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Weathering Definition:
Weathering is the disintegration and decomposition of rocks on the earth's surface. An essential feature of the process is that it affects rocks in situ; no transport is involved.
The combined weathering process results in weakening, fragmentation (by Mechanical process) and decomposition (by Chemical process) of bedrock at and near the earth's surface.
In other words, weathering is a process by which rocks are broken down and decomposition by the action of external agencies such as wind, rain, temperature changes, plants and bacteria.
Characteristics:
1) Weathering is related to the weather and climate of a place.
2) It is a very slow process.
3) It is a static process.
4) Weathering occurs in situ.
Influencing factors:
a) Rock structure or nature of rock
b) Climatic condition
c) Topographic condition
d) Natural vegetation cover
Difference Between weathering and Erosion:
|
Subject |
Weathering |
Erosion |
|
Progress |
It is a very slow process so landforms changes slowly |
The very rapid process so Landforms changes rapidly. |
|
Concept |
The disintegration and decomposition of rocks on the earth's surface is known
as weathering. An essential feature of the process is that it affects rocks
in situ; no transport is involved. |
In Erosional process by which the rock fragments which are produced by
weathering are removed from their place of origin by dynamic forces of the
river, glacier, sea, waves, etc. is called erosion. |
|
Nature |
Static process |
Dynamic process |
|
Movement |
Weathering occurs in situ so transport is not involved. |
Weathered rock fragments are moved from one place to another. |
|
Influencing
Factors |
Elements of weather like temperature, rainfall, ice, moisture etc. |
Erosional forces and transporting forces like river, glacier, sea
waves etc. |
|
Process
|
Mainly three processes-Mechanical or physical, Chemical,
Biological |
Processes involve–Collision, Abrasion or Corrosion, Friction,
Solution |
Erosion:
Definition:- The process by which the rock fragments produced by weathering are removed from their place of origin by dynamic forces of the river, glacier, sea, waves, etc. is called erosion.
Characteristics:
1) Erosion is related to dynamic forces like rivers, glaciers, seas, wave works.
2) It is a very dynamic process.
3) Landforms changes rapidly.
4) Height of the landforms is reduced due to the removal of weathered materials.
5) Rock fragments are moved from one place to another.
- Classification of weathering:
A) Mechanical B) Chemical C) Biological
A)Mechanical or Physical Weathering:
Definition:
It includes all processes where solid rock is broken into fragments and disintegrated in situ but is left chemically unchanged. It is the most widespread process.
Classification:
1) Mechanical weathering by thermal tension
2) Mechanical weathering by crystal growth (Frost and Salt)
3) Mechanical weathering by pressure release.
4) Mechanical weathering by biological Agents.
1) Mechanical weathering by thermal tension:
a) Block disintegration: -
Definition: In a hot and dry atmosphere when joints and cracks develop in rocks by successive processes of expansion and contraction and ultimately break down into smaller blocks.
Formation: This type of disintegration is caused by exposure to an extreme range of temperature. A much diurnal range of temperature can be seen in a desert region. Where we see extreme hot in the daytime and very cold at night due to rapid longwave radiation. The rock successively expands and contract in response to the hot and cold condition. On the other hand, unequal expansion and contraction have been seen in the upper and lower portions of a big rock mass. So stress occurs on rock mass and ultimately fractures developed in fresh, massive rock and then break into smaller blocks. Fragmentation and shattering are the further products of block disintegration.
Note: However block disintegration is occurring not only for temperature changes, rather the release of load from the top is also responsible for this weathering.
Region- This kind of weathering can be seen in hot regions like deserts.
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b) Spheroidal Withering or Onion Weathering:
Definition: Homogeneous rock and round-shaped rocks due to intense heat and pressure release surface rock separated (peeled off) in succession from the original rock mass, leaving behind the spheroidal body then it is called spheroidal or onion weathering. This process is called exfoliation.
Formation: Rock is a bad conductor of temperature. In day time meanly in the desert region (where the range of temperature is high) in a round shape, homogeneous rocks becomes intensely heated. The upper layer of the rock expanded much from the lower layer because a temperature gradient occurs from outside towards the core of the rock. So surface layer expanded more due to direct exposure of heat but lower layers relatively less expanded or expended in different co-efficient. Unequal stress is generated in the rock mass. Ultimately surface layer pulls away from the lower layer in succession from the original rock mass leaving behind the spheroidal body.
Example: For example, we take Granite rock with a diameter of 30.49 meters. When Granite is heated and its temperature rises up to 65.6°C the diameter of the rock increases by 2.55cm (approx.). Then rock temperature falls at night the rock contacts by 2.55 cm. (approx.)
Note: Chemical action is more effective in exfoliation than physical action of temperature changes.
Region: In hot desert and semi aired regions exfoliation occurs much.
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C) Granular Disintegration:-
Definition: In heterogeneous rocks which are composed of different course mineral grains may break up into grains to differential contraction and expansion caused by insolation. This process is called granular disintegration.
Formation: In heterogeneous rocks which are composed of different course mineral grains. Different minerals have different coefficients of expansion. Due to the variation of heat, various minerals expand and contract at different rates compared to others. Different expansion of minerals at the same temperature develops stress in the rock mass. When unequal stress crossed the limit rocks break up in grains along cracks.
Example: In sandstone, at the same temperature Quartz expands mote then mica.
Region: In desert or semi aired region sound of pistol shot can be heard which indicate granular disintegration of rock.
Note: According to Griggs outer sells of the rocks are shattered due to sudden light showers in a hot desert region. According to him, this process is more effective than temperature changes. He remarked after experiments that small cracks are developed at the outer surface of the highly heated rocks when light drizzles suddenly strike the rock surface. He did his experiment with the showering of cool water on the heated rock. There he found cracks developed on the rock surface. The repetition of this process causes Spalling and Granular disintegration of rock.
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2) Mechanical weathering by crystal growth (Frost and Salt):-
a) Mechanical weathering by crystal growth by Frost:
Frost is the strongest agent for the disintegration of rocks mainly in mountain regions. We know when water freezes, it expands about one-tenth of its volume. In daytime in high altitudes when the glacier melts produce water. Water gets into crevices, cracks, pores in rocks but at night water turns into ice. Frozen water forms wedges of ice. Thus cracks are widened and exert great pressure of about 125kg to the square cm. ultimately rock fragmented.
Note: This type of weathering is very common in High Mountain regions since large hills are reduced to a heap of screes.
Scree or Talus:- Often broken rocks formed by weathering roll down by gravity and accumulate at the foothills. Such accumulated loose sharp angle rock debris at the foot of a mountain is called Scree or Talus.
Felsenmer or Block spade – When foothills are covered by Scree or Talus then it is called Felsenmer.
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b) Mechanical weathering by crystal growth by Salt:
Salts are abundant in the rocks of dry regions or semi-desert areas. Scanty rainfall (less than 25 cm annual rainfall) does not remove all salt from the surface. In semi-desert areas, salt is present in the aerosol which is formed by evaporation of wave spray in distant saline water mass (oceans) washed down in the ground or on rocks during occasional rain. When rock moisture evaporates salt turns into crystals. The developed crystals in pores and cracks cause local stress and tension. Since rock granules are out from their own position. Ultimately rock fragmented.
3) Mechanical weathering by pressure release –
This type of weathering is found in underlying rocks which are buried under thick covers of overlying rocks. When overlying rocks are disintegrated after that gradually denudated then underlying rocks are slowly exposed to the atmosphere. This removal of overlying rocks or gradual unloading releases pressure from the underlying rocks developed joints and cracks and ultimately break up along these cracks and joints.
Note Sheeting:- This process refers to the developed cracks and fractures parallel to the ground of the underlying rock due to the release of a superincumbent load of overlying rock. This type of disintegration is found in massive sandstone, Granite etc.
4) Mechanical weathering by biological Agents:-
This type of physical wreathing occurs by plant roots, insects and worms those who live in the soil, Burrowing animals like squirrels, rats, rabbits and humans.
Other Physical or Mechanical Weathering:
Dirt Cracking – Weathering because of dirt.
Boulder cleaving - Weathering occurs because of thermal expansion in boulders of granites and basalts.
Slaking – shattering of rocks due to alternate drying and wetting of rocks because of that successive expansion and contraction.
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