About

• Weathering is action of elements of weather and climate over earth materials.
• There are a number of processes within weathering which act either individually or together to affect the earth materials in order to reduce them to fragmental state.
• Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
• As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
• Weathering processes are conditioned by many complex geological, climatic, topographic and vegetative factors. Climate is of particular importance. Not only weathering processes differ from climate to climate, but also the depth of the weathering mantle

There are three major groups of weathering processes:

Chemical; (ii) physical or mechanical; (iii) biological weathering processes

Chemical Weathering Processes

• A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface and/or soil water and other acids.
• Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions
• Solution When something is dissolved in water or acids, the water or acid with dissolved contents is called solution.
• This process involves removal of solids in solution and depends upon solubility of a mineral in water or weak acids.
• On coming in contact with water many solids disintegrate and mix up as suspension in water.
• Soluble rock forming minerals like nitrates, sulphates, and potassium etc. are affected by this process.
• So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions. Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones are soluble in water containing carbonic acid (formed with the addition of carbon dioxide in water), and are carried away in water as solution.
• Carbon dioxide produced by decaying organic matter along with soil water greatly aids in this reaction.
• Common salt (sodium chloride) is also a rock forming mineral and is susceptible to this process of solution.

Physical Weathering Processes

• Physical or mechanical weathering processes depend on some applied forces.

The applied forces could be:

1. gravitational forces such as over burden pressure, load and shearing stress;
2. expansion forces due to temperature changes, crystal growth or animal activity;
3. Water pressures controlled by wetting and drying cycles

• Many of these forces are applied both at the surface and within different earth materials leading to rock fracture.
• Most of the physical weathering processes are caused by thermal expansion and pressure release.

Special Effects Of Weathering

Exfoliation

• Exfoliation is a result but not a process. Flaking off of more or less curved sheets of shells from over rocks or bedrock results in smooth and rounded surfaces
• Exfoliation can occur due to expansion and contraction induced by temperature changes.
• Exfoliation domes and tors result due to unloading and thermal expansion respectively

Significance Of Weathering

Details

• Weathering processes are responsible for breaking down the rocks into smaller fragments and preparing the way for formation of not only regolith and soils, but also erosion and mass movements.
• Biomes and bio- diversity is basically a result of forests (vegetation) and forests depend upon the depth of weathering mantles.
• Erosion cannot be significant if the rocks are not weathered.
• That means, weathering aids mass wasting, erosion and reduction of relief and changes in landforms are a consequence of erosion.
• Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc., which are of great importance for the national economy.
• Weathering is an important process in the formation of soils.
• When rocks undergo weathering, some materials are removed through chemical or physical leaching by groundwater and thereby the concentration of remaining (valuable) materials increases.
• Without such a weathering taking place, the concentration of the same valuable material may not be sufficient and economically viable to exploit, process and refine. This is what is called enrichment.

Several activating causes precede mass movements.

They are:

1. Removal of support from below to materials above through natural or artificial means;
2. Increase in gradient and height of slopes;
3. Overloading through addition of materials naturally or by artificial filling;
4. Overloading due to heavy rainfall, saturation and lubrication of slope materials;
5. Removal of material or load from over the original slope surfaces; (vi) occurrence of earthquakes, explosions or machinery;
6. Excessive natural seepage;
7. Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks;
8. Indis- criminate removal of natural vegetation.

Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three forms of movements.

The Erosion

• Can be defined as “application of the kinetic energy associated with the agent to the surface of the land along which it moves”
• Kinetic energy is computed as KE = 1/2 mv2 where ‘m’ is the mass and ‘v’ is the velocity.
• The work of the other two agents of erosion waves and ground water is not controlled by climate.
• In case of waves it is the location along the interface of litho and hydro sphere coastal region — that will determine the work of waves, whereas the work of ground water is determined more by the litho logical character of the region.
• If the rocks are permeable and soluble and water is available only then Karst topography develops.

Deposition

• It is a consequence of erosion.
• The erosional agents lose their velocity and hence energy on gentler slopes and the materials carried by them start to settle themselves.
• In other words, deposition is not actually the work of any agent.

Carbonation

• Carbonation is the reaction of carbonate and bicarbonate with minerals and is a common process helping the breaking down of feldspars and carbonate minerals.
• Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid.
• Calcium carbonates and magnesium carbonates are dissolved in carbonic acid and are removed in a solution without leaving any residue resulting in cave formation.

Hydration

• Hydration is the chemical addition of water. Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock.
• Calcium sulphate takes in water and turns to gypsum, which is more unstable than calcium sulphate.
• This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.

Oxidation And Reduction

• In weathering, oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.
• The minerals most commonly involved in this process are iron, manganese, sulphur etc.
• In the process of oxidation rock breakdown occurs due to the disturbance caused by addition of oxygen. Red colour of iron upon oxidation turns to brown or yellow.
• When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place.
• Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
• Red colour of iron upon reduction turns to greenish or bluish grey.

Unloading And Expansion

• Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses.
• Fractures will develop roughly parallel to the ground surface. In areas of curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs of rock.
• Exfoliation sheets resulting from expansion due to unloading and pressure release may measure hundreds or even thousands of metres in horizontal extent.
• Large, smooth rounded domes called exfoliation domes

Temperature Changes And Expansion

• Various minerals in rocks possess their own limits of expansion and contraction.
• With rise in temperature, every mineral expands and pushes against its neighbour and as temperature falls, a corresponding contraction takes place.
• Because of diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place regularly.
• This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic.

Freezing, Thawing And Frost Wedging

• Frost weathering occurs due to growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting.
• This process is most effective at high elevations in mid-latitudes where freezing and melting is often repeated.
• Glacial areas are subject to frost wedging daily.
• In this process, the rate of freezing is important. Rapid freezing of water causes its sudden expansion and high pressure.
• The resulting expansion affects joints, cracks and small inter granular fractures to become wider and wider till the rock breaks apart.

Salt Weathering

• Salts in rocks expand due to thermal action, hydration and crystallisation.
• Many salts like calcium, sodium, magnesium, potassium and barium have a tendency to expand.
• Expansion of these salts depends on temperature and their thermal properties. High temperature ranges between 30 and 50oC of surface temperatures in deserts favour such salt expansion.
• Salt crystals in near-surface pores cause splitting of individual grains within rocks, which eventually fall off.
• This process of falling off of individual grains may result in granular disintegration or granular foliation cause splitting of individual grains within rocks, which eventually fall off.
• This process of falling off of individual grains may result in granular disintegration or granular foliation.
• Salt crystallisation is most effective of all salt-weathering processes
• With salt crystal growth, chalk breaks down most readily, followed by limestone, sandstone, shale, gneiss and granite etc

Biological Activity And Weathering

• Biological weathering is contribution to or removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms.
• Burrowing and wedging by organisms like earthworms, termites, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.
• Human beings by disturbing vegetation, ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth materials.
• Decaying plant and animal matter help in the production of humid, carbonic and other acids which enhance decay and solubility of some elements.
• Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.