Download - Chemical and biological weathering lesson 5
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CHEMICAL WEATHERIN
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Chemical Weathering
Decomposition of rock in situ resulting from chemical change
Water (rain, fog, moisture, ocean etc) is the key medium for chemical weathering.
Common in warm humid climate (high temperature & high rainfall)
Weathered rocks by this process have a dull pitted appearance
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Five main processes of chemical weathering
1. Oxidation
2. Hydration
3. Hydrolysis
4. Carbonation
5. Solution
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Oxidation Oxygen dissolved in
water reacts with mineral iron in the rock
The rock is converted to iron oxide
Always seen as reddish-brown crust
This conversion weakened the mineral bonding and consequently the whole structure of the rock
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Hydration
Involves in the expansion of minerals resulting from the absorption of water
E.g. Clay minerals are capable of absorbing large quantities of water into their crystal structure
When this happen, they exerts stresses within the rock which may eventually cause it to break apart
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Hydrolysis
Involves a chemical reaction
between hydrogen ions in water with certain minerals in the rock
E.g. Weakly acidic rainwater causes feldspar to be converted into white powdery clay (kaolin) which breaks down easily
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Hydrolysis
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Kaolinite mine
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Carbonation Is a form of solution weathering
Common in rocks composed of calcium carbonate such as limestone and chalk
Rainwater mixed with carbon dioxide form carbonic acid
The acid water change the calcium carbonate to soluble calcium bicarbonate which is then easily washed away
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Solution Simply the
dissolving minerals in water
E.g Halite (rock salt) dissolve in water
Quartz dissolves at an extremely slow rate
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Spheroidical Weathering
A type of chemical weathering
Creates rounded boulders and domed monolith
Typically granitic in composition
Rock forms at great temperatures and pressures
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Spheroidal weathering
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BIOLOGICAL WEATHERING
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Biological Weathering
Caused by the action living organisms
Can lead to physical break down of rock (Biophysical)
Also chemical attack (Biochemical weathering)
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Biophysical Weathering
1. By plant roots
2. By animals such as rodents and earthworms
3. By human activities
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Biophysical
By plant roots:
When roots penetrate into cracks, they will usually break rocks apart
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By animals:
Help to widen and loosen the joints in rocks and expose underlying rocks to weathering processes as they burrow through the ground
Biophysical
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By human activities:
Clearing of land for cultivation
Construction and mining activities
This activities break up rocks and expose them to the weathering processes
Biophysical
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Biochemical
Organic acids from dead plants and root can help to decompose rock
Chelation is the effect of organic acid on rock
Waste and organic acids from dead animals help to weather rocks chemically
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Human activities:
Industrialisation can cause pollution such emission of sulphur dioxide which can produce acid rain
Accelerate the chemical weathering process of hydrolysis, carbonation and solution
Biochemical