Exogenetic Processes: Mechanical, Chemical, and Biological Weathering (copy)
Mechanical Weathering
Definition: Mechanical weathering is the disintegration of rocks into smaller particles without any change in their chemical composition.
Thermal Expansion and Contraction:
Minerals contained within rocks expand when exposed to heat and contract when the ambient temperature decreases.
Different minerals react differently to changes in temperature; certain minerals expand significantly more, while others expand to a lesser degree.
Consequently, variable expansion and contraction rates occur across different mineral grains within the same rock.
Due to continuous alternating cycles of contraction and expansion, tension repeatedly develops, increases, and decreases among the rock particles.
Over time, this fluctuating internal stress leads to the development of cracks within the rocks, eventually causing them to break apart.
Frost Action (Freezing of Water):
Occurs in geographical regions where temperatures drop below for prolonged periods.
Water accumulates within the existing cracks and crevices of rock structures.
As the temperature drops below , the trapped water freezes into ice.
The volume of water expands upon freezing, generating massive outward pressure and tension within the rock walls.
This elevated pressure forces the cracks wider, causing the rocks to shatter into fragments.
Chemical Weathering
Definition: Chemical weathering is the process of rock decomposition resulting from alterations in their internal chemical composition.
Carbonation:
Atmospheric rainwater mixes with carbon dioxide present in the air during precipitation, resulting in the formation of dilute carbonic acid.
Rocks rich in minerals such as limestone dissolve easily when exposed to these dilute carbonic acid solutions.
Solution:
Water dissolves specific soluble minerals contained directly within the rock matrix upon contact.
The dissolution of these internal minerals and alkalis weakens the bonding structure of the rock, making it brittle and susceptible to rapid breakdown.
Oxidation:
Takes place in rock types that contain substantial amounts of iron.
When iron within the rock comes into direct contact with water, a chemical reaction occurs between the iron and dissolved oxygen.
This chemical reaction creates a reddish-colored layer across the rock surface, which is referred to as rust.
Oxidation occurs primarily in regions experiencing high rainfall.
Biological Weathering
Definition: Biological weathering is the process by which rocks are broken into small fragments and fine particles through the active processes of living organisms, including plants, animals, microorganisms, and human beings.
Plant Activity:
Plant roots extend into the natural joints and crevices of rocks in search of subsurface moisture.
As the root systems mature and grow larger in diameter, they exert increasing pressure and internal tension against the surrounding rock.
This wedging force progressively expands the fractures, causing the rock to split and break.
Animal and Insect Activity:
Burrowing animals such as mice, rabbits, and rats, alongside insects constructing anthills, excavate holes into ground and rock formations.
These subterranean activities disrupt and weaken the structural integrity of the rock, causing it to loosen and disintegrate into smaller pieces.
Flora and Microorganism Growth:
Biological organisms such as algae, moss, lichens, and other specialized flora grow on damp rock surfaces.
The physical presence and metabolic processes of these organisms contribute directly to the physical disintegration and chemical breakdown of the underlying rock.