Introduction
Weathering refers to the breakdown and decay of rocks that occur in situ (on the spot) under the influence of atmospheric agents, such as temperature, water, and organisms. Weathering is not the transportation of material as is the case for erosion.
Types of Weathering
1. Physical (Mechanical) Weathering
Disintegration of rock without change in chemical composition.
- Unloading: When overlying layers of rock are removed, this releases pressure and causes expansion, which may create cracks (as in exfoliation domes).
- Temperature Changes:
- Block disintegration: Rocks break into blocks due to alternate heating and cooling, common in deserts.
- Granular disintegration: Granite disintegrates in to grains.
- Frost Action (Frost Wedging): Water in cracks freezes, expands (~9% volume increase), and widens joints. Common at high altitudes and high latitudes.
- Salt Weathering: Salt crystals grow in the pores, thus fracturing the rock. Common in dry and coastal areas.
- Insolation Weathering: This is the differential expansion and contraction caused by temperature differences over the course of a day.
2. Chemical Weathering
Decomposition of rocks through chemical reactions that alter mineral composition. Dominant in warm, humid environments.
- Solution: Minerals are directly dissolved in water (such as rock salt, gypsum).
- Carbonation: Carbon dioxide combines with water to form carbonic acid, which dissolves limestone — the basis of karst topography.
- Hydration: Water is absorbed by minerals and the minerals expand, thereby reducing the strength of the rocks.
- Oxidation: Minerals, particularly iron-bearing minerals, will combine with oxygen, creating reddish-brown coatings of oxides (rusting effect) which will weaken the rock.
- Hydrolysis: Water reacts with minerals like feldspar to form clay minerals (e.g., kaolin).
3. Biological Weathering
Breakdown of rocks due to the activity of living organisms.
- Mechanical biological action: Root wedging due to plants and burrowing done by animals.
- Chemical biological action: Organic acids produced by lichens, moss and microbes lead to decomposition, as do human activities such as agriculture and construction.
Factors Controlling Weathering
- Climate: Temperature and rainfall determine dominance of physical vs chemical weathering.
- Rock Structure: Joints, fractures and bedding planes are sites where weathering agents can enter.
- Mineral Composition: Minerals that are soluble in water will chemically weather rocks more quickly.
- Slope and Relief: Steep slopes promote physical weathering; gentle slopes retain moisture, favouring chemical weathering.
- Vegetation Cover: Improves biological weathering and retention of moisture.
Significance of Weathering
- Produces regolith and soil, essential for agriculture and ecosystems.
- Gathers rock debris for erosion and mass movement.
- Forms characteristic landforms:
- Exfoliation domes (physical weathering)
- Karst landscapes (chemical weathering – carbonation)
- Laterite soils (chemical weathering – leaching in tropics)
- Tors and inselbergs are formed by differential weathering of granite.
- Influences denudation rate and landscape evolution.
Weathering in Different Climatic Regions
- Humid Tropical Regions: High temperature and abundant moisture favour chemical weathering. Extensive deep weathering profiles and clay mineralisation are common.
- Arid Regions: Chemical weathering is limited by low moisture, thermal expansion, salt crystallisation and mechanical disintegration are significant.
- Cold Regions: Frost weathering and repeated freezing & thawing results in angular rock fragments.
- Temperate Regions: Physical and chemical processes are both present and seasonal variations affect their relative importance.
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