Lecture 10 - Weathering Flashcards
1
Q
what is weathering:
A
- the breakdown, but not removal, of geomaterials (rocks and regoliths*) on Earth’s surface.
- *regolith = layer of loose rocks and dust covering solid rock on planets, moons, and asteroids.
2
Q
why is weathering important:
A
- essential precursor to almost all other geomorphological processes.
- without weathering there would be no soils, no soils = no plants, no plants = no people.
- weathering shapes Earth’s surface and creates landscapes and landforms that we observe and study in geography.
3
Q
how does weathering link with other Earth surface processes:
A
- weathering + mass wasting + erosion = denudation.
- weathering = Earth surface process of rock and mineral breakdown, dissolving, and transformation in preparation for transport, weathering can be chemical, biological, or physical, weathering is a slow repetitive process, critically occurs in situ.
- mass wasting = downslope movement of material (soil, sediment, rock) mostly driven by force of gravity, but often triggered by other processes, also known as slope movement or mass movement.
- erosion = Earth surface process of wearing away, removal, and transport of rocks, soils, and minerals by the wind, water, ice, and gravity.
- denudation = includes all process (weathering, erosion, and mass wasting) that work to reduce the relief, both of individual landforms and topography, a key process in geomorphology.
4
Q
weathering and equilibrium:
A
- equilibrium is constant relation between input and output or form, towards which a landform tends or around which it fluctuates in time.
- rocks are forms under conditions that are very different to surface, therefore are not in equilibrium with surface conditions.
- weathering occurs to create more stable minerals and forms.
- rock forms from magma in subsurface, outward pressure balanced by inward pressure of soil/overburden.
- overburden lost through weathering and erosion, outward pressure no longer balanced and exfoliation/fracturing occurs.
5
Q
weathering and minerals:
A
- every mineral has unique chemical composition and atomic arrangement = every mineral will resist and respond to weathering in a different way.
- majority of Earth’s crust is made up of small number of minerals, most common of these are feldspar and quartz (both silicates) and make up ≈90% of the crust.
- Bowen’s reaction series described in early 20th century and orders igneous rock by temperature of formation and resulting stability and susceptibility to weathering.
- other controls such as age of rock, exposure to weathering processes, and local conditions mean weathering patterns and rates in series are not always followed.
- weathering of minerals generally follows expected patterns.
- arrows show course of weathering for typical rock types, lengths of arrows indicate relative rates of change.
- clay minerals and solutions are ultimate product of chemical weathering.
6
Q
importance of water (H2O):
A
- water has ‘triple point’.
- can freely change between 3 states under normal Earth surface conditions = solid, liquid, gas.
- state changes of water facilitate wetting and drying.
- also facilitate changes in volume e.g. water expanding as it becomes ice.
- covalent bond between O and H (shared electron) is uneven (O attracts electrons more strongly than H) = water is polar molecule with asymmetric charge distribution.
- polarity allows water to separate other polar molecules, making water an excellent solvent.
- dipole-dipole interactions work in same way as magnets.
- water can undergo self-ionisation (break apart) to produce: 1 - positively charged hydrogen ions (H+)(cation)(acidic), 2 - negatively charged hydroxyl ions (OH-)(anion)(basic).
- water is amphoteric = it can act as either acid or base in chemical reactions.
- H3O+ = hydronium ion.
7
Q
different types of weathering:
A
- physical weathering (disintegration) - application of external stress or release of accumulated internal stress, reduces size of material, does not change composition.
- chemical weathering (decomposition) - involves water, changes composition of material (dissolves minerals, changes chemical composition).
- biological weathering (not always considered as separate category) - breakdown of material by living things by either physical or chemical weathering processes.
8
Q
what is physical weathering:
A
- mechanical processes that cause rock and regolith disaggregation into smaller fragments, through application of external stress or release of accumulated internal stress.
- breakdown takes place because of physical processes e.g. temperature and pressure changes, abrasion.
- processes are typically slow and repetitive, and disintegration is caused by resulting stress and fatigue.
9
Q
processes - freeze-thaw:
A
- repeated cycles of freezing and thawing occur until rock eventually breaks apart as cracks propagate.
- factors affecting freeze-thaw weathering: rock type, porosity, internal structure, water absorption.
10
Q
processes - salt weathering:
A
- associated with growth and expansion of salt crystals in pores and fractures.
- accumulation of salts is needed to generate enough stress to cause failure.
- regular and frequent volume changes from repeated solution and crystalisation, hydration and dehydration, expansion and contraction, cause fatigue.
- process favours warm, humid, sheltered environments.
- thermal expansion of salts can be greater than enclosing rock.
11
Q
processes - insolation (thermoclasty):
A
- temperature change causes expansion and contraction of rocks.
- changes can occur over different scales (seasonal, diurnal, hourly - sun/shade).
- cracks and exfoliation occur.
- repeated stress causes fatigue and failure.
- thermal shock can also occur e.g. during a fire.
12
Q
processes - dilation (pressure release):
A
- typically occurs when rocks formed at depth (intrusive igneous rocks e.g. granite) become exposed.
- pressure released and rock expands forming cracks.
- shear stress is parallel to surface, resulting in sheet joints.
13
Q
what is chemical weathering:
A
- chemical processes that dissolve, transform, and remove more soluble rocks, sediment, and minerals.
- requires presence of water.
- generally occurs at surface of rocks therefore surface area, porosity, and permeability are critical.
14
Q
processes - solution:
A
- simple process where soluble mineral comes in contact with water and results in dissolution.
- water is effective solvent because is polar molecule (has both + and - charges).
- controls are pH, Eh (redox potential), temperature, saturation of solvent.
- halite = rock salt (NaCl).
- NaCl is ionic compound.
- has chloride anion (-) and sodium cation (+).
- NaCl + H2O > Na+ + Cl- + H2O.
15
Q
processes - hydrolysis:
A
- reaction between mineral and slightly acidic water.
- substitution of H+ in acidic water for cation (+) in solid minerals - proton donation.
- simplified reaction for orthoclase = 2KAlSiO8 + 2(H+OH-) > Al2Si2O5(OH4) + 4SiO2 + K2O.
- orthoclase + ionised water > kaolinite (clay) + quartz + soluble potassium oxide.
- dring hydrolysis, cation are selectively replaced within crystal lattice, known as congruent dissolution.
- monovalent e.g. Na+K+ > divalent e.g. Ca2+Mg2+ > polyvalent e.g. Al3+Fe3+.
16
Q
processes - carbonation:
A
- when minerals react within carbonic acid.
- best known example = weathering of limestone, another example = carbonic acid in fizzy drinks eroding teeth.
- CO2 dissolves in water (rain) to create carbonic acid (H2CO3).
- carbonic acid reacts with limestone (CaCO3) to produce soluble calcium bicarbonate, is then washed away.
17
Q
processes - hydration:
A
- process where water is added to chemical structure of mineral.
- produces new and generally relatively weak mineral compounds.
- opposite of hydration is dehydration, where water is removed.
- example = absorption of water by anhydrite which forms gypsum.
- reaction causes 35% increase in volume making gypsum less stable.
- repeated hydration and dehydration weakens structure of rocks, making them more susceptible to physical weathering.
18
Q
processes - oxidation:
A
- reaction of minerals with oxygen in presence of water.
- consists of addition (oxidation) or removal (reduction) of oxygen to/from mineral producing new mineral compounds.
- example = conversion of pyrite into hematite and sulphur as ferrous iron (Fe2+).
- iron becomes red when oxidised (rusting).
- oxidised minerals are less resistant to weathering.
19
Q
what is biological weathering:
A
- breakdown of rocks and minerals by living organisms.
- can be both chemical and physical.
- physical examples = boring and burrowing animals, growth of plant roots, footpath erosion.
- chemical examples = reaction with CO2 from respiration and oxygen from photosynthesis.
20
Q
weathering in combination:
A
- weathering processes tend to act in combination or sequence.
- e.g. physical weathering processes create cracks and increase surface area, exposing more rock to chemical weathering processes.
- this increases rate of weathering.
21
Q
controls over weathering:
A
- weathering is competition between environment and material.
- key controls:
1. moisture (H2O).
2. rock and mineral types.
3. temperature.
4. biological activity.
5. surface/topography.
6. substrate properties (permeability, joints, porosity).
7. time.
22
Q
climate and weathering:
A
- no weathering process can be ascribed to a particular climatic zone.
- however, environmental conditions found in climatic zones control dominance and intensity of chemical vs. physical weathering (Peltier, 1950).
- Strakhov’s model also describes how depth and intensity of weathering relates to climate.
23
Q
importance:
A
- Earth is closed system where materials cycle between lithosphere, atmosphere, hydrosphere, and biosphere.
- weathering is key process that helps transform matter in Earth system cycles (1 - rock cycle, 2 - carbon cycle, 3 - nutrient cycle).
24
Q
rock cycle:
A
- in rock cycle, weathering is one of processes that creates and alters rocks and minerals.
- other denudation processes (erosion, mass wasting) move rocks through cycle.
25
Q
carbon cycle:
A
- in carbon cycle, weathering is one of processes that draws carbon out of atmosphere by carbonation.
- carbon is then deposited on sea floor where it joins lithosphere.
26
Q
nutrient cycle:
A
- in nutrient cycle, weathering is one of processes that releases nutrients into soil.
- rocks break down into regolith and combine with organic matter forming soils.
