Weathering and Soils Flashcards
What is Weathering?
The break up (physical disintegration) and decomposition (chemical alteration) of solid rock by surface processes
Solid rock → loose unconsolidated sediment and/or dissolved ions (weathering does not involve transport)
where does weathering occur?
at interface: solid earth meets hydrosphere, atmosphere, biosphere
Why is weathering important?
Provides sediment
-Contributes to soil formation
-Wears down mountains built by plate tectonic processes
-Shapes landforms
Important role in carbon cycle (reactions consume CO2, produce carbonate)
Some weathering processes (e.g. of mining waste piles) lead to toxic run-off
what are some products of weathering?
Loose debris (regolith)
Secondary minerals
Ions in solution
Weathering also leads to rounded surfaces and is essential for most landform & landscape development
what are the two types of weathering processes?
1. Physical (disintegration → smaller pieces) –no change in chemistry –increases surface area 2. Chemical (decomposition/alteration) –breaking of bonds –reactions alter or ‘stabilise’ minerals
both processes often work simultaneously
water is usually required
Physical/mechanical weathering increases the surface area exposed for …
chemical weathering
what are the two main processes of physical weathering?
(1) Ice wedging
2) Pressure unloading (exfoliation
what is ice wedging?
Water seeps into fractures and other openings
Freezes → expands volume by 9%
–exerts huge amount of stress…
–wedging, heaving, shattering
Significant in climates with freeze-thaw cycles – with each freeze, the rock is wedged further apart
what is pressure unloading?
Rocks formed at depth expand & ‘relax’ when exposed at surface
‘Unloading’ → exfoliation
Sheeting, exfoliation
Rock layers break off like an onion
what is chemical weathering?
Breakdown of minerals through chemical reactions with the atmosphere or hydrosphere
what are the three main processes of chemical weathering?
(1) Dissolution
(2) Hydrolysis
(3) Oxidation
what are the products of chemical weathering?
soluble ions (e.g.Ca2+) new secondary minerals e.g. clays, oxides
what is dissolution?
Minerals completely dissolve into soluble ions in the presence of water
e.g. calcite dissolves in weak acid
what is hydrolysis?
minerals react with water
H+ ‘attacks’ and replaces other cations in mineral
→ alteration to form new clay minerals
what is oxidation?
Iron (Fe) bearing minerals react with oxygen
e.g. rusting of solid iron → iron oxide
What minerals does oxidation affect?
Fe-bearing silicate minerals e.g. olivine, pyroxene, biotite, garnet
Sulfide minerals e.g. Pyrite FeS2 (when exposed to oxygen and water → sulfuric acid (H2SO4) + iron & hydrogen ions → acid mine drainage)
what is biological weathering?
(physical/chemical)
Tree roots: wedge into rock fractures → growth exerts sufficient pressure to split rock
Animal burrowing, boring
Lichens: secrete acidic compound
How does weathering shape landforms?
All landscapes experience weathering
Landforms result from differential weathering
–at the mineral scale up to rock outcrop scales
–more resistant ≈ ridges;
–weaker ≈ valleys
what creates karst topography?
dissolution of carbonate
e.g. Limestone pavement, W. Ireland or Karst towers, S. China
What controls weathering rates?
How susceptible is the rock?
Do the conditions promote weathering?
what are the primary controls of weathering rates?
rock type & mineral composition
climate (temperature, precipitation)
what are the secondary controls of weathering rates?
rock texture (grain size, porosity) topography (relief, aspect, drainage) vegetation & soil chemistry
chemical weathering is strongest with…
high precipitation
high temp.
physical weathering is strongest with…
moderate precipitation
low temp.
