L10: Weathering and introduction to Alpine slopes Flashcards
Key factors in terms of the ability of rock to be weathered are…
- Tensile strength (resistance to being broken down).
- Porosity (how much water the rock can hold)
- Presence of structures (lines of weaknesses)
What does physical weathering mean?
Give some examples…
In situ disintegration by mechanical stress.
a) Frost weathering (shattering) by 9% volume increase with phase change.
b) Segregation ice and hydrofracture.
c) Thermal stress
d) Salt weathering
e) Pressure release
Frost wedging…
Crack in rock of mountain.
Crack fills with water, which freezes in winter.
Ice wedge expands, block of rock is forced off.
WRONG!
To get pressure that exceeds the tensile strength of a rock, what is required?
Water freezing in a closed cavity in a material rigid enough to prevent expansion will exert enough pressure.
But needs to freeze in closed???
How do you get a low enough temperature for freezing in frost wedging?
Zero curtain delays the fall in temperature, so the latent heat is going to delay.
Pressure of partial freeing lowers freezing point.
Impurities in water lower freezing point.
Surface tension on small particles delays freezing.
Effective freezing requires -4oC.
What is ice plugging?
An ice plug at the surface of a cavity might render it effectively “closed”.
Surface cracks could be closed by downward annual freezing.
Rapid surface freezing might plug a crack in short term.
Plug formation needs temperature drop of 1oC every 10 mins.
How does ice segregation occur?
Migration of water towards a freezing front through pores.
Growth of ice lenses can initiate and widen micro cracks and so fracture rock.
What are controlling factors in ice segregation?
Rate of temperature change- not too fast so whole system freezes, slow enough to allow migration.
Porosity- pore too big it will just freely drain.
Moisture- too much = latent heat, saturation, too little = lack of migration.
Two sided freezing most efficient.
How does thermal contraction cracking occur?
Rapid cooling or heating of the surface few cms causes a steep thermal gradient with deeper rock and compressive (contraction) and tensile (expansion) stresses.
Ultimately rock fracturing will over through fatigue or shock.
What is salt weathering?
Stresses that are formed through salt crystals growth by:
a) Growth within pores and microcrack.
b) Thermal expansion of salt crystals.
c) Swelling through hydration.
What is salt weathering highly dependent on?
The degree of saturation and position with regards to the surface (evaporation).
What is pressure release (dilatation)?
Release of the overburden of rock (or ice) otherwise known as dilatation (relaxation).
Most rocks are not elastic so stresses cause fracture and exaggeration of existing joints.
These are then exploited by the other physical processes already mentioned.
What does chemical weathering do?
Makes relatively insoluble minerals more soluble.
Reduces strength of bonds making rocks more vulnerable to physical processes.
Key factors of cold chemical weathering?
Climate in terms of precipitation (leaching) and temperature (rate of chemical reactions).
pH of environment (for example, snow and precipitation can be slightly acidic).
Chelation (break up of mineral bonds by acid, for example in organic material).
What is biological weathering?
Constrained by temperatures and flora.
Micro organisms such as lichen, algae and fungi.
Contribute to surface weathering and vulnerability to chemical and physical weathering by biochemical (detaching grains and flakes) and biochemical (chemical alteration of minerals).
