downwards
o Cannot expand up because top is frozen, so it expands sideways putting pressure on the
rock (ice = 9% bigger volume than water)
- Exfoliation (peeling away of layers of rock)
o Big changes in temperature between day and night cause the outer layer of the rock to
expand and contract.
o Weakens outer layer causing it to ‘peel-off’.
Chemical weathering
- Changes chemical composition of rocks ∴ weakening and breaking it.
- Different types:
o Carbonation
o Oxidation
o Hydrolysis
- Carbonation
o Water in atmosphere and CO2 come into contact and form carbonic acid (H2CO3)
o Acid comes into contact with rocks containing calcium carbonate (CaCO 3)
o Rain changes insoluble calcium carbonate into soluble calcium bicarbonate
▪ Dissolves in rainwater and washes away.
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, - Oxidation
o Occurs in rocks containing iron (rusting)
o Water contains O2 which reacts with iron to form iron oxide.
o Changes structure of rocks making it weaker ∴ it breaks down more easily.
- Hydrolysis
o Silicates in rocks absorb water and change into clay.
o Clay washes out of rock.
o Rock becomes unstable and may crumble.
Biological weathering
- Plants grow in cracks in rocks causing them to expand.
- Animals burrow and dig in soil ∴ exposing rocks and weakening the soil.
- Plants and animals contain chemicals that cause chemical weathering.
- Lichen (green/red) grows on rocks.
o Roots penetrate between grains in rock.
o Produces acid that breaks down minerals in rock.
Erosion and deposition
- Erosion --> wearing away of land through natural forces (movement)
- Deposition --> the depositing of materials created by weathering and erosion (on land)
Rivers
- River flows over the land, it carries weathered materials called a load (sand, stones, and silt)
o Load (sand and stones) bounce along the bottom of river wearing away the riverbed and
sides)
o Load piece sizes get smaller as journey continues as force of river wears away the rocks.
Upper course of river
- Waterfalls and rapids
o Waterfall is a sudden drop in a rivers gradient.
River flows over bands of The river undercuts the harder River forms a plunge pool below
harder and softer rock rock leaving an overhang whilst the waterfall. Overhang rocks
the sifter rock is more quickly eroded collapse into the plunge pool
o Rapids are where the water flows faster in a river’s course (without sudden drop in height
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