GENERAL GEOLOGY

What is Weathering and Its Types.

Weathering is the set of all processes that decay and break up bedrock by a combination of physically fracturing or chemical decomposition.

Weathered outcrop.

There are three types of weather:
1. Physical 
2. Chemical 
3. Biological 

1.Physical  Weathering:-

This process refers to the mechanical disintegration of rock in which their mineralogical composition is not changed. Some of the important processes of physical weathering are:-
• Exfoliation:-in this case thin sheets of rocks split off due to differential expansion and contraction during heating and cooling over the diurnal temperature range.
• Crystal growth:-the soluble constituents of the rocks or minerals enter the rocks through fractures and joints along with water with the evaporation of water the solution is precipitated to form crystal or crystalline aggregates and as they grow they exert large expansive stress which helps in breaking up some rocks.
• Freezing of water:-when water freezes it expands of its volume. In nature, water enters into cracks of rocks. Upon freezing it expands and exerts great pressure on the walls of cracks. As a result, the rocks break into pieces. This process called frost wedging.
• Differential expansion:-rock forming minerals expand when heated but contract when cooled where rocks surface are exposed daily to intense cooling by longwave radiation at night the resulting expansion and contraction of mineral grains tend to break them apart.

2.Chemical Weathering:

Chemical weathering or decomposition is a process in which rocks are broken down by the chemical decay of minerals. During chemical weathering, a set of chemical reactions act on rocks which change their minerals to more stable form. manly three processes are notably responsible for chemical weathering are
• Hydration:- the term hydration refers to the chemical union of water with a mineral. In this processes, certain minerals take up water which leads to changes in the minerals composition rocks. Due to the absorption of water, the minerals expand causing more stresses within the rock.
• Example:-the altering of anhydrite to gypsum and hematite to limonite as the show

CaSo4 +2H2O—– CaSo4.2H2O
Andhdrite + water —– gypsum

Fe2O3 + nH2O —– Fe2O3.2H2O
Hematite + water —– limonite
• Oxidation:- the presence of dissolved oxygen in water in contact with mineral surface leads to oxidation which is the chemical union of oxygen atoms with atoms of other metallic elements. oxygen has a particular affinity for iron composition and these are among the most commonly oxidized materials
• Example:- oxidation of pyrite
FeS2 + nO2 +mH2O——->FeSO4——>Fe2(So4)3—–>Fe2O3.nH2O
Pyrite+ oxygen+ water—–>ferrous sulphate——->ferric sulphate—–>limonite
• Carbonation:-carbon dioxide dissolves in water to form carbonic acid. Carbonic acid is an effective weathering agent. Granite is the most abundant continental rock under the influence of carbonic acid it weathers into the clay.
• Example:-
H2O+ CO2——->H2CO3
Water+ carbon dioxide——->carbonic acid
2KAlSi3O8 + H2CO3 + H2O——->K2CO3 +Al2Si2O5 (OH)4 + 4Sio2
Orthoclase + carbonic acid+ water—–>potassium carbonate+ kaolinite+ silica

3.Biological Weathering:-

the processes of weathering which are mainly related to the activities of plants, animals, and organisms like bacteria etc. Are known as biological weathering.
(1) bio-physical processes  (2) biochemical processes
Factors affecting weathering:- A number of natural factors affect weathering in various ways and control the degree of weathering to a major extent. Some of the major factors that affect weathering is as follows
• climate
• topography
• structure, texture, and minerals composition of rocks
• vegetation cover
• time
Agents of weathering:-the principle agents of transformation of rock in the zone of weathering are water, wind, gases, like oxygen, carbon dioxide etc. Acid, gravity, a variation of temperature, organisms etc.

ALSO, SEE EROSION

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