WEATHERING OF ROCKS - EROSION
Doc Brown's Chemistry - Earth Science & Geology Revision Notes
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INDEX OF EARTH SCIENCE PAGES
GCSE/IGCSE level chemistry revision
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3. Weathering of Rocks - Chemical and Physical Erosion
The different ways by which rocks are weathered
and eroded are discussed e.g. physical weathering by wind, rain, waves, ice,
heat from the sun, chemical weathering by acidified run off water and acid rain
and biological weathering by plants-roots descriptions etc. Examples of rock
formation erosion and associated processes of transportation and deposition are
3. Weathering of Rocks
all wear away eventually!
ALL rock formation, whether they
be igneous, metamorphic or sedimentary, do weather and erode away by one means
or other (chemical or physical), but the rate of weathering or erosion can vary
considerably from rock type to rock type.
(3a) Weathering of Rocks
the process of breaking them up into smaller fragments or even dissolving away and it can occur in many different ways.
physical weathering examples
- Liquid water from rain or melted
snow/ice runs into rock cracks. When the temperature falls below 0oC,
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.
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.
examples (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
+ CO2(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.
examples e.g. the action
and pressure of growing plant roots expanding in the cracks of rocks.
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.
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
- 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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