factors affecting weathering

Question 1

climate (temp)

Answer 1

Physical: heating and cooling
- heating and cooling of rock = expand and contract, weakening the rock.
- physical process which when repeated, = breakdown of the rock.
- common in desert, large diurnal temperature ranges cause rocks and minerals to expand during the day (40◦ C) and contract during the night (temps drop below 0◦C due to rapid radiation cooling due to cloudless skies) resulting in disintegration.
- Different rocks will have different albedos e.g. light-coloured granite = higher reflectivity than dark basalt.
- However, granite = better conductor, will take heat into the subsurface. limestone not a good conductor, so surface layers heat up quickly, more subjected to heating and cooling effects

Chemical: Hydrolysis
Van’t Hoff’s Law states that chemical reactions increase by 2-3 times for every 10 degrees celsius increase
- chemical weathering process, occurs on rocks with orthoclase feldspar, notably granite.
- Feldspar reacts with acid water and forms kaolin (or ‘china clay’), silicic acid and potassium hydroxyl.
- The acid and hydroxyl are removed in the solution, leaving behind kaolin.

Question 2

climate (precip)

Answer 2

Physical: salt crystal growth
- Water entering cracks and pores in rock evaporates, leaving salts that expand by up to 300%, putting pressure on the rock, eventually disintegrating.
- most effective salts are sodium sulphate, magnesium sulphate, sodium carbonate and calcium chloride
- most effective at temperatures of around 27 degrees C, temperature fluctuations can cause rocks to expand
- It is therefore most effective in the Tropics

Chemical: hydration
- Hydration is a physio-chemical process as minerals change when they absorb water
- expansion and contraction (repeated heating and cooling) lead to increased stress, leads to becoming more susceptible to further breakdown (granular disintegration)
- e.g when anhydrite is changed to gypsum – when it absorbs water it expands by 0.5%.
- Some shales and mudstones can expand by 1600 % when clay minerals absorb water.

Question 3

rock type

Answer 3

Physical: freeze-thaw
- water trapped in pores/fissures/cracks of rock (not possible if impermeable rock)
- water expands when freezing, exerts pressure, rock breakdown.
- repeats, more cycles = more disintegrate
- common in periglacial as temp oscillates around 0 degrees
Chemical: Carbonation
- dissolving of CO2 in water, producing carbonic acid, which reacts w/ calc carbonate in rocks to make calcium bicarbonate, washed away in solution (not possible if no cal carb in rocks)
- effectiveness related to pH of water, iron soluble when pH < 4.5

Question 4

vegetation

Answer 4

Physical: Vegetation root action
- growth of roots in joints and crevices of rocks forcing them apart as roots develop.
- more cracks/joints in the rock = more the roots can exploit them.
- Common in tropical areas and worst in polar regions. Need wet/warm conditions.
- ↑ in highly vegetated areas, esp large vegetation e.g. trees (roots capable of breaking rocks apart)
Chemical: chelation
- weathering of rocks through the action of organic substances, e.g. bacteria and humic acids.
- humic acids will aid chemical weathering such as carbonation.
- Chelation where there is more vegetation, in environments with high productivity, e.g. in tropical rainforests.
- more common in rocks susceptible to chemical weathering with acidic water, such as limestone and granite.
- Heavily jointed and/or porous rocks also be more susceptible as allow water to penetrate