ZIMSEC O Level Geography Notes: Physical Weathering
- Physical weathering is the disintegration of a rock into smaller particles by mechanical processes and without any changes in the chemical composition of the rock.
- It is more likely to occur in arid climates such as deserts, arctic regions and areas that have little to no vegetation.
- Physical weathering typically produces sand soils.
- Physical weathering is also known as mechanical weathering.
- Physical weathering processes include exfoliation, frost shattering, pressure release and thermal shattering.
- Rocks expand when heated and contract when cooled much like all solid materials.
- In deserts and areas with scant vegetation, diurnal temperatures can exceed 50°C.
- During the excessive day heat the rocks expand.
- The outer layers of the rocks expand faster than the inner layers.
- At night the when temperatures drop, in deserts evening temperatures can and usually fall to below freezing point, the rock contracts and again the outer layers cool faster than the inner layers.
- This results in stresses being set up within the rock causing outer layers of the rock to peel off like the rings of an onion.
- Thus the process is sometimes called onion weathering.
- Also several different minerals withing the rock expand and contract at different rates depending on their physical characteristics.
- This will result in granular disintegration with the rock.
- This is the process that was thought to result in the creation of exfoliation domes like Domboshava.
- This form of weathering is also known as thermal expansion or insolation weathering.
NB At Ordinary Level you are supposed to accept this method of weathering as fact. The occurrence of this method of weathering in the real world is nowadays seriously doubted after a series of experiments by people like David Griggs and others.
- It occurs in rocks that have crevices and joints and where there is limited vegetation cover and temperatures revolve fluctuate around 0°C.
- During the day when temperatures are warmer, water enters the crevices or joints.
- When temperatures fall at night the water freezes leading to breakdown:
- Ice occupies more space than water i.e as the temperature of water falls below 4°C it starts to expand.
- As the water freezes within the rock it attracts more small particles of water which forms more ice crystals and expands further, fueling the process.
- With each freeze and thaw cycle the joints expand until the rocks shatter and fall off the main rock as blockfields,scree and talus.
- The process is also known as frost wedging
- Saline(water containing salt) water enters pore spaces in rocks.
- As it evaporates salt crystals are likely to form.
- This process usually occurs in deserts where water is drawn to the surface of rocks (sandstone rocks) by capillary action.
- As the crystals become larger they exert stress upon the rocks causing it to disintegrate ( granular disintegration).
- Salt crystallization occurs on costs where there is a ready supply of salt water.
- The process results in the development of weathering pits.
- Intrusive granite landforms such as batholiths are formed deep below the surface and under intense pressure due to the weight of the overlying overburden.
- If the overburden is removed by denudation processes there is a substantial reduction in pressure.
- The reduction in pressure causes fractures to develop especially on the top layers of the rock.
- These cracks develop parallel to the surface in a process that is referred to as sheeting.
- This results in the formation of exfoliation domes like Domboshava
To access more topics go to the Geography Notes page.