Mass movement (mass wasting) is the movement of material downslope due to gravity. If the shear stress of a slope overcomes the shear strength of material and friction, movements occur. Mass movements requires a triggering mechanism which weakens the immobile overlying material and stresses the slope enough for movement to occur. Triggering or causing mechanisms include:
- Earth movements such as earthquakes, volcanoes, folding and faluts
- Water which lubricates cracks & joints or adds weight to the overlying material
- Animal action e.g burrowing
Humans on the other hand can cause or trigger mass movements. For example:
- Noise vibrations from large vehicles such as bulldozers
- Uprooting trees
- Undercutting slopes for buildings and roads
- Read more: Human impact on slopes
In addition, mass movements depend on:
Unstable slopes such as along faults or slopes undercut by a river can result in rapid mass movements. In addition, slopes situated at earthquake prone areas are more prone to mass movements.
Steep slopes promotes fast movements that occurs instantaneously because of shear stress of the slope; conversely on gentler slopes movements tend to be slow e.g soil creep.
Water ContentWater 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 may help to bind and stabilize soil movements. Lack of vegetation means mass movements are more likely to occur.
Cohesion simply means how materials are well bonded together (jointed). Incohesive materials such as sands are more prone to movement than clays which have shear strength and resist movement. Once clays are saturated they often move en mass rather than individual grains.
Types of Mass movements
Include soil creep, talus creep and solifluction. They occur on gentler slopes of about 6o.
Soil creep is the slowest of all mass movements (1-2mm in the humid temperate and 10-20mm /year in the tropics). Soil creeps are mainly caused by wetting and drying or by freezing and thawing. Both of these processes adds weight to the soil which triggers their partial movement. Upon wetting the soil piles (heaves) up and subsequent drying contracts the soil and falls causing it to move. In freeze thaw scenarios the soil freezes and piles up and thaws upon warmed causing movement. The process of soil piling up and moving at right angles to the slope is called heaving. Soil creep tend to tilt trees, poles and burst walls as soil continue piling up at the wall foot.
Talus creep is similar to soil creep but involves movement of coarse stones.
This type of mass movement occurs in periglacial areas where the extreme cold temperatures not only freezes the surface, but also freezes the underlying bedrock. This makes it impossible for rainwater or melted water to infiltrate downwards, instead, it moves on top of the impermeable layer collecting any debris or soil in the path. This creates solifluction lopes or sheets.
Flows are usually faster and holds large amounts of water. They include mudflows, earthflows, avalanches, lahars.
Mudflows are saturated clays moving fast on steep slopes. They mostly occur when intense rainfalls completely saturates the soil which adds weight and causes the debris to move.
Earthflows are similar to mudflows but are slow because they mainly transport coarse material.
This is a type of mudflow comprising volcanic debris and water, e.g. 1980 Mt St Helens (USA) eruption caused widespread lahars which destroyed several properties and forests.
Avalanches are snow movements mixed with boulders and earth (soil) down a mountain. Earthquakes and volcanic eruptions disturbs the snow capped mountains and triggers the flow or thawing of snow in summer. In addition, human activities such as skiing can also result in these mass movements.
Slides are moderate to fast and contain considerable amounts of water. Depending on the water content they can reach 300km/hr. Water attacks areas of weakness by increasing pore pressure which triggers movement. Water in slides is mainly used to attack lines of weakness while the moving body is dry. Slides include translational/planar, slumps/rotational slide and rock slides.
Translational slide/ Landslide
These are blocks of land that slide along a slip plane. Usually water attacking lines of weakness can cause the block of land to slide down. Water increases soil pore pressure and the shear stress of the slope is overcomed by the shear strength of the material resulting in a landslide.
Slumps are blocks of land sliding and resting on top of another on a curved slope. The former block rests on top of the latter block; meaning the first block rests on the second, the second on the third and so on. Blocks of land may pile up until a hard resistant scarp is reached where the last block rests. Slumps are not as fast as landslides.
Involves rocks sliding down along a parent rock with joints and bedding planes.
Rockfalls are extremely fast and mostly happen instantaneously. They occur on steep scarps where boulders readily falls or when the scarp itself is weathered along lines of weakness by weathering such as freeze-thaw. They can also be triggered by earth movements. Other falling material besides rocks include debris falls and ice falls.