1.2 WEATHERING Flashcards
What is weathering?
The disintegration and decomposition of rocks and sediments by mechanical/chemical processes acting at/near Earth’s surface in situ.
What is erosion?
Process whereby particles primarily of rocks or soil are detached and transported by wind/water/ice.
Mechanical/Physical Weathering?
Physical disintegration of a rock into smaller fragments, each with the same properties as the original. Occurs mainly by temp/pressure changes.
Chemical Weathering?
Alteration/change in composition of a mineral due to the action of chemical agents.
What types of mechanical/physical weathering are there?
- Frost wedging: water percolates into joints; ice occupies more volume than water so breaks apart.
- Salt wedging: similar mechanisms to above, but due to growth of salt crystals. Typical of arid regions.
- Thermal expansion/contraction: different minerals expand and contract at different rates; requires extreme diurnal temp changes.
- Exfoliation: spalling of surficial layers due to release of lithostatic pressure & expansion as rock (esp. plutonic igneous) is exhumed. Also occurs as a result of chem. weathering e.g. along joint systems.
- Root wedging: interaction between biology and solid rock.
What are the main processes of chemical weathering?
- Dissolution
- Hydrolysis
- Hydration
- Oxidation
- Reduction
DISSOLUTION (sometimes just “solution”)
Chem. reactions between minerals and acidic/alkaline water.
- most rainwater slightly acidic due to dissolved CO2 - calcite in limestones dissolves.
Process of limestone dissolution is also known as CARBONATION.
- first carbon dioxide dissolves in water to make carbonic acid
- then carbonic acid reacts with calcite to form calcium and bicarbonate ions
Water percolates down joints of limestones, widening them.
Solution of material also commonly driven simply by polar nature of the water molecule.
HYDRATION
Absorption of water
e.g. anhydrite (CaSO4) accepts 2H2O to become gypsum (CaSO4 2H2O)
HYDROLYSIS
Chemical reaction between H+ & OH- ions of water and ions of mineral
e.g. orthoclase (potassium feldspar)
orthoclase is replaced by kaolinite as cations lost.
In weathering of silicates both water and carbonic acid are commonly involved e.g. olivine.
Silicate weathering consumes CO2 - AND IS THEREFORE MAJOR LONG-TERM CONTROL ON ATMOS. CO2 CONC.
OXIDATION (loss of electrons)
Chemical reaction with oxygen often dissolved in water- commonly oxidation then hydration:
the fate of all iron bearing silicates e.g. Fe pyroxene from basalts/gabbros etc.
REDUCTION (gain of electrons)
Commonly driven by microbial action in sediment pore waters.
Driven by demand for oxidants for metabolism. Microbially mediated.
REDOX 1
- once oxygen is used up, other oxidants are used
- this is reflected in change in ion concentrations in pore waters
- thickness of brown layer in deep sea sediments is related to the flux rate of organic carbon
REDOX 2
Bacterially mediated reactions:
- aerobic respiration
- nitrate reduction
- manganese reduction
- iron reduction
- sulphate reduction
- methanogenesis
WEATHERABILITY SERIES FOR COMMON IGNEOUS SILICATE MINERALS (GOLDICH)
Least to most stable: - olivine - augite - hornblende - biotite - k-feldspar - muscovite - quartz most stable are slowest weathering & most mature
FACTORS THAT INFLUENCE CHEMICAL WEATHERING
- Mineral composition; goldich dissolution series (weathering susceptibility)
- Climate; temp, contrasts, rainfall etc
- Living organisms; bioturbation, acid production/mineral decomposition
- Time
chemical weathering products= clays, metals ores, rounding of boulders (chemical exfoliation)
CHEMICAL WEATHERING - SOILS
REGOLITH: layer of unconsolidated particulate material covering bedrock
SOIL: part of regolith containing weathered rock, water, air and humus (org matter derived from decay of plant material)
O horizon: org debris, humus
A horizon: humus mixed with minerals derived from bedrock
B horizon: accumulation of dissolved material and fine clays, hardpan
C horizon: weathered and broken bedrock
SOIL TYPES
PEDALFERS: developed in humid, temperate regions
PEDOCALS: develop in dry warm regions like western U.S.
A horizon is thin
B horizon contains minerals such as calcite that would never precipitate in moister environments. These form nodules of calcite (caliche)
LATERITE: soil formed in tropics where rainfall so intense all soluble minerals completely leached, even silica. Rainfall so heavy & decomposition so fast that there’s no significant O horizon. A horizon may be thick but lacks soluble materials - typically rich in insoluble iron and aluminium oxides incl. bauxite (al ore). B horizon is absent.
