The unconcetrated movement of water along slopes as sheetwash or concetrated in rills
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 depend on: Slope stability: 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. Slope Angle: Steeper slopes provide fast movements and have more shear stress; conversely on gentler slopes movements tend to be slow e.g soil creep. Water Content: Water provides lubrication to the soil which adds weight and triggers mass movements. Vegetation: Vegetation may help to bind and stabilize soil movements.Lack of vegetation means mass movements are likey to occur. Cohesion/Shear Strength: Cohesion simply means how materials are well cemented (jointed). Incohesive materials such as sands are more prone to movement than clays which have shear strength and resist moving. Animal Action: Animals such as worms may burrow the soil causing it to disintegrate and move downslope
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 temperates and 10-20mm /year in the tropics). Soil creeps are mainly caused by wetting and drying or by freezing and thawing. Both these processes add 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 thawing 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 creepTalus 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 freeze 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 saturate the soil ,adding weight and causing the debris to move.
Earthflows are similar to mudflows but are slow because they mainly transport coarse material.
This is a type of mudflow where volcanic debris is completely saturated and moves downslope. e.g 1980 Mt St Helens, USA eruption caused widespread lahars.
Avalanches are snow movements mixed with boulders and earth(soils) down a mountain. Earthquakes and volcanic eruptions disturbs the snow capped mountains and triggers the flow or thawing of snow in summer.
Slides are moderately fast and contain considerable amounts of water. Water attacks areas of weakness and triggers movement. Slides include translational/planar, slumps/rotational slide and rock slides.
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.
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.
Involves rocks sliding down along a parent rock with joints and bedding planes.
RockFalls are extremely fast and almost happen instantaneously.They occur on steep scarps where material readily falls or when the scarp itself is weathered along lines of weakness and the weathered rocks fall. They can also be triggered by earth movements. Other falling material besides rocks include soil and ice.