Introduction to Weathering and Erosion

Question 1

Weathering definition

Answer 1

The breakdown of rock to form transportable material

Question 2

Weathering can be cause by complex interaction between…

Answer 2

• Tectonics

- Physical, chemical and biological processes

Question 3

Factors that control weathering rates

Answer 3

1) Properties of rock - stability of minerals and structures within rock
2) Climate - rainfall and temperature
3) Presence of soil - soil formation speeds weathering, changes pH
4) Length of exposure time
5) Topography - steeper/higher = higher weathering rate

Question 4

Two types of weathering

Answer 4

• Mechanical Weathering: no change in chemical composition just disintegration into smaller pieces
• Chemical Weathering: breakdown as a result of chemical reactions

Question 5

Under what conditions is mechanical weathering prominent?

Answer 5

Arid and/or cold climates

Question 6

What are the agents of mechanical weathering

Answer 6

• pressure release
• water: freeze-thaw cycles
• water: wetting and drying
• water: crystallisation of salt in cracks
• heat: thermal expansion and contraction

Question 7

How does this promote further weathering

Answer 7

increase surface area exposed to weathering processes promoting further weathering

Question 8

How does pressure release occur

Answer 8

• Rocks form at depth and are confined by pressure
• As a rock mass is uplifted it is brought closer to the surface
• the confining pressure decreases, rock mass moves out of equilibrium and expands forming joints
• virtually all rock masses at or near the surface contain joints

Question 9

What happens in the exfoliation process

Answer 9

As rock is uncovered it expands (due to lower confining pressure) resulting in exfoliation and the rock breaks apart in layers parallel to the surface

Question 10

Forst Wedging

Answer 10

• Rock breakdown is caused by expansion of ice in joints

- shattered rocks common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart

Question 11

Wetting and Drying occurs in…

Answer 11

many fine-grained sedimentary rocks e.g. mudstones, contains clays

Question 12

Wetting and Drying process

Answer 12

• clays absorb significant amounts of water which causes them to expand
• clays release water as they dry causing them to shrink
• repeated wetting and drying causes repeated expansion and shrinking which breaks up the rock into small pieces

Question 13

Salt Crystallisation process

Answer 13

• saline solution seeps into joints in rocks
• moisture evaporates precipitating salt crystals
• crystals expand when heated
• expansion exerts pressure on confining rock
• also occurs when solutions decompose rock e.g. limestone

Question 14

Thermal Expansion

Answer 14

Extreme range of temperatures can shatter rocks in desert environment due to the repeated swelling and shrinking of minerals with different expansion rates shatters rock

Question 15

Role of physical weathering

Answer 15

• Reduces rock material to smaller fragments that are easier to transport
• increases the exposed surface area of rock making it more vulnerable to further weathering
• the increase in joint formation also leads to a pathway for water

Question 16

Chemical Weathering

Answer 16

• involves chemical interactions between water, air and rock

- water interacts with minerals creating a mix of chemical reactions and products

Question 17

Types of Chemical Weathering

Answer 17

• Dissolution
• Hydrolysis
• Oxidation
• Hydration

Question 18

Dissolution process

Answer 18

• Complete removal of mineral from the rock and is strongly influenced by pH and T
• e.g. Carbonation and hydrolysis

Question 19

Carbonation

Answer 19

• atmospheric CO2 dissolves in rainwater forming weak carbonic acid
• carbonic acid reacts with calcium carbonate (in limestone) forming calcium bicarbonate which is carried away by water
• biological activity in soils also generates substantial CO2
• process very effective within joint systems
• widens and deepens

Question 20

Acid Rain

Answer 20

• unpolluted rainwater weakly acidic
• in polluted atmosphere SO2 and nitrogen oxides present
• react in rainwater to form stronger acids

Question 21

Hydrolysis

Answer 21

• Chemical reactions where water changes composition and size of minerals in the rock
• Common in granitic rocks

Question 22

Hydration and Oxidation

Answer 22

• Hydration is the chemical reaction where water is added to the crystal structure of a mineral and if it occurs in joint expansions it can cause rock to disintegrate
• Oxidation is the reaction of minerals with oxygen which manifests as red oxidised rock coatings and red soils

Question 23

Chemical stability of minerals

Answer 23

• Hight T + P minerals (e.g. olivine, pyroxene) are least stable
• Low T + P minerals (e.g. quartz) are very stable
• Controlled by silicate structure)

Question 24

Biological Weathering

Answer 24

• Can be both chemical and mechanical in nature

- For example roots split rocks apart (mechanical) or roots produce acids that dissolve rocks (chemical)

Question 25

Mechanical weathering is enhanced when…

Answer 25

… there are frequent freeze-thaw cycles

Question 26

Chemical weathering is most effective in…

Answer 26

… areas of warm, moist climates

Question 27

Chemical weathering is least effective in…

Answer 27

… polar regions (water is locked up as ice) and arid regions (little water)

Question 28

Effects of weathering

Answer 28

• strength decrease
• loss of elasticity
• density decrease
• increases in porosity and therefore moisture absorption

Question 29

Erosion

Answer 29

The detachment and transportation of weathered surface particles under the action of:

• water (fluvial, surface erosion)
• wind (aeolian erosion)
• glacial ice
• waves (coastal erosion)
• gravity (mass wasting)

Question 30

What materials are susceptible to erosion

Answer 30

any weathered residual products or any unconsolidated deposit

Question 31

When is soil erosion most active and what does it lead to

Answer 31

• where it is difficult for rainfall to infiltrate into the ground and this results in flow over surface and removal of soil
• the faster the flow the greater the erosion
• removal of topsoil so less water and nutrients for plant growth which means less vegetation and more erosion

Question 32

What can subsurface soil erosion cause

Answer 32

piping which undermines structures causing failure