3.2 Weathering Flashcards
What are the different types of physical weathering?
freeze-thaw
heating/cooling processes
salt weathering
pressure release (dilatation)
vegetation root action
What is freeze-thaw weathering?
- aka frost action
- due to FREEZING TEMPERATURES
- water gets into the cracks of rocks, freezes and volume expands by 9%
- this repeated action puts pressure on a rock, eventually causing it to shatter and break off
Where?
- plenty of moisture
- where temps frequently fluctuate above/below freezing point
- polar periglacial areas and in mountainous alpine areas
Products:
- scree : fragments of rock fall to bottom of cliff
What happens in heating/cooling processes (thermal fracture)?
-In areas with a high diurnal temperature range can cause rocks to be broken down and weathered away
- eg the hot desert with temperature ranges of 40-50 degrees Celsius
-Rocks expand rapidly when hot (druing the day) and contract rapidly when cold (at night)
-Rock is a poor conductor of heat so stresses only occur in the outer layers of the rock.
-This can cause peeling or flaking of the skin of outer rock to occur- known as exfoliation
-Moisture is required for this
-Different coloured and types of minerals absorb and emit heat at different rates.
- A pale-coloured rock like granite will reflect more heat than a dark-coloured one like basalt/Where:
- most effective in dark crystalline rocks, which absorb heat
- requires wide diurnal temp - greater than 25 down to 0
- often in hot deserts
What happens in salt weathering?
Where:
- in areas with temp around 26-28
- occurs due to salt crystal growth inside cracks/pores in rocks
- when salt water gets into cracks and evaporates, it leaves behind salt crystals from the solutions
- as these crystal accumulate over time, the build up of pressure expands the gap in the rocks
- eventually causing rocks to break off/disintegrate
What happens in pressure release (dilatation)?
- takes place when heavy overlying rocks are removed by erosion
- the release of this pressure and weight causes the exposed rocks to expand which may cause them to split apart parallel to the rock surface
- the removal of melting ice sheets/glaciers from an area is a primary cause
What happens in vegetation root action?
- roots and seedlings can grow into joints/bedding planes where there may be moisture and washed in soil
- extreme pressure can be exerted by the growth of roots as roots need more space to grow, which can cause the rocks to break apart
What are the different types of chemical weathering?
hydrolysis
hydration’
carbonation
What happens in hydrolysis?
- breaking down of rock when it reacts with water
- rocks can break down when they are soluble in water
e.g mineral feldspar found in granite is converted into a white powdery clay called kaolin or China clay - rain water (with the aid of carbon dioxide) breaks up into
positive and negative (H+ and OH-) ions - chemically combines with minerals within rocks (silicate and carbonates) and breaks it up forming both soluble and insoluble parts
-The soluble parts are removed in solution
(by water)
What happens in hydration?
Where:
- minerals that have capacity to take up water
- as they absorb/soak up the water they expand and change their chemical composition/new mineral which often makes them weaker and less resistant to erosion
- e.g anhydrite+water= gypusmexpands by about 0.5%
- CASO4 + H2O = CASO42H2O
What happens in carbonation?
- when CO2 dissolved in water (carbonic acid) reacts with rocks and breaks them down
- limestone and chalk
- precipitation falling through the atmosphere absorbs CO2 to form a weak carbonic acid
- e.g limestone
- this acidulated rainwater will react with any rock containing calcium carbonate converting it to soluble calcium bicarbonate which then dissolves
Where:
- more active in cold conditions, as cold water can hold more CO2
What factors affect weathering?
climate
rock type
rock structure
vegetation
relief
How does climate affect weathering?
- temperature and precipitation control type/speed of weathering processes
- e.g physical weathering, water is required for freeze-thaw as well as temperatures that move above/below freezing
- in exfoliation, high temps needed for rock surfaces to expand and these need to fall for surface to contract
- chemical weathering most effective in areas with both high precipitation and high temperatures
- e.g alpine climates where temps fluctuate more rapidly than in polar environments, freeze-thaw weathering is more sever
- salt crystal growth favours dry climates, allowing water to evaporate to leave salt crystals
HOWEVER:
- climates vary with altitude - e.g
mountain range situated in the tropics may have a success of climates from temperature to arctic as height increases
- climate changes over time so particular weathering processes/features should not necessarily be linked to present-day conditions
How does rock type affect rate of weathering?
3 reasons:
1. differences in the chemical composition of rocks. e.g limestone is composed of calcium carbonate, which makes it susceptible to chemical weathering through solution/carbonation
2. differences in the cements that hold sedimentary rocks together> e.g some cements are iron oxide-based and are very susceptible to oxidation, whereas silica-based cements made of quartz are very resistant to oxidation
- presence or absence of pores will affect its vulnerability to weathering processes. Porous rocks (chalk) will have a high proportion of cracks, while non-porous rocks (slate) do not (as more energy required to break down the rock)
- rocks with natural weakness areas, such as layered sedimentary rocks, are also more vulnerable to weathering
How does vegetation affect rate of weathering?
- increased levels of CO2 from plant respiration forms Carbonic acid when dissolved in water and increases rates of carbonation
- rates of physical weathering will decrease due to thermal insulation of the vegetation which leads to decreased freeze thaw weathering
- direct biological weathering, through growth of plant roots into joints and along bedding planes wedging rocks apart
How does relief affect rates of weathering?
- has an indirect affect on climate:
- e.g mountainous areas : height of the land may cause freezing temperatures and freeze-thaw action on exposed rocks
- in areas of Steep relief, slope processes such as slides and flows, can expose rock to various weathering processes
- in contrast, lowland areas, rocks may be protected by the thick layers of soil/weathered material
- the accumulation of water at the base of slopes may also provide more water for chemical processes to take place (hydrolysis and hydration)
- aspect of slope may also affect rate
- in Northern Hemisphere physical weathering greater on north-facing slopes (experience **longer periods of freezing temps having longer periods of time in the shadow/lack of sunlight compared to south-facing slopes)
- opposite in southern hemisphere
