Weathering
Sedimentary Processes and products
Weathering Processes
The syllabus says:
Describe and explain the weathering processes producing soluble products and insoluble residues by:
Chemical:
– Hydrolysis
– Hydration
– Oxidation
– Carbonation
Mechanical:
– Exfoliation
– Frost shattering
Biological processes.
Definition of weathering:
The break up of a rock in place "in situ", involving the gradual decay of the materials of the rock structure leading to the whole rock structure being weakened .
The rock eventually collapses and the residual minerals can be left in place.
Weathering occurs in two major and one minor form: physical, chemical and biotic .
Definition of erosion:
Involves the break up of the rock by various agents of erosion (river, sea, ice and wind) with the particles then being transported and eventually deposited by that agent of erosion.
This breaks up the rock into smaller and smaller pieces.
Differential
expansion
and contraction :
Occurs where there is a large diurnal T range, best seen when water is present.
Most effective where there are a lot of minerals e.g. metallic, expand/contract at different rates thus weakening the rock.
This make the surface layer peel off
“exfoliation” and is common in basalt and granite.
Pressure release, offloading:
Granites for example form at great depth and are under P .
Erosion means that the granite is exposed at the surface.
The rock experiences P release and it expands .
Because it is solid it takes up the expansion by fracturing parallel to the surface.
Pressure release, offloading:
These are sheet joints (we mentioned them at the end of the
2831 section when we were talking about joints).
The rock may then experience exfoliation with the layers peeling away.
Frost shattering/Freezethaw weathering
Water can collect in cracks in the rock.
In cold areas a T fluctuation above and below freezing is needed.
The water freezes and expands about 9%.
This expansion pushes the grains apart causing granular disintegration.
The rock cracks/fractures .
This is repeated again and again.
1.
To operate well there are 2 main factors:
There needs to be water available.
There needs to be heat .
The water provides reactive solutions , which can produce new minerals by chemical reactions.
The heat speeds up the reactions, increases solubility etc.
Important reactions include:
Hydration (addition of water)
Hydrolysis (reactions involving water).
Oxidation (addition of oxygen)
Carbonation (reactions in the presence of
CO
2
.
These produce:
– solutions with dissolved elements.
– new minerals like clay minerals
– residual minerals like quartz.
Hydration
Very simply: the chemical combination of water with another substance.
Hydrolysis
This is a reaction between silicate minerals and water.
It can change the mineral and produce ions in solution .
Water is incorporated into the mineral lattice .
Feldspar breaks down to clay minerals plus ions in solution .
Oxidation
The processes of combing oxygen to a structure.
The most common example is where iron rich minerals oxidise and form iron oxides (rust).
Carbonation
These are simply reactions of minerals in the presence of CO
2
There can be physical break up of the rocks by the action of plant roots or by burrowing animals.
Acids can be produced especially during the breakdown of plants, which may cause chemical reactions.
Weathering Products
Physical weathering will tend to produce angular fragments that can vary in size .
With Chemical weathering there can be:
– Ions in solution
– New minerals
– Residual minerals
The detail of the products will depend on the chemical process and the mineral involved.
Chemical Weathering
Products
MINERAL
Olivine
Pyroxene
Amphibole
Mica
Feldspar
Calcite
Quartz
Serpentine
Clay
Minerals
Oxidised iron minerals
if iron present
Dissolved silica
Dissolved salts
Weathered only in extreme conditions
Climatic Controls on
Weathering
1.
2.
From what you now know about weathering you are aware that the climate controls the type of weathering.
For chemical weathering to operate what 2 conditions need to be met?
Warm
Wet
Give an example of such an environment:
Rain forest/sub tropical
Climatic Controls on
Weathering
What conditions are required for freeze/thaw weathering:
Alternating above freezing and below freezing.
Would this operate in a desert?
No.
What conditions are required for differential expansion and contraction:
Diurnal T change.
Could this happen in a desert.
Yes.
