The two main categories of weathering each have their own generalised needs to provide optimum conditions for weathering to occur quickly. The presence of water is a large importance; if the rock is too dry there will be no moisture to freeze in the pores. The conditions required for physical weathering, with temperatures fluctuating around zero degrees and a medium level of rainfall, are those typically found in areas of high and middle latitude, which is why these affect upland Britain.
Mass movement is the downward movement of weathered material including soil, loose stones and rocks under the influence of gravity but excludes movements when material is carried by ice, water or wind, although mass movement often needs water to assist in the affect. The three main types of mass movement are soil creep, earth flows and landslides.
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Soil creep is the slowest of downhill movements and takes place at a rate of less than 1cm a year. However, unlike faster movements, it’s an almost continuous process. Soil creep occurs mainly in humid climates where there is a vegetation cover. There are two main causes of soil creep, both resulting from repeated expansion and contraction. During times of repeated rainfall, when the moisture causes an increase in the volume and weight of the soil, therefore causing it to expand. In following dry periods the soil will then contract. When the ground freezes it produces ice crystals increasing the volume of the soil by 9%. As the soil expands, particles are lifted at right angles to the slope in a process called heave. When the ground later contracts, these particles fall back vertically under the influence of gravity and so move down slope. The freeze thaw process works best in climates, which experience a large diurnal range in temperature. Britain rarely experiences weather that allows the freeze thaw process to occur and the rare occasions that it does would only have a very small effect on soil creep. Vegetation removal and consequent lower water use may increase soil water levels causing an increase in pore water pressure within the soil profile and therefore an increased likelihood of soil creep.
Figure
The effects
of soil creep
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Earth flows occur on slopes of 5-15° when the soil becomes saturated and therefore increases its weight and begins to flow downwards at speeds less than 15km per year taking any debris with it.
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Landslides are the movement of large masses of rock and land. Rocks, which have bedding planes roughly parallel to the angle of the slope, are more susceptible to landslides. Landslides are common in many costal areas and are less likely to occur in upland Britain so they are not a physical activity that plays much part in the lowering of interfluves.
The climate, rock structure and the type of soil all have large influences on mass movement.
- The effect of climate: Heavy rain adds volume and mass to the soil, heavy rain occurs more in upland Britain than low land Britain and is therefore one of the main factors contributing to mass movement. Heavy rain also increases the erosive power of the rivers at the base of the interfluves and therefore removes more material and makes the slope less stable and more susceptible to mass movement.
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The effect of rock structure: An impervious underlying rock will cause the topsoil to become saturated more quickly therefore adding more weight to the soil and making mass movements more likely. Steep slopes are more likely to suffer mass movement than gentler ones. In Britain most slopes are above 5° and few are over 40. Areas of alternating hard/resistant rocks and soft/less resistant rocks are more likely to experience movement. The presence of joints, cracks and bedding plains can allow increased water content and so lead to sliding.
- The effect of soil type: The thinner the soil the more unstable it tends to be. If the soil is thin it can only hold up a certain amount of vegetation therefore if there is a high amount of vegetation on an area of thin soil mass movement is far more likely. A porous soil is less likely to become saturated.
Here is a diagram, which illustrates a slope as a dynamic open system:
Interfluves are lowered by the process of vertical erosion, caused by the flow of the river. This causes the slopes down to the river to become steeper and therefore debris from mass movement moves at an increased speed. The water then washes away the debris that has fallen into the river leaving the rock exposed to weathering. There are four main processes of erosion.
- Corrasion: This occurs when the river picks up debris and rubs it along the riverbanks wearing them away by abrasion. This occurs most effectively in times of flooding.
- Attrition: As the bed load is moved downstream boulders can collide into other material and the impact can break rock into smaller pieces.
- Hydraulic action: The force of the water moving downstream causes water to be pushed into cracks in the rock. The pressure in the crack increases and in time breaks the rock down.
- Corrosion: Is caused by the chemical composition of the water and occurs continuously.
These erosion processes all work together to gradually wear away at the river bed and banks therefore lowering the interfluves, making the sides of them steeper and weakening the lower part of them, causing the process of mass movement to take place at an increased speed.
Human activity including deforestation, farming, acid rain, which is increased by pollution, and tourism all affect the speed of erosion, weathering and mass movement. Agriculture and forestry, as well as housing, industrial development, and road construction, partially or wholly destroy the protective canopy of vegetation that is often found in these areas and greatly speed up erosion of certain kinds of soil.
- Deforestation causes reduced interception and therefore causes an increased runoff, an increase in the amount of water reaching the river at one time which will cause the river to flow faster and therefore soil erosion will occur faster. If people re afforest this will slow down the affect that deforestation has.
- The grazing of animals and ploughing of soil remove protective vegetation cover, speeding up weathering.
- Acid rain is a form of chemical weathering and is increased by human activities such as power generation and transport as it increases the amount of toxic gases released into the atmosphere that then form acids in solution in rainwater.
- By building on the slopes of the interfluves it adds weight to them and therefore can increase the process of mass movement. When roads are built on the interfluves heavy traffic can cause vibrations that will weaken the slope and result in mass movement of the loosened debris.
- Tourism also has a large effect on lowering interfluves in upland Britain. If causes soil erosion by tourists walking on the land. Car parks, roads and footpaths are built in tourist areas, these all contribute to the physical activities of mass movement, weathering and erosion. For example vegetation is removes to build such things leaving it uncovered, which increases the likelihood of weathering occurring.
Natural processes and human activity are both responsible for the lowering of interfluves in upland Britain to an average extent. All three physical processes, weathering, erosion and mass movement work together in a continuous cycle which all contribute to the lowering of interfluves. Each process has a knock on effect making the other one more effective. Human activity probably has a larger effect on the lowering of interfluves in upland Britain, as it enhances the possibility of natural processes occurring, therefore if these human activities talked about above did not occur then the natural processes would take place at a slower rate.
