Weathering: Types and Rate of Weathering

Weathering: Disintegration of rocks

in situPhysical

Weathering Size and shape of rocks

altered but not the chemical composition

Angular and coarse end-products

Some involvement of water in the processes (e.g. freeze-thaw and salt crystal growth)

Effective in extreme climate conditions

Chemical Weathering

Physical structure and chemical composition of rocks altered

Finer end-products (e.g. clay and sand)

Water (rainwater, seawater) is important as the main solvent in all processes

Effective in vegetated areas and hot and wet climate

Chemical Weathering

Hydrolysis

Carbonation

Oxidation

Solution

Karst Landscape

Insolation Weathering

Pressure Release

Salt Crystal Growth

Freeze-Thaw Action/ Frost Shattering

Factors affecting type and rate of weathering

Factor 1: Rock Hardness

Rock Hardness- dependent on minerals that form rocks and how minerals are cemented together

Igneous stronger than Sedimentary rocks. Why?

Least important factor since all rocks ultimately experience weathering once they are exposed to weathering agents

Factor 2: Mineral composition

Influence rock resistance to chemical weathering

Weaker minerals: calcium carbonate, olivine and pyroxene

Stronger minerals: quartz

Influence also physical weathering like insolation weathering because of differences in coefficient of expansion and contraction of minerals

Factor 3: Grain size of rocks

Coarse-grained rocks weathered down faster than fine-grained rocks

When minerals susceptible to changes are altered (e.g. feldspar), larger gaps are formed, weakened structure, larger gap provides bigger surface area for further chemical attack

Factor 4: Lines of Weakness of rocks

Lines of weakness (e.g. cracks, joints, bedding planes) found in rocks make them more susceptible to both physical and chemical weathering. How?

The more well-jointed the rock is, the faster the rate of physical, chemical and even biological weathering

Factor 5: Relief

On flat land, soil and weathered materials can retain water which seeps through them and acts on the underlying rocks chemically

On slopes, mass movements, like landslides and slumping, expose more underlying rocks for physical weathering.

Chemical weathering less effective on steep slopes as water flows away

Factor 6: Vegetation cover

Vegetation cover can retain rainwater and keeps underlying rocks moist for chemical weathering to take place

Biological weathering common too

Factor 7: Human activities

How do human activities contribute to weathering?

Weathering regions of the world in relation to mean annual

temperature and precipitation (Peltier,1950)