3.2 Weathering Flashcards
1
Q
What is weathering
A
- The breaking down of rock which occurs in situ (in place)
- Divided into physical weathering and chemical weathering
2
Q
What is physical weathering
A
- Involves the physical breaking down/fragmentation of rock into larger blocks and eventually into grains or individual minercal crystans
- Little or no chemical change
3
Q
Explain the process of freeze thaw
A
- Water collects in cracks in the rock
- When temperature drops below 0ºC it freezes and expands by 10%
- The pressure created results in the progressive weakening of the rock and its eventual failure
- This repeated process breaks apart the rock such as slopes in areas of high altitude
4
Q
Where is freeze thaw most likely to occur and why?
A
- The daily freeze-thaw cycles experienced during winter in cool maritime regions are more effective thant the greater but less frequent cycles encountered in cold continental regions
- It gives rise to slopes of frost shattered material, or screes, below the most exposed rock faced in high altitudes
5
Q
Explain the process of heating and cooling
A
- The process of thermal expansion and contraction of rock due to rising and falling temperatures
- It occurs during the daily cycle of heating and cooling and sets up stresses in the rock causing it to disintegrate
6
Q
Where does heating and cooling usually occur
A
- In hot deserts where a lack of cloud cover results in a pronounced difference between peak daytime and nighttime temperatures i.e. a large diurnal temperature range
7
Q
Explain the process of salt crystal growth
A
- Occurs when salt crystals such as sodium carbonate and magnesium sulphate grow within spaces in a crock
- It can be caused when saline water enters cracks in the rock and evaporates
- The growing crystals then prise the rock apart especially when hot - sodium sulphate and carbonate expand by 300%, and when temperature rises salt crystals expand exerting pressure on the rocks
- It is effective in semi-arid areas where salt solutions dry quickly, as well as coastal regions
8
Q
Explain the process of pressure release (dilation)
A
- It is an unusual type of weathering in that it is not caused by elements of weather
- It occurs either when erosion removes a heavy covering of rock (overburden), or when huge ice sheets melt at the end of a glacial period
- The removel of grate weights allow the rock below to expand, producing sheet joints, or pseudo-bedding planes, parallel to the ground surface
- These joints may then encourage further weathering
9
Q
Explain the process of organic action
A
- Occurs due to the breaking up of rock by plant roots and burrowing animals
- Roots tend to exploit existing cracks and lines of weakness and, as they thicken, they exert increase pressure and cause rocks to fracture
- Building foundations are at risk from roots of larger trees
- Burrowing animals can also break apart partailly weathered rock, exposing it to further weathering
10
Q
What is chemical weathering
A
- The process by which mineerals are chemically altered or dissolved
- It is most common in humid climates with a dense vegetation cover
11
Q
How is carbonic acid formed and why is this important
A
- It is produced by CO2 disolving in rainwater
- H20 + CO2 = H2CO3
- This is the major chemical weathering agent
12
Q
Explain the process of hydrolysis
A
- Carbonic acid releases hydrogen ions from water which then combine with certain minerals in rock, causing them to disintergrate (i.e. rocks react with water)
- This process is common in rocks that contain the mineral feldspar, such as granite
- As the process proceeds permeability increases exposing more mineral surface to the process, the mineral grains will eventually weaken leading to spalling or crumbling
13
Q
Explain the process of hydration
A
- This process occurs when water is absorbed into the crystal structure of certain minerals and causes chemical changes
- For example, anhydrite (a form of calcium sulphate) absorbs water to become gypsum, whcih is a very soft mineral
- Apart from chemical changes, the absorption of water may also lead the rock to swell, creating stress and leading to physical weathering
14
Q
Explain the process of carbonation
A
- Occurs mainly with limestone
- On contact with calcium carbonate (the main component of limestone), carbonic acid creates calcium bicarbonate
- H2CO3 + CaCO3 = Ca(HCO3)2
- Calcium bicarbonate is readily dissolved in water therefore is quickly removed
- This occurs with many different rocks as well
15
Q
Explain why climate affects the type and rate of weathering
A
- Heat speeds up chemical reactions (+10ºC = double the reaction rate). NOT ONLY TEMPERATURE BUT ALSO CYCLES
- Lower temperatures may be better e.g. for carbonation where CO2 dissolves better into water
- Warm wet climates favour chemical weathering whereas cold dry climates lead to more physical weathering
- Freeze thaw in mountainous areas rather than polar
- Salt crystal growth in dryer climates due to evaporation
16
Q
Explain why rock-type affects the type and rate of weathering
A
- Chemical composition - the solubility and stability of the chemical components are very important - GRANITE EXPOSED TO HYDROLYSIS DUE TO PRESENCE OF FELDSPAR
- Another example is LIMESTONE CONSISTS OF CALCIUM CARBONATE THERFORE SUSCEPTIBLE TO CARBONATION
- The nature of cements in sedimentary rock - grain structure - IRON OXIDE BASED CEMENTS ARE PRONE TO OXIDATION
- Colour - darker rocks absorb more insolation
17
Q
Explain why rock structure affects the type and rate of weathering
A
- Joints and bedding planes - acts as routes for weathering agents such as water, carbonic acid (or others) and roots
- Permeable and porous rocks are more easily affected since water is able to flow through and more surface area of the rock is exposed to weathering
- Hardness - harder rocks require more energy to break down
- Exposure of weak layers
18
Q
Explain why vegetation + human activity affects the type and rate of weathering
A
- Vegetation can prevent some physical weathering by providing protective cover
- It accelerates other types such as root action and burrowing animals
- Moisture content, root depth and acidity of humus can affect weathering
- Increase sulphur emissions leads to a lower pH of rainwater, leading to more weathering
19
Q
Explain why relief affects the type and rate of weathering
A
- The direction in which a slope faces will affect the temperature
- In the northern hemisphere, south-facing slopes reveive more insolation therefore much warmer. In winter, south-facing slopes experience regular, even daily, cycles of temperatures above and below freezing
- Vegetation cover may vary with aspect, being denser in south-facing slopes
- Slope angle for removal of weathered material can be important
20
Q
Explain how temperature affects the type and rate of weathering
A
