Category: The Fundamentals of Physical Geography
Topic: Physical Weathering
NCERT notes on important topics for the UPSC civil services exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. This article talks about the Physical Weathering.
Physical Weathering Processes
- Physical or mechanical weathering processes are influenced by some applied forces.
- The applied forces are:
- Gravitational forces like shearing stress, load, and overburden pressure.
- Expansion forces due to crystal growth, animal activity or temperature variations.
- Water pressures regulated by drying and wetting cycles.
- Many of these forces are applied both at the surface and within different earth materials leading to rock breakage.
- Most of the physical weathering processes are caused by pressure release and thermal expansion.
Unloading and Expansion
- Elimination of covering rock load because of sustained erosion causes vertical pressure release with the result that the upper layers of the rock enlarge producing fragmentation of rock masses.
- Fractures will occur roughly parallel to the ground surface.
- In areas of curved ground surface, arched fractures incline to create massive sheets or exfoliation slabs of rock.
- Exfoliation sheets causing from expansion due to pressure release and unloading may measure hundreds or even thousands of metres in horizontal extent.
- Big, smooth rounded domes are called exfoliation domes.
Temperature Changes and Expansion
- Several minerals in rocks possess their own limits of contraction and expansion.
- With an upsurge in temperature, all minerals enlarge and thrust against its neighbour and as temperature drops, a corresponding shrinkage takes place.
- Due to diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place repeatedly.
- This process is effective in high elevations and arid climates where diurnal temperature variations are extreme.
Freezing, Thawing and Frost Wedging
- Frost weathering happens due to development of ice within openings and cracks of rocks during recurrent cycles of melting and freezing.
- This process is effective at high elevations in mid-latitudes where melting and freezing is frequently recurrent.
- Glacial regions are subject to frost wedging every day.
- In this course, the rate of freezing is significant.
- Hasty freezing of water causes its high pressure and rapid expansion.
- The resulting expansion affects joints, cracks and small intergranular fractures to become wider and wider till the rock breaks apart.
- Salt crystallisation is most effective of all salt-weathering processes.
- Salts in rocks enlarge due to hydration, crystallisation and thermal action.
- Various salts like sodium, barium, calcium, potassium, and magnesium, have an inclination to enlarge.
- Enlargement of these salts relies on temperature and their thermal properties.
- High-temperature ranges between 30 and 50 degrees Celsius of surface temperatures in deserts support such salt expansion.
- Salt crystals in the adjacent surface pores cause splitting of single grains within rocks, which ultimately drop.
- This process of dropping of individual grains may result in granular disintegration or granular foliation.
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