Definition: Weathering is the decomposition and disintegration of rocks in place.The term has been originated from the word weather, and includes chemical-, biological-, and physical (mechanical) weathering. When different components of weather such as temperature, humidity and rainfall (etc.) act on the rocks and cause decay and disintegration, it is known asweathering, provided no large-scale transport of loosened material is involved.

Characteristics of Weathering
  • It is a rock altering process
  • It involves the change of rocks from a massive to a fragmented state
  • It is a static process as it does not involve the transport of weathered rocks


Why Weathering is Important?Weathering breaks down rocks and facilitates mass wasting and erosion. It contributes to the process of soil formation and is important for landscape evolution, as it causes a general lowering of land, creates and modifies minor landforms and cliff recession.



Physical or Mechanical WeatheringDefinition: the disintegration of rocks into smaller pieces by physical processes like thermal tension - expansion and contraction, crystal growth, pressure release and moisture swelling.
  • It does not involve any change in chemical composition
  • Mechanical weathering is most likely to occur in areas of bare rock, where there is no vegetation to protect the rock from the extremes of weather, commonly found in desert and periglacial region


Types of Mechanical Weathering
  • Freeze thaw action or frost shattering
  • Disintegration
    • Block disintegration
    • Granular disintegration
  • Exfoliation
  • Salt crystal growth
  • Pressure release or unloading


Freeze Thaw Action or Frost Shattering

  • Occurs in cold climate and alpine region
  • Water enters into the cracks and joint of the rocks during the warmer day and freezes at night. When the temperature falls below the freezing point, water turns into ice and it expands, exerting pressure on the rock wall and joints. At 0 degree C water expands by 10% of its volume and may exerts pressure up to 2100kg/cm2
  • Repeated freezing and thawing widens the cracks and causes rocks to break off. The fragmented rock-particles accumulate at the foot of the cliff is called scree and develop fan shaped mounds called talus cone


Disintegration (Block Disintegration, Granular Disintegration, Exfoliation)
  • Dominating process: Thermal expansion and contraction
  • Common in Hot desert region where there is large diurnal temperature range
    • Rocks heats up by day and contract at night
Block disintegration: well-jointed blocks break down into large rectangular shaped blocks under the action of alternate heating and cooling (may be aided by frost action)Granular disintegration: is caused by thermal tension aided by absorption of water, which causes various rock-forming mineral to expand and contract at different rates and to break off in fragments. A large mass of coarsely grained igneous rock can break up into granular particles by this processExfoliation (also known as onion peeling weathering): occurs in very warm climate with non-vegetated bare surface, which is repeatedly heated and cooled. Outer layer heats up and expands more rapidly than the inner layer. This process sets up stresses within the rock and cause the surface layer to peel of like the layer of an onion

Salt Crystal Growth or Haloclasty
  • Occurs in areas of warm climate when water evaporates leaving salt crystal behind, that may expand 300% by volume and cause weathering


Pressure Release or Unloading
  • This happens when overlying rocks are removed by erosion, allowing underlying rocks to expand and fracture
  • Same effect may occur due to the removal of great weight, such as glaciers
  • Fracturing of sand stones due to release of pressure is called cambering



Chemical Weathering
Chemical weathering involves the gradual decomposition of rocks and changes in chemical composition. It is a extremely slow process and commonly occurs in warm and humid climate.

Types of Chemical Weathering
  • Carbonation-solution
  • Oxidation
  • Hydration
  • Hydrolysis


Carbonation-SolutionCarbon dioxide (Co²) dissolved in rain forms a weak carbonic acid (H²Co³). This solution reacts with calcium carbonate (CaCo³). Rocks, such as limestone and chalk contain calcium carbonate and after reaction forms calcium-bicarbonate or calcium hydrogen carbonate which is water-soluble and is removed by percolating water.Chemical weathering causes joints to widen and deepened. Deep grooves formed in limestone rocks are called grikes, which are separated by flat-topped ridges called clints.

Oxidation
  • Occurs when rock, forming iron bearing minerals, reacts with oxygen dissolved in water and produce reddish brown rust, so that the whole rock mass gets decomposed very soon
  • Iron compounds oxidized from ferrous to ferric state


Hydration
  • It is the process whereby certain minerals absorb water, expand and change
  • Hydration occurs when minerals incorporate water into their molecular structure
  • This causes swelling, crumbling and decaying of rocks
  • It is vital in decomposition of silicate clay mineral


Hydrolysis
  • Water molecules dissociates (splits into smaller parts) into H+ and OH- ions (ions are charged particles), which stimulate chemical reaction with rock forming minerals
  • Hydrolysis may produce a special type of exfoliation called spheroidal weathering
  • Large granite masses are sometimes weathered by the process of hydrolysis and solution, even below the surface at a depth of 200m or more, as water/solution enters via pores, joints and cracks – this creates tors



Bio-Physical Weathering and Bio-Chemical Weathering
Bio-physical weathering Involves colloid plucking through root penetration, activities of burrowing animals and man made causes like mining, construction, etc.
Bio-chemical weathering Chelation: a complex organic process, where humic acid is formed, which is capable of extracting metallic ions (these are otherwise insoluble)Microbial weathering: organic acids like lactic and oxalic acids attack minerals like olivine, keolite

Peltier Diagrams
  • A Peltier diagram shows the ranges of different weathering types as plotted against temperature and rainfall
  • It gives a broad picture of the climatic range of mechanical (physical) and chemical weathering
  • It is expressed as strong, moderate or weak chemical or mechanical weathering
    • Strong chemical weathering will exist in areas with abundant rainfall and high temperatures
    • Strong mechanical weathering will exist where there are diurnal fluctuations across zero degrees and sufficient moisture to allow freeze thaw
    • Where there is little moisture and very low temperatures little weathering will take place
  • Although the diagram gives an indication of the chemical or mechanical nature of the weathering, it does not indicate the particular processes taking place, as no account is taken of the factors other than temperature and precipitation



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Fun Game on weathering
http://www.purposegames.com/game/physical-weathering-quiz