Weathering and erosion

Physical/mechanical weathering

• The break down or disintegration of rock without any chemical alteration

• Breaks up bedrock into smaller blocks, increasing surface area, making chemical weathering more efficient

• More likely to occur in areas devoid of vegetation, such as deserts, high mountains and arctic regions

Insolation weathering (what?)

• Exfoliation: Stresses cause the outer layers to peel off

• Block disintegration: Where rocks splits along joints/cracks/lines of weaknesses

• Granular disintegration: Disintegrate grain by grain

• Common in well-jointed rocks (basalt, granite)

Insolation weathering - exfoliation (how?)

• In day time, the absence of clouds in tropical deserts & the overhead sun produces high day time temperatures. The sun’s heat causes the outer layers of the rock to expand.

• At night time, with no clouds, rocks cool quickly by radiation cooling. Cooling causes the outer layers of rock to contract.

• Different minerals expand and contract at different rates causing stresses along mineral boundaries.

• Repeated daily heating & cooling causes outer layers of rock to be weathered.

• Layers or shells of rocks peel away.

Conditions for Insolation weathering

• Common in deserts where cloud cover and vegetation cover are minimal

• Daily range of temperature can exceed 50˚C (high in day, low at night)

• Rocks containing different minerals (darker coloured minerals absorb more heat and will expand more than lighter coloured minerals

• Joints/lines of weaknesses in rocks

Salt crystal growth (what?)

Physical weathering caused by the growth and thermal expansion of salt crystals

Salt crystal growth (how?)

• Salt solution enters pores in surface

• As water cools or evaporates, salt crystals form in the rock.

• When the rocks are heated, the salt crystals expands, putting pressure on the rock.

• Over time, this splinters the rock into fragments.

• After this (haloclasty) occurs, the rock can be split apart or be left with many pits formed by salt crystals (honeycomb weathering).

• Granular disintegration - split in small pieces

Conditions for salt crystal growth

• Salt

• Water

• Presence of joints, fractures, bedding planes, pore spaces

• Coastal environment (salt in seawater)

• In hot desert environments (high evaporation due to high temperatures, results in salt precipitates)

Chemical weathering

• Actual decomposition of rock

• Involves reaction between air and water and minerals in rocks

• Minerals may combine with oxygen or carbon dioxide from the atmosphere, or dissolve and combine with water

• More likely to take place in warmer, more humid climates where there is vegetation cover

• Usually forms clays

Oxidation (what?)

• When certain metallic elements in rock minerals combine with oxygen to form oxides.

• Causes red or yellow coloration in rocks

• Rocks crumble more easily

Oxidation (how?)

• Occurs through contact between minerals and water containing dissolved oxygen from the soil or atmosphere

• Oxygen reacts with iron in minerals to form iron oxide minerals

Conditions for oxidation

• Air, specifically oxygen

• Iron mineral present in rock

• Heat to speed up chemical reactions

Carbonation (what?)

Rock minerals reacting with carbonic acid

Carbonation (how?)

• Carbonic acid is formed when water combines with carbon dioxide

• Rainwater contains carbon dioxide in solution, which produces carbonic acid. Carbonic acid reacts with rocks containing calcium carbonate and breaks down or dissolved minerals in the rock

• The dissolved rock is removed in solution by running water.

Condition for carbonation

• Water

• Calcium carbonate in rock (limestone)

• Humid climates (both tropical and temperate)

Biological weathering definition

The breakdown of rocks and minerals by the action of living organisms such as plants, animals and microorganism

Biological weathering - physical actions

• Animals digest, abrade and mix mineral grains

• Burrowing allows water and air into soil

• Roots of trees - grows into crack and joints, widen and deepen joints, loosen rock structure

Biological weathering - chemical actions

• Humid acids from decaying plants

• Respiration - CO2 + H2O are acidic

• Organic acids from plants and micro-organisms

Factors influencing the type and rate of weathering

• Rock characteristics

  • Hardness - minerals that for. the rocks and how the minerals are cemented together

  • Structure

    • Joints: increase the surface area of rock exposed to physical and chemical weathering

    • Fracture or separations in rock that occur without displacement of the sides

  • Composition: darker coloured minerals vs lighter coloured minerals (rate of expansion and contraction → insolation weathering)

  • Grain size: coarser-grained rocks wether faster than fine-grained rocks

  • Permeability: depends on grain size, shape and packing of grains

  • Porosity: volume of water which can be held within a rock or soil

• Climatic elements

  • Physical weathering increases with increasing temperature fluctuations most effective in temperature regions and desert areas

  • High temperatures allow for faster chemical reactions, high rainfall allows for more water available for carbonation

  • Rainfall, temperature

• Vegetation

  • Produces organic acids from partial decay of organic matter which contribute to chemical weathering

  • Plant roots can enter crevices and break up (biological weathering) exerting enough pressure to widen and/or deepen creaks to break the rock apart → expose greater surface area of rocks to other weathering processes

• Relief

• Human activities