3.3 Slope processes and development

Limestone scenery is unique on account of its: l permeability l solubility in rain and ground-water

Limestone consists of mainly calcium carbonate. Because of their permeability, limestone areas are often dry on the surface and are known as Karst landscapes. Carboniferous limestone has a distinctive bedding plane and joint pattern, described as massively jointed. These features act as weaknesses allowing water to percolate into the rock and dissolve it. One of the main processes affecting limestone is carbonation–solution. The process is reversible, so under certain conditions calcium carbonate can be deposited in the form of speleothems (cave deposits such as stalactites and stalagmites) and tufa (calcium deposits around springs). Limestone is also affected by freeze–thaw, fluvial erosion, glacial erosion and mass movements.

As the joints and cracks are attacked and enlarged over thousands of years, the limestone’s permeability increases. Clints and grikes develop on the surface of the exposed limestone. Large areas of bare exposed limestone are known as limestone pavements. Depressions can range from small-scale swallow holes (or sinks) to large dolines up to 30 m in diameter. These are caused by the solution of limestone but can also be formed by the enlargement of a grike system, by carbonation or fluvial activity, or by the collapse of a cavern. Rivers can disappear into these holes, hence the term ‘sink’. Resurgent streams arise when the limestone is underlain by an impermeable rock, such as clay.

Stalactites develop from the top of the cave, whereas stalagmites are formed on the base of the cave.

16 What are the two main theories about tor formation? 17 What is equifinality? 18 What are the main processes affecting limestone? 19 Explain the formation of swallow holes.

Tested

Revised

Geological structure is an important influence on slope development. This includes faults, angle of dip and vulcanicity: l Faulting can produce steep valley sides, as in a rift valley. l Folding can produce either steep or gentle slopes depending on the angle of dip. l Vulcanicity produces intrusions of resistant igneous rock. For example, Great

Whin Sill is a harder and more resistant rock than the surrounding dolomite, and so has produced a steep slope.

Rock type and character influence whether a rock is affected by weathering, and to what extent it can resist the downslope force of mass movement. Resistance is largely physical. Regular jointing can increase the risk of movement, as well as the amount of water that enters a rock.

Many slopes are shaped by climate, which affects the types and rates of processes that operate in a region and when they occur: l In arid regions, slopes are jagged or straight due to mechanical weathering and sheetwash.

A slope is an inclined surface (hillslope). It can also refer to the angle of inclination.

Slopes that comprise many different types of rocks are often more vulnerable to landslides due to differential erosion. The softer rocks get worn away and can lead to the undermining of harder rocks.

l In humid areas, slopes are frequently rounder, due to chemical weathering, soil creep and fluvial transport. l In the humid tropics accelerated chemical weathering dominates. This is due to the hot, wet conditions and the availability of organic acids. Deep clays are produced, favouring low slope angles.

Regolith is the superficial and unconsolidated material found at the Earth’s surface. It includes soil, scree, weathered bedrock and deposited material. l Its unconsolidated nature makes it prone to downslope movement. The extra weight of a deep regolith will increase the likelihood of instability. l Clay-rich regoliths are particularly unstable because of their ability to retain water.

Soil structure and texture will largely determine how much water it can hold. Clay soils can hold more water than sandy soils. A deep clay on a slope where vegetation has been removed will offer very little resistance to mass movement.

Aspect refers to the direction in which a slope faces. In some areas, past climatic conditions varied depending on the direction a slope faced.

Vegetation can decrease runoff through the interception and storage of moisture. Deforested slopes are frequently exposed to intense erosion and gulleying. However, vegetation can also increase the chance of major landslips.

Climatic geomorphology is a branch of geography that studies how different processes operate in different climatic zones to produce different slope forms or shapes.

Some students state that north-facing slopes are colder than south-facing slopes. This is only true in the northern hemisphere – the reverse is true in the southern hemisphere.

20 Briefly describe two ways in which geology affects slope development. Answer on p.215 Tested

Mass movements (Figure 3.5) include: l very slow movements, such as soil creep l fast movements, such as avalanches l dry movements, such as rock falls l fluid movements such as mud flows

Flow

River Wet

Mudflow

Earthflow

Revised

Mass movements are large-scale movements of the Earth’s surface that are not accompanied by a moving agent such as a river, glacier or ocean wave.

Solifluction

Landslide

Heave Slow Seasonal soil creep Talus creep Rockslide

Figure 3.5 A classification of mass movements

Dry

Slide Fast

The likelihood of a slope failing can be expressed by its safety factor. This is the relative strength or resistance of the slope, compared with the force that is trying to move it. The most important factors that determine movement are gravity, slope angle and pore pressure.

Gravity has two effects: l It acts to move the material downslope (a slide component). l It acts to stick the particle to the slope (a stick component).

The downslope movement is proportional to the weight of the particle and slope angle. Water lubricates particles and in some cases fills the spaces between the particles, which forces them apart under pressure.

Heave or creep is a slow, small-scale process, which occurs mostly in winter. It is one of the most important slope process in environments where flows and slides are not common. Talus creep is the slow movement of fragments on a scree slope. Individual soil particles are pushed or heaved to the surface by (a) wetting, (b) heating or (c) freezing of water. About 75% of the soil creep movement is induced by moisture changes and associated volume change. Freeze–thaw and normal temperature-controlled expansion and contraction are also important in periglacial and tropical climates. Particles move at right-angles to the surface as it is the path of least resistance. They then fall under the influence of gravity when the particles have dried or cooled, or when the water has thawed. Net movement is downslope. Heave forms terracettes.

Falls occur on steep slopes (>40°), especially bare rock faces where joints are exposed. The initial cause of the fall may be weathering, such as freeze–thaw or disintegration, or erosion prising open lines of weakness. Once the rocks are detached they fall under the influence of gravity (Figure 3.6). If the fall is short it produces a relatively straight scree. If it is long, it forms a concave scree. Falls are significant in causing the retreat of steep rock faces and in providing debris for scree slopes and talus slopes.

Lines of weakness Free face 70°

Fall

Slope failure is caused by caused by two factors: l a reduction in the internal resistance, or shear strength, of the slope l an increase in shear stress, that is, the forces attempting to pull a mass downslope Both can occur at the same time.

21 State one difference between a rockslide and a mudflow. 22 Define the term mass movement. 23 Suggest how mass movements can be classified. 24 Define the terms shear strength and shear stress.

Tested

Bounce

Talus slope

Roll

24°

Straight 32°

Figure 3.6 Falls

Concave 24°

Slides occur when an entire mass of material moves along a slip plane. They include: l rockslides and landslides of any material, rock, or regolith l rotational slides, which produce a series of massive steps or terraces

Slides commonly occur where there is a combination of weak rocks, steep slopes and active undercutting. They are often caused by a change in the water content of a slope or by very cold conditions. As the mass moves along the slip plane it tends to retain its shape and structure until it impacts at the bottom of a slope (Figure 3.7). Slides range from small-scale slides close to roads, to largescale movements killing thousands of people.

Slip planes occur: l at the junction of two layers l at a fault line l where there is a joint l along a bedding plane l at the point beneath the surface where the shear stress becomes greater than the shear strength

Slide plane Scar

Slide plane

Detached block

Debris from an earlier slide

Velocity at each depth

Ground surface

Base of slide plane

Figure 3.7 Slides

Loose rock, stones and soil all have a tendency to move downslope. They will do so whenever the downslope force exceeds the resistance produced by friction and cohesion. When the material moves downslope as a result of shear failure at the boundary of the moving mass the term landslide is applied. This may include a flowing movement as well as straightforward sliding. Landslides are very sensitive to water content. This reduces the strength of the material by increasing the internal pressure. This effectively pushes particles apart thereby weakening the links between them. Moreover, water adds weight to the mass, increasing the downslope force.

Slumps occur on weaker rocks, especially clay, and have a rotational movement along a curved slip plane (Figure 3.8). They have a higher water content than landslides, and involve smaller particles. Clay absorbs water, becomes saturated, and exceeds its liquid limit. It then flows along a slip plane. Frequently the base of a cliff has been undercut and weakened by erosion thereby reducing its strength.

Weak rocks such as clay have little shear strength to start with and are particularly vulnerable to the development of slip planes. The slip plane is typically a concave curve and as the slide occurs the mass will be rotated backwards.

By contrast, flows are more continuous, less jerky, and are more likely to contort the mass into a new form (Figure 3.9). The material involved is predominantly made up of fine particles, such as deeply weathered clay.

Sand

Scar

Clay Back tilted slope (terrace)

Possibility of mudflows at toe

Curved or arcuate slip plane

Figure 3.8 Slumps

Scar

e.g. Aberfan 21 October 1966 where 144 people were killed, 116 of them school children

Highly fluid; lacks cohesion; saturated with water Over 33% of the material is fine-grained Toe of flow spreading out

Velocity at each depth Ground surface

Figure 3.9 Flows

Base at flow

Avalanches are rapid movements of snow, ice, rock or earth down a slope. Snow and ice may pick up rocks and/or earth. In steep mountainous areas, a rock avalanche suggests a large-scale movement of material, whereas a rock fall could be of individual rocks. l They are common in mountainous areas: newly fallen snow may fall off older snow, especially in winter (a dry avalanche), while in spring partially melted snow can move (a wet avalanche), often triggered by skiing (Figure 3.10). l Avalanches frequently occur on steep slopes over 22°, especially on northfacing slopes where the lack of sun inhibits the stabilisation of snow. l Debris avalanches are rapid mass movements of sediments, often associated with saturated ground conditions.

The speed of a flow varies – mudflows are faster and more fluid that earthflows, which tend to be thicker and deeper. A higher water content will enable material to flow across more gentle angles.