WEATHERING

Def:

Weathering is the process of weathering, disintegration or fragmentation of rocks into small particles.

Rocks on the earth’s surface are weathered by different type where we get type of weathering as follows:

1. physical/mechanical weathering
2. chemical weathering
3. biological weathering

                                  1. PHYSICAL / MECHANICAL WEATHERING

This is type of weathering whereby the rocks disintegrate in small particles without chemical reaction.

Physical weathering occurs in different type such as:

1. temperature change (exfoliation)
2. frost action
3. alternate wet and dry
4. biotic factors
5. burrowing animals

A. temperature change (exfoliation)

This is the type of physical weathering where by temperature cause the rocks to disintegrate, when temperature increase rocks expand during the day and when temperature falls during the night rocks decrease in size so this process when continues every day cause the rocks to break into pieces, the broken rocks are know as exfoliation

B. frost action

This is the type of physical weathering of the rocks caused by frost, when water penetrates in fissures of the rocks and when temperature cools and falls below 0˚c it causes water to freeze in fissures, since freezing water/ice has high volume than melting water then cause expansion of rocks and development of cracks then fragmentation of rocks.

C. alternate wet and dry

Rocks are combination of different minerals, all minerals absorb and loss water in different rate, this different wet and dry in minerals cause cracks between the margins of one or more elements.

D. biotic factors

Is the type of physical weathering caused by plants as plants grow cause the roots to expand and penetrates in rocks then develop cracks inside the rocks hence disintegration

E. burrowing animals

These animals are such as rats, rabbits etc. they disintegrate rocks when they dig burrows hence physical weathering of rocks occurs.

2. CHEMICAL WEATHERING.

This is the process of weathering of rocks through chemical reaction, chemical weathering occurs in different types as:

1. solution
2. oxidation
3. carbonation
4. hydration
5. hydrolysis
6. biotic factors

A. solution

This is the type of chemical weathering occurs when soluble rocks such as limestone, salt rocks etc.dissolve in water to form solution.

B. oxidation

This is the type of chemical weathering, it occurs when oxygen is added in rocks that contain iron and form rust on it, when these rocks react with oxygen they form oxidized iron that some times known as hematite.

E.g. Oxygen        +         Iron         →         Hematite (oxidized iron)

C. carbonation

When carbon dioxide combine with rain water it forms weak carbonic acid and this reacts with soluble calcium carbonate (lime stone) to form soluble calcium hydrogen carbonate

(Calcium bicarbonate).

CO₂            +            H₂O              →             H₂CO₃

Then:

H₂CO₃            +            CaCO₃             →             Ca (HCO₃)_2.

When carbonic acid reacted with the surface of the lime stone it forms two features

i.                     Grikes

ii.                   Clint

D. hydration

This is the type of weathering where by rocks absorb water then swell, this swelling causing rocks to develop cracks hence disintegrate. Also when hematite rocks absorb water they form new rocks known as limo-nite.

H₂O            +           Hematite         →            Limo-nite

E. hydrolysis

This is the type of chemical weathering occurs when hydrogen reacts with different minerals in rocks to from different kind of rocks e.g. when hydrogen reacts with feldspar new rock known as clay is formed.

Feldspar         +         H                  →              Clay

 biotic factors

is the process where animals release some chemical which ultimately react with rocks leading to its disintegration e.g Humic acid released by vegetation after decay. This acid reacts with rocks.

DIFFERENT BETWEEN PHYSICAL AND CHEMICAL WEATHERING

1. physical weathering involves break down of rocks without any change in chemical and  mineral structure of rock while in chemical weathering is break down of rocks in which chemical change are experienced.

2. physical weathering is influenced by temperature change and hence common in arid areas where the diunal range of temp is high. While in chemical weathering is mainly influenced by water and hence is common in equatorial region where rainfall is high throughout the year.

3. physical weathering concentrates on upper layer of the earth therefore is shallow while chemical weathering even deeper layers where percolating rain water can reach.

4. physical weathering has got six processes while chemical weathering has got five processes.

5. chemical weathering is slower disintegration because of slow reaction of gases in water while physical weathering may be quicker e.g man cracking rock.

FACTORS AFFECTING WEATHERING

1. Climate

     Temperature and rainfall are two major weather element, in certain climatic region receive a certain type of weathering e.g in equatorial region with high temperature experience fact and deep weathering.

     Chemical weathering is most common in area of high rainfall while physical weathering is common in low rainfall.

2.  Relief

     This refers to the nature of the land surface, chemical weathering is common in mountain region where the wind ward side receive high rainfall.

     In the steep slope the weathering takes place at high speed because it is the center of denudation and it experienced with physical weathering due to the high surface run of all the weathered material is falling downward lead to its disintegration.

     In gentle slope the weathering is in low speed because it is the place of deposition and it is affected by chemical weathering due to down ward movement of water.

3. Nature of rock

     Colored rock has effect on weathering, the rock which contain black mineral absorbs temperature so that the physical weathering takes place. E.g. olivine s are dark.

     The rocks with minerals like granite (composed of quarts, feldspar) resist weathering than the rock contains grabbo (composed of augite and lime soda) are affected faster by chemical weathering.

     Weakness of the rock which has the line of weakness such as crack or joint affected much by weathering.

4. Time

     When the process of weathering takes place in a day or night but after a very long time the weathering will be more but if it is shorter the weathering will be less.

5. Living organism (Flora/Fauna)

    At the area which have dense vegetation the plants add nitric acid within its roots so when it penetrates within the rock the acid reacts with rock affecting weathering compared to the place with low vegetation.

    Animal affect weathering through living or dying.

EFFECT OF WEATHERING

1. Soil formation

    Since weathering breaks the rocks into small particles which appear being mixed with organic matter leads to development of soil profile.

2. Modifying the landforms.

     Other features formed through weathering like exfoliation domes, clints, grikes etc.

3.  Provision of building materials to man.

     The scree and blocks can be used in construction of building the house, bridge and dams.

4.  Influence the rate of mass – wasting

     Where the rocks have been disintegrated so the erosion takes place very fast unlike the area where weathering takes place at low degree.

5. Tourist attraction

     Some landforms resulted by weathering process have good visual impression so are very attractive to tourists.

     Example granite tors like Kit mikage in seme hills in Kisumu district