WEATHERING OF ROCKS
Weathering is the gradual process whereby rocks are broken down by the action of external factors or agents such as temperature, wind, water, pressure, roots of plants, oxygen and carbonic acid.
TYPES OF WEATHERING
There are three main types of weathering:
1. physical/mechanical weathering 2. Chemical weathering 3. Biological weathering
PHYSICAL WEATHERING
This is the physical or mechanical breakdown of rocks into smaller particles without any change in the chemical composition of the minerals in the rocks.
Causes of physical weathering of rocks:
i) Temperature: Changes in temperature causes rocks to expand (with heat) and contract (with cold). As this happens over and over again, the structure of the rock weakens because different minerals in rocks expand and contract at different rates. Over time, the cracks and breakdown.
ii) Water / river or tidal action: Moving water such river or stream carry rock particles along its course. These particles hit against each other causing them to break.
water can also move into rocks and expand (ice) when temperatures drop to freezing points. This causes rocks to break.
iii) Wind: Strong winds carry tiny rocks and particles which have abrasive action on the surface of exposed rock causing them to break down.
iv) Living organism (animals and plants): The action of man (farming and walking), animals (creating hiding places in soil), and plant roots getting into cracks and growing bigger can cause rocks to break down.
v) Geological processes: Earthquakes and other earth movements cause rocks to break down into small particles
vi) Growth of salt crystals in rock crevices: In arid regions salt crystals may grow within cervices of rocks to set up pressures that assist in the breakdown of such rocks.
CHEMICAL WEATHERING
This is the breakdown of rocks into smaller particles such that there is a change in the chemical composition of the mineral constituents. It occurs through the following chemical processes.
i) Hydration: this involves the rigid attachment of water to rock minerals making them soft and porous and containing water of crystallization. This produces new compounds. Examples
a) 2FeO2 + 3H2O 2FeO2 3H2O
b) Al2O3 + 3H2O Al2O3 3H2O
ii) Hydrolysis: This is the process whereby water reacts with minerals in rocks. The metallic cation of some compounds is replaced by hydrogen ions from water resulting in disintegration of the rocks.
a) 3MgFeSiO4 + 2H2O H4Mg3Si2O9 + 3FeO + SiO2
b) CaSiO3 + H2O H2SiO3 + Ca(OH)2
c) H4Al2Si2O9 + 5H2O 2Al(OH)3 + 2H4SiO4
iii) Oxidation: In this process, oxygen (from atmosphere and that contained rain water) combines with rock minerals to produce different compounds which dissolves easily. For example, when iron (II) oxide combines with O2, it becomes iron (III) oxide which breaks easily. 2FeO + 1/2O2 2 FeO3
iv) Carbonation : The decomposition of organic matter in the soil and respiration by plant roots both releases carbon dioxide into the soil. The carbon dioxide reacts with certain rock minerals to produce carbonates and bicarbontes which are relatively more soluble and therefore easily carried away.
a) K2Al2Si6O16 + 2H2O + CO2 H4Al2Si2O9 +K2CO3 + 4SiO2
In addition, atmospheric carbon dioxide dissolves in water to form weak carbonic acids which attacks and dissolves rock minerals such as calcite and dolomite.
a) KOH + CO2 K2CO3 + H2O
b) CO2 + H2O H2CO3
v) Reduction : This is the process in which oxygen is removed from some rock minerals due oxygen diffiency, thereby resulting in the disintegration of rocks. Eg Fe3+ Fe2+
vi) Solution: In this reation,water on rock surfaces and crevices (narrow openings) dissolves dissolves soluble rock minerals causing the rock to breakdown . The most suitable minrals are the sulphates and chlorides and the least soluble is silica. NaCl + H2O NaOH + HCl
Water also forms carbonic acids which hepls to dissolve rocks. CO2 + H2O H2CO3
BIOLOGICAL WEATHERING
Biological weathering invoves the disintegration (breakdown) of rocks into smaller units due to the activities of living organisms such as man, animals, palnts and micro organisms.
Causes of bilogical weathering
1. Human activities such as farming, mining, building and road costruction break down rocks.
2. The passage of tree roots into cracks and cervices (narrow openings) and subsequent increase in size, widen the crevices and cause rocks to break.
3. The burrowing action and feeding habits of anials such as earthworms, termites, crabs and rats create spaces in soft rocks allowing air and water to enter which causes disintegration of rocks.
4. Respiring organisms in the soil produce carbon dioxide which reacts with soil water to produce carbonic acids that breaks down rocks.
5. Movement of living organisms such as man, elephants, horses, and cattle exerts pressure on rocks thereby weakening and breaking them down eventually.
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