Unit 1 - Weathering

Weathering

• A group of processes that breaks down rocks, soils and minerals into smaller pieces through contact with the earth’s atmosphere, biota (Fawn & Flora) and waters

• Movement of eroded materials does not occur

• Process is usually very slow

  • Can take thousands of years

• Responsible for the creation of soil and many other unique features

  • Like the fish river canyon in southern Namibia

Physical Weathering

• The breakdown of rocks into smaller components without changing their chemical composition

• Some things that can cause physical weathering:

  • Changes in temperature

  • The freezing and thawing of water

  • Ground Ice (any type of ice that exists within the ground, particularly in permafrost regions)

  • Wetting and drying of rocks

• Three main types of physical weathering:

  • Granular (Small grain) disintegration

  • Exfoliation

  • Block Separation

Granular (Small grain) disintegration

• Physical Weathering

• Occurs when rocks break up into small grains or pieces

• Forms sand or very small pieces of gravel

Exfoliation

• Physical Weathering

• When rocks weather by peeling off in sheets rather than breaking up into small pieces of gravel

• This can happen in thin layers on large rocks

Block separation

• Physical Weathering

• Occurs when rocks break up into large fragments related to rock joins

• The main cause of this is frost shattering

  • Takes place when ice is formed in the joins of rocks

  • The ice expands and breaks up rocks

Chemical Weathering

• The breakdown of rocks into particles with a different mineral composition or make-up than the original rock

• Water, oxygen and carbon dioxide are agents of chemical weathering

• Common examples of chemical weathering:

  • Solution

  • Hydrolysis

  • Hydration

  • Oxidation

  • Carbonation

Solution Weathering

• Chemical Weathering

• When the minerals or soluble chemicals in the rock dissolve directly in water

Hydrolysis Weathering

• Chemical Weathering

• Occurs when there is a chemical reaction between the minerals in the rock and hydrogen in rainwater

• Eg:

  • During hydrolysis feldspar (a rock-forming mineral) in granite changed to a clay mineral which breaks easily

Hydration Weathering

• Chemical Weathering

• Occurs when minerals in rocks absorb water and expand

• The expansion of minerals causes the rock to break up

Oxidation Weathering

• Chemical Weathering

• Occurs when rock minerals react with dissolved oxygen to produce red or yellow oxides or hydroxides

• Red and yellow soils found in tropical areas are the result of this process

Carbonation Weathering

• Chemical Weathering

• Occurs when carbon dioxide dissolves in water to form a weak solution of carbonic acid

• Carbonic acid can weather carbonate rocks such as chalk and limestone

Biological Weathering

• Involves living things

• It involves physical actions such as:

  • The breaking of rocks by plant root growth or animal burrows

• It also involves chemical actions such as:

  • The excretion of acids

    • Some organisms secrete acids that can chemically weather rocks and minerals

• Another form of biological weathering is the decomposition of organic matter such as animals, plants and leaves, which can release corrosive substances into the environment

Impact of human activities on weathering

• Humans can increase the rate of weathering on rocks or buildings

• Examples:

  • The burning of fossil fuels or release of chemicals into the air makes the atmosphere acidic

    • When it rains, acid rain can eat into limestone or buildings made of limestone and break it down by chemical weathering

  • When making roads people use dynamite to blast and break rocks into smaller pieces

  • During mining operations people dig up large rocks and break them up

    • Quarrying is also another activity that contributes to weathering