Mass movement (mass wasting) is the movement of material downslope under to gravity. If the shear stress of a slope overcomes the shear strength of material and friction, movements occur. Mass movements require a triggering mechanism that weakens the immobile overlying material and stresses the slope enough for movement to occur. Triggering mechanisms include:
- Earth movements such as earthquakes, volcanoes, folding and faluts
- Water that lubricates cracks and joints, or adds weight to the overlying material
- Weathering
- Undercutting by rivers
- Animal action, e.g burrowing
Humans on the other hand can cause or trigger mass movements. For example:
- Building on slopes
- Noise vibrations from large vehicles such as bulldozers
- Deforestation
- Overloading slopes e.g debris from mines
- Undercutting slopes for buildings and roads
- Read more: Human impact on slopes
Factors Affecting Mass Movements
Slope Stability
Unstable slopes such as along faults or slopes undercut by a river can result in rapid mass movements. In addition, slopes situated in earthquake prone areas are more prone to mass movements. Building on slopes overloads the slope and becomes more unstable.
Slope Angle
Steep slopes are more prone to rapid movements, conversely, on gentler slopes movements tend to be slow e.g. soil creep.
Water Content
Water lubricates the soil which adds weight and triggers mass movements. In addition, it increases soil pore pressure resulting in the moving body to overcome the shear stress of the slope. Water plays a pivotal role in different types of mass movements.
Vegetation
Vegetation may help to bind and stabilize soil movements. Lack of vegetation means mass movements are more likely to occur.
Cohesion/Shear Strength
Cohesion simply means how materials are well joined together. Incohesive materials such as sand are more prone to movement than clays which are well joint and tend to resist movement. Once clays are saturated they often move en-mass rather than individual grains.
Types of Mass movements
They are many types of mass movements. Some of their differences are subtle. Furthermore, some mass movements are said to be complex, meaning that they evolve from one type to the other.
We are going to list the most commonest types of mass movements.
Creep & Heaving
Soil Creep
Soil creep is the slowest of all mass movements (1-2mm in the humid temperate and 10-20mm /year in the tropics). It is mainly caused by freezing and thawing or wetting and drying of soil. Upon freezing, the soil piles up at right angles with the slope, and subsequent thawing drops the soil causing it to move downward. The process of soil piling up at right angles to the slope is called heaving. Soil creep tends to tilt trees, poles, and burst walls as soil continues piling up at the wall foot. Soil creep can create step-like ridges called terracettes.
Talus creep
Talus creep is similar to soil creep but involves the downward movement of rock fragments along the rectilinear slope.
Debris refers to more coarse particles typically above 2mm.
Earth refers to fine soil grains such as clay and silt that are below 2mm
Slides
Slides involve material moving (sliding) down over a slip surface. The material typically slides down en-mass, i.e. as a block of soil or rock. However, material may not always slide together as a unit and may disintegrate into other forms like earthflows.
Water attacks areas of weakness causing the slope to fail. Water in slides is mainly used to attack lines of weakness while the moving body may be dry. Slides include translational/planar, slumps/rotational slides, and rock slides. They are moderately fast.
Note that the term landslide is usually interchanged with mass movements referring to all types of mass movements. However, landslides are a type of mass movement typically involving material (soil/rock) that slides down a slip/failure plane as a unit or block.
Translational Slide
These are blocks/masses of land that slide along a flat slip plane. They are caused by water attacking lines of weakness, slope overloading, or undercutting. The mass (block) can move as a unit and can deform or disintegrate into a flow.
Slump/Rotational Slide
Rotational slide or slump involves a block of material sliding down over a curved failure plane. The moving mass slides inwards. Typically, slumps occur on slopes where a thick layer of soft soil such as clays slides over a firmly cohesive or impervious layer. They are caused by water attacking lines of weakness along fractures, or undercutting at slope foot such that there’s no support at the bottom. The main slump can detach into sub-slumps, or ultimately deform into an earthflow.
The head scarp is a hard layer on which the mass rests when sliding.
Rock Slides
Involves rock blocks sliding down along a parent rock with joints and bedding planes.
Spreading
Lateral Spread
Lateral spread occurs on gently sloping surfaces where coherent mass of land overlays a liquefied soil layer. The top land slides over the liquefied layer and tensional forces may rip the mass into separate distinct units. Lateral spreading is usually initiated by an earthquake.
Flows
Flows are a continuous down movement of waste, either dry or wet. They are often caused by torrential rainfalls lubricating loose soil or can be accompanied by a landslide which disintegrates into a flow of loose material.
Flows can move fast or slow depending on the amount of water present.
Solifluction
This type of mass movement occurs in periglacial areas with freeze-thaw cycles. Water flows down on an impervious frozen bedrock collecting debris in the path. This creates tongue-like lobes and sheets.
Mudflows/ Debris Flow
Debris flows are a mixture of water and sediments comprising soil, rock, and other loose materials moving downslope at speeds of about 4km/hr. They mostly occur when intense rainfalls completely saturate loose debris which adds weight causing the debris to move. If water content is high they are termed mudflows, though the difference is trivial. Debris flows tend to follow a channelised path such as a valley or a man-made drainage path.
An unvegetated patch of land is more susceptible to cause debris flows. In California, burnt areas from wild fires are likely to accompany debris flows.
Dry debris flows have little water content.
Earthflows
Earth flows consist of a slow-moving thick mass of fine sediments such as silt and clay. The speed depends on the water content available; fast if water is abundant. Most earthflows evolve from a slump that disintegrates into a flowing thick mass of fine sediment.
Lahars
This is a type of mudflow comprising volcanic debris and water. For example, The 1980 Mt St Helens (USA) eruption caused widespread lahars which destroyed several properties and forests.
Avalanche
Avalanches are snow movements mixed with boulders and earth (soil) down a mountain. Earthquakes and volcanic eruptions disturb the snow-capped mountains and trigger the flow, or thawing of snow in summer. In addition, human activities such as skiing can also result in these mass movements.
Falls
Rock Falls
Rockfalls are extremely fast and mostly happen instantaneously.
They occur on near vertical steep scarps where boulders fall vertically through the air under gravity. They are caused by weathering such as freeze-thaw or by earth movements.
Other falling material besides rocks include debris and earth falls along river banks and ice falls.
Topples
Topples are similar to rockfalls with a subtle difference. Instead of rocks falling vertically as in rockfalls, they fall in a forward rotation fashion. Imagine a stack of standing books; when pushed at the back they all fall directly on their faces.
Toppling can be caused by water attacking cracks and joints, weathering, overloading the slopes, or earth movements. Debris and earth can also topple over scarps or river banks.
References & Further Reading
- Varnes, 1978, Slope Movements Classification
- 10 Mass wasting, Open Geology
