WEATHERING refers to the process of weakening, breaking up, and disintegration of the rock that forms the surface of the ground and that lie exposed to the weather.
weathering is divided into 3 types namely; physical or mechanical weathering, chemical weathering, and biological weathering
physical or mechanical weathering
Mechanical or physical weathering is any of the various weathering processes that cause the physical disintegration of the exposed without any change in the chemical composition of rocks.
6 processes of physical/mechanical weathering.
Frost action (freeze/thaw effect)
this is the breaking of rock due to pressure exerted by freezing water into rock cracks or joints.
When water freezes its volume increases. alternate freezing and thawing, therefore, exert pressure on the sides of cracks and joints in rock and this leads to the weakening and disintegration of rock.
Pressure release
this occurs when pressure is released when rock buried deep down in the earth’s surface is exposed due to erosion.
Deep in the crust for instance where a pluton might be emplaced, rock experiences high confining pressure equal to the weight of all underlying rock.
If an area is uplifted and country rock originally around the pluton is removed by erosion, the pluton experience a progressive declining confining pressure.
The rock body responds by expanding outward once the pressure is low enough, usually, once the plutons are completely exposed.
The expansion of the rock body causes it to break along fractures more or less parallel to its surface called sheet joins. The sheet thus produced can fall away or exfoliate or spall. The result is a dome-shaped rock body called an exfoliation dome. A good example of an exfoliation dome is a stone mountain in central Texas.
Temperature change (thermal expansion and contraction)
as any material is heated it expands and vice versa as it is cooled it contracts.
In materials like a rock with many crystals, this expansion and contraction can cause mechanical fracturing.
In rock such as granite, where many minerals exist and are oriented in many directions the effects of crystal expansion are of great importance.
Mechanical weathering due to temperature change is common in arid and semi-arid regions, particularly in the hot deserts.
Alternate wetting and drying
this occurs when the rock absorbs a certain amount of water but some absorb more water than others which causes them to swell, and when they become dry they shrink and start to disintegrate.
This type of weathering is common especially in West Africa coastal rocks, which wet and dry up alternatively through rising and falling of tides. It is common also along river banks and sea cliffs
Salt crystal growth
this causes the disintegration of rock when saline solution seeps into cracks and joints in the rock and evaporate leaving salt crystals behind.
These salt crystals expand as they are heated up, exerting pressure on the confining rocks.
This type of mechanical weathering is associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization.
It is also common along coasts.
The action of plants and animals
plant and animals break the rock into smaller pieces in a variety of ways.
Plant develop strong roots that grow into existing rock cracks and as the roots grow they expand and add pressure to the crack until small pieces of rock begin to flake away.
Also burrowing animals, like moles and rabbits, dig holes that expose the new rocks to the effects of weathering
The holes allow water and other weathering agents to reach the rock layer that had been covered by soil.
chemical weathering
Chemical weathering is the weakening and subsequent disintegration of rock by chemical reactions.
The following are 5 processes of chemical weathering
Hydrolysis
This is the chemical breakdown of substances when combined with water. Therefore hydrolysis is the breakdown of rock due to the reaction between rock and water.
The most common example of hydrolysis is feldspar which can be found in granite changing to clay. When it rains water seeps down into the ground and comes into contact with granite rock. The feldspar crystals within the granite react with the water and are chemically altered to form clay minerals that weaken the rock.
Oxidation
This is the weakening of the rock due to its reaction with oxygen. Oxidation is the process that causes rust. It occurs when oxygen in water reacts with the mineral iron in rocks, which causes them to rust. Oxidation is effective in presence of moisture. An example of oxidation is when the ferrous oxide is oxidized to ferric oxide which is reddish-brown in color.
Carbonation
this is the mixing of water with carbon dioxide to form carbonic acid. It occurs in limestone and chalk regions where the weak carbonic acid reacts with limestone or chalk to form calcium hydrogen carbonate which dissolves in water. The process of carbonation is responsible for the formation of surface and subterranean limestone features in the karst region.
Hydration
is the absorption of water in the mineral structure. A good example of hydration is the absorption by anhydrate resulting in the formation of gypsum. Hydration expands volume and also results in rock deformation.
Dehydration
This is the removal of water from rock or mineral structures. A good example of dehydration is the removal of water from limonite, resulting in the formation of hematite.
biological weathering
biological weathering is weakening and subsequent breakdown by plants, animals, and microbes of rocks
growing roots of plants can put stress or pressure on a rock causing it to weaken and disintegrate
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