Discuss the various processes of physical weahtering of the rocks

Physical weathering refers to the disintegration of the rocks due to temperature variations,frost actions,wind etc. there are 6 such processes:-

a)Low temperature based weathering:-

1)Freeze-thaw weathering:-

this is the process which occurs when the water freezes and expands in volume within the rocks.

a)by freezing of 'in-place' water:-When water freezes a 95% volumetric expansion takes place because water molecules take a hexagonal arrangement which occupies more space than their relatively close packed distribution in liquid phase.this expansion generates a pressure of 207 MPa (a theoretical pressure),at -22 degree Celsius under the optimum conditions,against the containing walls in a closed system.this pressure is much greater than rock tensile strength,which is rarely more than 10 MPa.Even if theoretical pressure is not reached,the actual stress generated are sufficient to shatter rocks.

b)By water migration and ice growth:-This model is dynamic.Water and ice can co-exist in a rock at temperatures <=0.As per Gibbs free energy concept the two phases are stable when their free energies are equal.this equalizing incentive is given by play of factors like salt presence adsorption effects and capillary effect.A free energy gradient forms and the transfer of mass tends to work along the gradient and the water moves to a freezing surface.The movement depends on the rock permeability, temperature and water availability.The gradient can be expressed as  a suction force on water.The presence of cracks in the rocks may be exploted bu pressure called stress intensity factor.Stress intensity factor is affected by rock geometry and ice pressure.Stress in tensity factor is restricted by tensile strength of rock called fracture toughness.When stress intensity factor is greater than fracture toughness the cracks begin to grow and mechanical weathering occurs.

c)Hydration shattering:-The idea is that individual polar water molecules will get oriented to cahrged mineral surfaces in such manner that one end of a molecule bonds to the mineral surface while the other end projects away.These molecules may not readily freeze at sub-zero temperatures as there may be insufficient  energy for them to be reoriented.An alignment of molecules is thus developed and their rigidity increases as temperature falls.Repulsion forces set up in cracks <5 4="" and="" below="" by="" degree="" diameter.the="" elsius="" enhances="" expansion="" fall="" helped="" in="" is="" magnitude="" micro-metres="" nbsp="" o:p="" repulsion="" resulting="" temperature="" water="" weathering.="" with="">

d)Ice crystal growth:-Laboratory experiments show growth of pollycrystalline ice from a melt has been measured in the laboratory and found to be about 20 Pa. In nature certain conditions have to be met for the process to work effectively like- slow rate of freezing,allowing water movement into zones of crystallization and permeable texture which encourage water migration.the process is consistent with water migration theory.the pressure is at it's maximum in the direction of greatest resistence.

e)Hydraulic pressure:-The magnitude of the presure depends on the rate of water movement whic is controlled by freezing from the surface and greater the distance lower is the freezing rate.The effectiveness of the process depends upon the weahtering objects size.

2)Salt weathering:

a)thermal weathering:-For weathering to occur,thermal expansion coeffecient of a salt should exceed that of the surrounding rock.for example:-sodium chloride produces a volumetric expansion of 1% at about 50 degree celcius which is greater than enclosing rocks.The process may be preectly effective in granite rocks which may be under internal stress.

b)Hydration pressure:-When salts hydrate they expand in volume.Higher degree of hydration has a higher pressure,for this to hapen temperature must reach transition point where anhydrous salt become fully hydrated.

c)Crystallisation pressure:-Salts present in the solution state crystalize when water evaporates through rock cracks.The growth of crystals give rise to ctystallization pressure which is excess of the tensile strength of the rocks which give weathering.

3)Wetting and drying weathering:-The process is closely related to hydration shattering.Certain rock properties like-presence of clay minerals,structural weakness,low tensile strength and pore size and distribution.Rain in hot desert causes numerous cracks on heated rocks and the process repeatation causes spalling and granular disintegration.

4)By temperature change weathering:-Rocks are exposed to 2 sources of heat:-

a)sun;

b)natural fires.

This process related,thus to the concept of temperature change and the rate of temperature change.Temperature changes of rocks by the insolation depends on the  following factors:-

a)latitude;

b)altitude;

c)aspect;

d)time of the year;

e)cloud cover nature;

f)wind speed;

g)air temperature.

   Bush fire are frequent events but when they occur they occur their can raise rock temperatures to >800 degree Celsius within about 10 minutes and is termed as thermal shock and has destructive effects on rocks and accelerate weathering.However,rock properties like-albedo,thermal conductivity(low in rocks), coefficient of elasticity which is indirect measure of rock's resistance to weathering.

5)Pressure release weathering:-

a)Sheet jointing involving change of compressive stress:-As surface erosion removes overlying rocks.the rock unit beneath the land surface which sis under high compressive stress will now tend to move in the direction of stress reduction and tensile stress associated with the expansion exceeding the tensile strength of the rock which results in the failure.It began with the extension of initial parallel crack(as per Grifith theory of crack development).It is most effective in brittle,crystalline rocks of high elasticity.

b)Sheet jointing involving mainly tensile stress:-In quarries in hard rock typically revelas the development of open joints parallel to and at the top of side and back walls.This variety of sheet jointing develop where tensional forces on a unit of rock exceeds it's resistance to where resistance to the tensile stress is the lowest.

6)Stress corrosion cracking:-This is caused by the combined action of chemical action and physical stress at a point.The mechanism may be effective at a crack tip in a silicate when water is present.In this case,the chemical processes is hydrous which brings about the replacement of strong structural bonds by the weaker links.