the agents and processes of biological weathering

Biological weathering is the decomposition of rocks due to activities of flora and fauna and several chemicals exuded by them or is formed due to their activity.Important biological agents include -bacteria,lichens, algae,fungi,plant roots and organic matter in a state of decay organisms assist rock and mineral breakdown in 2 main ways:-

a)by exerting physical stress on the rocks.example-expansion and contraction of plant organic tissue during wetting and drying.

b)Release of acids ,carbon di oxide,complexing agents,protons and electrons as part of their life process.

a)Faunal weathering:-

Fauna include foxes,burrowing animals ,termites etc. small organisms play more decisive roles in rock and soil weathering.micro=organisms including bacteria release the carbon-di-oxide increasing carbon-di-oxide content in the soil air between the percentage 0.5%-20% resulting weakly acidic soil solutions enhancing chemical weathering.

 nitrificaton of ammonium ions by bacteria involve release of hydrogen ions(protons).

the concentration of H+ ions increase causing acidification and weathering.

            Metal oxidising bacteria like metallogenium bacteria which forms desert varnish,T.Ferroxidans.The impressive activity of certain bacteria to leach metals from ore has been used commercially.for example-In 1971,15% of U.S.A.'s copper was produced by bacterial leaching.

           Lichen(a composite organisms whose body contains a fungus and algae growing together),Rock colnising lichens are sub-divided into endolithic(i.e.,growing within rocks example in fracture) and epilithic(those growing on the surface)varieties.Mechanical breakdown by lichens is brought about by 2 ways:-

a)By hydraulic penetration:-It grows along existing micro-cracks and grains to a depth of 1mm - 12mm causing tensile stress excess than the tensile strength of the rocks causing weathering.

b)Expansion of thalli and hypahe by water absorption:-Most lichens can increase their water content by 150%-300% and those whose thalli contains high proportion of gelatine can show increased stress upto 3900% which sets up damaging levels of tensile stress due to physical expansion.

                            Lichens emit organic acids which is effective in mineral alteration.Ex:-oxalic acid(soluble to the degree of 143gl-1).Effectiveness of the organic acids depend upon it's solubility in water.

   algae and cyanobacteria contribute to both physical and chemical weathering.the rock dwelling types of algae(like the lithophytic algae) are the greatest interest from a weathering point of view.

    Mucilage surrounding the algaeal cells would take up a large quantity of water during wet periods generating an expansionary force (about 20 times the dry volume  causing removal of already weakened flakes.algae also aids desert varnish formation.

b)Floral weathering:-

the upper zone of the regolith,dominated by plant roots,called rhizosphere where roots bring about rock and mineral breakdown by physical and chemical methods.A common belief that the growing roots through the rock microcracks cause their rapture has been proven false mechanically.this is so because the measured axial growth pressure of the plant roots is 3MPa,which can fracture weakened rocks.the effective tensile stress is set up by radial pressure which is 1/3rd to1/4th of the axial amount.However,this force in application is even lower.

formula:-

Maximum force exerted=largest vertical cross-sectional area*radial pressure by the root

this area would be much less than potential fracture surface area and thus,distributing a weak force over a large area.

a Plant root is a complex microsystem that emits  and absorbs the substances as part of it's life process.Rhizosphere is the effective micro-system for weathering .

c)working of the decaying plant and animal matter:-

An advanced stage of decay is humus formation (a complex mixture of amorphous and colloidal substances.the decay process which is essentially oxidation yields carbon-di-oxide and humic acids and fulvic acids.they form chelate which are soluble and aid weathering.Ex:-fulvic acid chelate with Al3+ and Fe3+.

d)anthropogenic weathering:-

'technological man' lashed with modern technology has become the most powerful weathering and erosion agent.Blasting of hills and ridges for mining,road building,dam construction etc. result in fast disintegration of geomaterial.