Introduction to Weathering and Erosion Flashcards
Weathering definition
The breakdown of rock to form transportable material
Weathering can be cause by complex interaction between…
- Tectonics
- Physical, chemical and biological processes
Factors that control weathering rates
1) Properties of rock - stability of minerals and structures within rock
2) Climate - rainfall and temperature
3) Presence of soil - soil formation speeds weathering, changes pH
4) Length of exposure time
5) Topography - steeper/higher = higher weathering rate
Two types of weathering
- Mechanical Weathering: no change in chemical composition just disintegration into smaller pieces
- Chemical Weathering: breakdown as a result of chemical reactions
Under what conditions is mechanical weathering prominent?
Arid and/or cold climates
What are the agents of mechanical weathering
- pressure release
- water: freeze-thaw cycles
- water: wetting and drying
- water: crystallisation of salt in cracks
- heat: thermal expansion and contraction
How does this promote further weathering
increase surface area exposed to weathering processes promoting further weathering
How does pressure release occur
- Rocks form at depth and are confined by pressure
- As a rock mass is uplifted it is brought closer to the surface
- the confining pressure decreases, rock mass moves out of equilibrium and expands forming joints
- virtually all rock masses at or near the surface contain joints
What happens in the exfoliation process
As rock is uncovered it expands (due to lower confining pressure) resulting in exfoliation and the rock breaks apart in layers parallel to the surface
Forst Wedging
- Rock breakdown is caused by expansion of ice in joints
- shattered rocks common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart
Wetting and Drying occurs in…
many fine-grained sedimentary rocks e.g. mudstones, contains clays
Wetting and Drying process
- clays absorb significant amounts of water which causes them to expand
- clays release water as they dry causing them to shrink
- repeated wetting and drying causes repeated expansion and shrinking which breaks up the rock into small pieces
Salt Crystallisation process
- saline solution seeps into joints in rocks
- moisture evaporates precipitating salt crystals
- crystals expand when heated
- expansion exerts pressure on confining rock
- also occurs when solutions decompose rock e.g. limestone
Thermal Expansion
Extreme range of temperatures can shatter rocks in desert environment due to the repeated swelling and shrinking of minerals with different expansion rates shatters rock
Role of physical weathering
- Reduces rock material to smaller fragments that are easier to transport
- increases the exposed surface area of rock making it more vulnerable to further weathering
- the increase in joint formation also leads to a pathway for water
Chemical Weathering
- involves chemical interactions between water, air and rock
- water interacts with minerals creating a mix of chemical reactions and products
Types of Chemical Weathering
- Dissolution
- Hydrolysis
- Oxidation
- Hydration
Dissolution process
- Complete removal of mineral from the rock and is strongly influenced by pH and T
- e.g. Carbonation and hydrolysis
Carbonation
- atmospheric CO2 dissolves in rainwater forming weak carbonic acid
- carbonic acid reacts with calcium carbonate (in limestone) forming calcium bicarbonate which is carried away by water
- biological activity in soils also generates substantial CO2
- process very effective within joint systems
- widens and deepens
Acid Rain
- unpolluted rainwater weakly acidic
- in polluted atmosphere SO2 and nitrogen oxides present
- react in rainwater to form stronger acids
Hydrolysis
- Chemical reactions where water changes composition and size of minerals in the rock
- Common in granitic rocks
Hydration and Oxidation
- Hydration is the chemical reaction where water is added to the crystal structure of a mineral and if it occurs in joint expansions it can cause rock to disintegrate
- Oxidation is the reaction of minerals with oxygen which manifests as red oxidised rock coatings and red soils
Chemical stability of minerals
- Hight T + P minerals (e.g. olivine, pyroxene) are least stable
- Low T + P minerals (e.g. quartz) are very stable
- Controlled by silicate structure)
Biological Weathering
- Can be both chemical and mechanical in nature
- For example roots split rocks apart (mechanical) or roots produce acids that dissolve rocks (chemical)
