Doc Brown's Chemistry - Earth Science Revision Notes

3. Weathering of Rocks - Chemical and Physical Erosion

Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

based on a GCSE Geological & Earth Science TASK SHEET * Earth Science Homepage * 5 multi-word fill GCSE worksheets + answers * GCSE Earth Science Quiz: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz

ANSWERS-NOTES 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. The Moon and Planets

3. Weathering of Rocks they all wear away eventually!

(3a) Weathering of Rocks means the process of breaking them up into smaller fragments and it can occur in many different ways.

• (i) physical weathering e.g.
  • Liquid water from rain or melted snow/ice runs into rock cracks. When the temperature falls below 0^oC, the water freezes and there is an expansion in volume in changing from water to ice. The resulting ice pressure cracks the rocks apart and the process can be repeated if the ice melts/thaws and re-freezes etc. So, many mountain sides have a very shattered appearance!
  • The continuous battering of rock surfaces with dust particles carried by the wind,
  • River water carrying rocks and battering into other rocks, which is why down a river the rocks/pebbles tend to become smaller and rounder.
  • Sea waves crashing on the seashore and against cliffs etc.
  • Layers of rocks flaking off when larger rocks expand and contract with extreme temperature changes.
  • The action of glaciers grinds rock material off the land and sides of valleys with tremendous power.
  • The physically weathered 'High Bridestones' in North Yorkshire, Northern England.
• (ii) chemical weathering (a sort of 'corrosion' of rocks) e.g.
  • Acid rain water will gradually break up even igneous rocks by slow chemical reactions. Some of the minerals dissolve and free up particles of the insoluble material. Even hard igneous rocks get weathered away eventually over millions of years to form sand grains. Rain is 'naturally' acid from - dissolved carbon dioxide from respiration and forest fires, nitrogen oxides from lightning flashes and sulphur dioxide from volcanoes. Pollution increases the acidity of air with extra nitrogen and sulphur oxides from fossil fuel burning.
  • Water running off from decayed and oxidised plant material is acidic e.g. peat water has a pH of 3.5.
  • Limestone rock (calcium carbonate) chemically dissolves away much quicker than most other rocks, even with just carbon dioxide in the water,
    • e.g. calcium carbonate + carbon dioxide + water ==> calcium hydrogencarbonate
    • CaCO_3(s) + CO_2(aq) + H₂O_(l) ==> Ca(HCO₃)_2(aq)
    • and the process is much faster in polluted acid rain, hence the rather worn appearance of medieval buildings in industrialised Europe made from the 'softer' sedimentary rocks limestone or sandstone.
• (iii) biological weathering e.g. the action and pressure of growing plant roots expanding in the cracks of rocks.

(3b) Erosion is the wearing away of the rock as a result of the weathering processes described above. Examples of erosion are the wearing away of mountains and the creation of river valleys and gorges.

• Transportation is the process by which the eroded weathered rock fragments are moved away from the erosion area.
  • This happens mainly due to 'falling under gravity' and then the rocks or fragments carried away by stream and river water as well as sand by wind.
  • There are powerful currents in the sea which transport huge masses of eroded material.
  • You find in rivers that as you go from high mountains to an estuary, the 'rocks' become smaller and more rounded the further they have travelled due to the constant collisions in the water chipping of the edges.
  • Glaciers also carry considerable eroded material away, particularly in the 'ice ages'
• Deposition will eventually occur giving rise to sediments or sand dunes in river delta's sea etc. The smaller (lighter) the particles, and the faster the current, the further they are carried. This means most deposition will occur in a slow moving but distant location e.g. fine silt deposits in estuaries.

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