World Geography Part 10 – Physical Weathering Processes

 


Physical Weathering Processes

  1. Physical or mechanical weathering processes depend on some applied forces.
  2. The applied forces could be
  3. Gravitational forces such as overburden pressure, load and shearing stress
  4. Expansion forces due to temperature changes, crystal growth or animal activity
  5. Water pressures controlled by wetting and drying cycles
  6. Many of these forces are applied both at the surface and within different earth materials leading to rock fracture.
  7. Most of the physical weathering processes are caused by thermal expansion and pressure release.
  8. These processes are small and slow but can cause great damage to the rocks because of continued fatigue the rocks suffer due to the repetition of contraction and expansion.

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Unloading and Expansion

  1. 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.
  2. Fractures will develop roughly parallel to the ground surface
  3. In areas of curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs of rock.
  4. Exfoliation sheets resulting from expansion due to unloading and pressure release may measure hundreds or even thousands of metres in horizontal extent.
  5. Large, smooth rounded domes called exfoliation domes

Temperature Changes and Expansion

  1. Various minerals in rocks possess their own limits of expansion and contraction.
  2. With rise in temperature, every mineral expands and pushes against its neighbour
  3. As temperature falls, a corresponding contraction takes place.
  4. Because of diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place regularly.
  5. This process is most effective in dry climates and high elevations
  6. Where diurnal temperature changes are drastic.
  7. As has been mentioned earlier though these movements are very small they make the rocks weak due to continued fatigue
  8. The surface layers of the rocks tend to expand more than the rock at depth and this leads to the formation of stress within the rock resulting in heaving and fracturing parallel to the surface.


Freezing, Thawing and Frost Wedging

  1. This process is most effective at high elevations in mid-latitudes where freezing and melting is often repeated.
  2. Glacial areas are subject to frost wedging daily. In this process, the rate of freezing is important.
  3. Rapid freezing of water causes its sudden expansion and high pressure
  4. The resulting expansion affects joints, cracks and small inter granular fractures to become wider and wider till the rock breaks apart.

Salt Weathering

  1. Expansion of these salts depends on temperature and their thermal properties
  2. High temperature ranges between 30 and 50 o C of surface temperatures in deserts favour such salt expansion.
  3. Salt crystals in near-surface pores
  4. Cause splitting of individual grains within rocks, which eventually fall off
  5. Salt crystallisation is most effective of all salt-weathering processes
  6. Sodium chloride and gypsum crystals in desert areas heave up overlying layers of materials and with the result polygonal cracks develop all over the heaved surface.
  7. With salt crystal growth, chalk breaks down most readily, followed by limestone, sandstone, shale, gneiss and granite etc.


BIOLOGICAL ACTIVITY AND WEATHERING

  1. 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.
  2. 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.
  3. 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.
  4. Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements.
  5. Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements.
  6. Algae utilise mineral nutrients for growth and help in concentration of iron and manganese oxides.

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