Weathering & Mass Movement
River valley processes
- Both fluvial and landscape processes shape the river and surrounding land in the drainage basin
- The landscape processes of weathering and mass movement shape land surrounding the river channel
- Fluvial processes shape the river channels and the landforms linked to them. They include:
- Erosion
- Transportation
- Deposition
- Most (about 95%) of a river's energy is used to overcome friction
- There is more friction in the upper course of the river where it is shallow and narrow
- The rest of the energy is used in erosion and transportation
- Energy in the river depends on the river discharge and velocity
- The greater the discharge and velocity the more energy a river has
Weathering and mass movement
- There are three types of weathering. These happen in-situ:
- Physical - rock is broken down into smaller pieces. This occurs due to changes in temperature - such as freeze-thaw and exfoliation
- Chemical - rocks disintegrate and dissolve in slightly acidic rainwater
- Biological - rocks are broken apart by the roots of plants
Freeze-thaw weathering
- There are a number of types of mass movement where large-scale movement of material occurs
- In river valleys there are two main types of mass movement:
- Slumping - where the slope is eroded by the river. This undercuts the slope causing large-scale movement of material down the slope
- Soil creep - the influence of gravity causes weathered material to slowly move down the slope towards the river
Factors affecting weathering and mass movement
- There are a range of factors which affect weathering and mass movement
- These factors are outlined in the table below
Factors Affecting Weathering and Mass Movement
| Factor | Effect |
| Climate |
In hot, wet climates, chemical and biological weathering are dominant |
| Weather |
Heavy rain increases mass movement |
| Slope |
Any slope over 5o experiences mass movement. The steeper the slope, the more mass movement there will be |
| Geology |
Different rocks have different levels of resistance to weathering |
| Altitude |
At higher altitudes, freeze-thaw weathering occurs frequently due to the low temperatures |
| Aspect |
Physical weathering is more common on a colder, north-facing slope due to a higher likelihood of freeze-thaw |
| Vegetation |
Roots bind the soil together which limits mass movement |
Worked example
Explain two factors which influence mass movement (4)
- Identify the command word
- The command word is 'explain'
- The focus of the question is 'mass movement'
- You can select any two factors from climate/weather, geology, vegetation, slope, altitude or aspect
- For the second mark for each factor you must explain why it increases or decreases mass movement
- Answer: (Any two factors and explanations from below)
-
- All slopes which have a gradient of more than 5o experience mass movement (1). The steeper the slope, the more mass movement (1)
- Where rock types are less resistant to weathering (1), there will be more mass movement as there will be more loose material (1)
- Vegetation decreases the amount of mass movement (1) as the roots bind the soil together holding the slope in place (1)
- On the north facing slopes there will be more physical weathering (1) leading to more mass movement as there will be more loose material (1)
- At higher altitudes freeze-thaw weathering may be more common (1) leading to more mass movement as there will be more loose material (1)
Exam Tip
Students often confuse weathering and erosion. Remember, weathering is the physical, biological or chemical breakdown of the rock where it is located - 'in-situ'. Erosion is the wearing away and movement of the material, usually by wind, water or ice.
