”The process of mechanical disintegration of and /or chemical decomposition of rocks that destroys their coherence and breaks them into smaller components and fragments is called weathering”.
Weathering is the decomposition of rocks, soils and their minerals through direct contact with the Earth’s atmosphere. Weathering occurs in situ, or ”with no movement”, and thus should not be confused with erosion, which involves the movement and disintegration of rocks and minerals by agent such as water, ice , wind and gravity.
Three important classifications of weathering processes exist. Mechanical or physical weathering involves the breakdown of rocks and soil through direct contact with atmospheric conditions such as heat, water, ice and pressure. Second classification, involves the direct effect of atmospheric chemicals, or biologically-produced chemicals in the break down of rocks, soil and minerals. The third classification (also known as biological weathering) involves agency of living organisms in disintegration of rocks.
The material left over after the rock breaks down combine with organic material creates soil. The minerals content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility while a soil weathered from a mix of rock types (as in glacial, eolian or alluvial sediments) often makes more fertile soil.
Types of Weathering:
a. Physical/ Mechanical Weathering:
”Mechanical weathering involves the physical disintegration of rock material without any change in its chemical composition”.
Mechanical weathering is the cause of disintegration of rocks. The primary process in mechanical weathering is abrasion-the process by which clasts and other particles are reduced in size. However, chemical and physical weathering often goes hand in hand. For example, cracks exploited by mechanical weathering will increase a surface area exposed to chemical action. Further more, the chemical action at minerals at cracks can aid the disintegration process.
Modes of Mechanical Weathering:
01. Thermal Expansion/ Isolation:
Thermal expansion, also known as onion-skin weathering, exfoliation, isolation weathering or thermal shock often occurs in areas like deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping greatly at night. As the rock heats up and expands by day, and cools and contracts by night, stress in often exerted on the outer layer. The stress causes the peeling of the outer layers of rocks in thin sheets. Though this is caused mainly by temperature changes, thermal expansion is enhanced by the presence of moisture.
Rock breakage due to thermal expansion.
02. Freeze Thaw Weathering/Frost Wedging and Frost Heaving:
Formerly believed to be the dominant mode, frost wedging may still be a factor for weathering of non-porous rocks, although rece
nt research has demonstrated it less important than previously thought. Frost action, sometimes known as ice crystal growth, ice wedging, frost wedging or freeze-thaw occurs when water in cracks and joints of rocks freezes and expands. Thus, ice produced by water frozen in the joints can produce about 1890 matric tons (21 hundred tons) of pressure for 0.1 meter square (1 ft.sq). This pressure is often higher than the resistance of most rocks and causes the rock to shatter.
When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by 9 percent when it freezes.
When the ice thaws, water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the joint further. Repeated freeze-thaw action weakens the rocks which, over time, break up along the joints in to angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks that are detached are of various shapes depending on rocks structure.
03. Pressure Release:
In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation.
04. Hydraulic Action:
This is when water (generally from Powerful waves) rushes into cracks in the rock face rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly release with explosive force. The explosive release of highly-pressurized air cracks away fragment at the rock face and widens the crack itself.
05. Salt Crystal Growth/ Haloclasty:
Salt crystallization are other wise known as haloclasty causes disintegration of rocks when saline solutions keep into to cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expands as they are heated up, exerting pressure on the confining rock. Salt wedging is caused by the crystallization of salts from evaporating water in same fashion as frost wedging but it is less significant in its total effect.
The salt which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more. It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts. An example of salt weathering can be seen in the honey-combed stones in sea walls.
b. Biological Weathering:
Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes. Growing plant roots can exert stress or pressure on rock. Although the process is physical, the pressure is exerted by a biological process (i.e., growing roots). Biological weathering occurs in following modes.
01. Through agency of Living Organisms:
Living organisms may contribute to mechanical weathering. Lichens and mosses grow on bare rock surfaces and create a more humid chemical micro-environment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface of the rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. Burrowing animals and insects disturb the soil layer adjacent to the bedrock surface thus further increasing water and acid infiltration and exposure to oxidation processes.
02. Through Agency of Plants also called Root wedging (Flora Bases):
Root wedging occurs when the root of a plant begins to grow into a crack or pore in a rock. As the plant grows larger, so too does its root, until the roots breaks the rock apart.
03. By Human Action:
Human activities like air pollution, quarrying and mining, road cuts, dam construction, underground atomic explosions, ploughing for agriculture result in disintegration of rocks.
c. Chemical Weathering:
Chemical weathering involves the decomposition of rocks by the alteration of rock-forming minerals. Chemical weathering involves the change in the composition of rocks, often leading to a break-down in its form. This type of weathering happens over a period of time.
Chemical weathering acts in following modes.
Rainfall is naturally slightly acidic because atmospheric carbon dioxide dissolves in the rain water producing weak carbonic acid. In unpolluted environment, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulphur dioxide and nitrogen oxide are present in the atmosphere. These oxides react in the rainwater to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulphur dioxide which comes from volcanic eruptions or from fossil fuels, can become sulphuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.
One of the most well-known solution weathering processes is carbonation; the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate such as lime stone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to from a weak carbonic acid which reacts with calcium carbonate (the lime stone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature and therefore is a large feature of glacial weathering.
The reaction is : H2O + CO2 = H2CO3.
CaCO3 + H2CO3 = Ca(HCO3)2
Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.
With in the weathering environment, chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2 (iron) and combination with oxygen and water to from Fe3+Hydroxides. This gives the affected rocks a reddish-brown colouration on the surface which crumbles easily and weakens the rock. This process is better known as rusting.
Hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. When rock minerals take up water, the increased vulume creates physical stresses within the rock. Hydration also allows for the acceleration of other decompositional reactions by expanding the crystal lattice offering more surface area for reaction.
Hydrolysis is the weathering reaction that occurs when the two surfaces of water and compund meet. It involves the reaction between mineral ions and the ions of water (OH- and H+), and results in the decomposition of the rock surface by forming new compounds and by increasing the pH of the solution involved through the release of the hydroxide ions. Hydrolysis is especially effective in the weathering of common silicate and alumino-silicate minerals because of their electrically charges crystal surfaces.
Effect of weathering in Various Climatic Regions:
The effect of the weather upon rocks vary according to the potency of the different climatic elements.
In equatorial latitudes, where both humidity and temperature are consistently high, chemical weathering is continuously active, and it is generally much more rapid and effective than the transport and removal of weathered material.
In desert areas, there is little weathering by ordinary leaching, but considerable mechanical weathering. The chemical weathering takes place by the drawing of strong solutions to the surface by capillary action.
In mid latitudes, frost is by far the most powerful agent, while solution, particularly in lime stone areas, exerts great effects.
Under polar conditions, great areas of permanent snow prevents any ordinary weathering, but where Nunataks project from ice-sheet, frost action is rampant. Chemical and organic agencies here seem to be negligible for their effects. Corbondioxide is more soluble at low temperatures than at high, and as the melted water has higher Carbonic Acid content, chemical weathering may be quite active under a glacier or at the edge of an ice-sheet.