CLASS NOTE OF GEOGRAPHY WEATHERING

WHAT IS WEATHERING Weathering is the general term applied to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of exposure near the earth surface

WEATHERING According to Ollier (1974) WEATHERING is the breakdown and alteration of materials near the earth surface to products that are more in equilibrium with newly imposed physico- chemical condition

CONTROLLING FACTORS Hardness of rocks and minerals. Presence of joints, foliation planes and Bedding planes Nature of ground slopes. Changing nature of humidity Chemical composition of rocks Floral effects Time

JOINTS IN BEDROCKS

CLASSIFICATION OF WEATHERING 1.Mechanical or Physical weathering 2.Chemical weathering 3.Biological weathering

MECHANICAL OR PHYSICAL WEATHERING A.Mechanical weathering due to crystallisation B. Mechanical weathering involving super incumbent pressure release C. Mechanical weathering due to change of temperature D. Mechanical weathering involving chemical change

MECHANICAL WEATHERING DUE TO CRYSTALLISATION 1. FROST ACTION 2. SALT CRYSTAL GROWTH

FROST ACTION Most important physical weathering in cold climates When water freezes in bedrock joints and bedding planes,it expands and can split rock apart Areas:i)Mountain summits ii)Arctic Tundra iii)Pore spaces of soil in cold climates

SALT CRYSTAL GROWTH Salt crystal growth can break rocks apart grain by grain, producing niches, shallow caves and rock arches

Mech weathering involving super incumbent pressure release Mech weathering due to unloading: Rock disruption due to relief of confining pressure of underlying rock Exfoliation is another term for unloading Areas: Rock brought near the surface by erosion of overlying layers, where rocks expand slightly in volume Expansion causes thick shells of rocks to break free from the parent mass below and form jointing and sheeting structure and exfoliation domes

Unloading

Mech weathering due to change of temperature Mech weathering due to expansion and contraction in rock forming minerals Due to intense heating of sun alternating with nighty cooling, it exerts powerful disruptive forces on rocks

Mechanical weathering involving Chemical change Mechanical weathering occurs due to hydration and oxidation, associated with changes in volume and resultant pressure in rocks.

Other causes Mechanical weathering also involves wind action: outer shells of rocks become loose due to temperature changes and are removed by strong winds

CHEMICAL WEATHERING Chemical weathering involves decomposition and disintegration of rocks and involves weathering away of minerals in rocks. Water vapour and water are the media of chemical reactions

Chemical weathering A. Solution B. Oxidation C. Carbonation D. Hydration E. Hydrolysis F. Chelation

Chem weathering-- SOLUTION First step in chem decomposition. It refers to the dissolution of soluble particles and minerals from the rocks with the help of water. Limestone is very much susceptible to sol process in the presence of CO 2 content of water and pH of the solution. CaCO 3 +H 2 O+CO 2 —Ca(HCO 3 ) 2

Chemical weathering-- OXIDATION Decomposition by oxidation changes strong rock into very weak regolith. Water mixed with atmospheric oxygen in contact with iron bearing rocks; iron oxidises to form ferrous oxides (FeO), then Ferric oxides(Fe 2 O 3 ) or Ferric hydroxides (Fe(OH) 2 ) 4FeO+3H 2 O+O Fe 2 O 3.3H 2 O… (Rusting of Rocks)

CHEMICAL WEATHERING

Chemical weathering--Carbonation Carbonation is the reaction of carbonate or bicarbonate ions with minerals. This process is also known as SOLUTION.

Chemical weathering-- Hydration Hydration is related to the addition of water to the minerals. It is an exothermic change

Chemical weathering-- Hydrolysis Hydrolysis is a chemical reaction between mineral and water, i.e. between Hydrogen (H) ions or hydroxyl (OH) ions and the ions of the mineral Rocks and water molecules decompose

Chemical weathering-- Chelation Chelation is a complex organic process– metallic cations are incorporated into hydrocarbon molecules. Chelate means a co-ordination compound in which a central metallic ion is attached to an organic molecule at two or more positions.

BIOLOGICAL WEATHERING OR BIOTIC WEATHERING In all types of weathering in all climatic regions biotic communities play some roles in one way or other A. Faunal weathering B. Floral weathering C. Anthropogenic weathering

Faunal weathering 1.Tranfer of soils from lower to upper and upper to lower horizons and activates weathering 2. Burrowing animals include gophers, prairie dogs, foxes, rabbits, jackals, termites, rats etc.

FLORAL WEATHERING Cracks are widened by root penetration and consequent root pressure (physical weathering). Dense vegetable cover generates distinct micro climate at the ground surface—initiate chemical weathering.

ANTHROPOGENIC WEATHERING The economic and technological man are most powerful weathering agent through mining,road and dam construction, deforestation etc.