Mass Movement
Mass movement is the name given to the process by which soil and
rock move down slope under the force of gravity. The strength and
cohesion of the soil helps to maintain the stability of a slope. When
gravity exceeds the resistance of the soil mass on a slope then
failure or mass movement occurs. Factors that affect slope stability
include slope angle, texture, mineral composition, weathering,
water content and vegetation cover.
The rate of mass movement can vary dramatically from catastrophic
falls that might occur over seconds to slow creep that occurs over
hundreds or thousands of years. Mass movement can occur in
many forms such as creep, slides, flows, topples and falls.
Solifluction
The rate of mass movement can vary dramatically from catastrophic
falls that might occur over seconds to slow creep that occurs
over hundreds of years. The picture below shows the scar of a
catastrophic landslide in glacial till on a steep slope. The slope
failure was provoked by the growth of ice crystals which weakened
adhesion between soil particles. (AJ)
Creep
Soil moves down slopes under gravity. The evidence for this can be
easily seen in many ways. One of the most common examples is the
formation of small terraces or natural ‘steps’ running across the slope.
The process responsible for these features is known as creep.
In cold climates, surface heave following the formation of ice crystals
in the soil is directed at right angles to the slope. When the ice thaws,
the soil particles drop in a vertical direction as a result of gravity. In
this manner, the soil mass is slowly displaced down slope through
cycles of lifting and falling. This process is referred to as frost creep.
Detachment slides occur on slopes when the active layer separates from the underlying
permafrost layer and slides down the slope. This process is illustrated in the above
photograph taken on Banks Island, Canada, where several hundred metres of the hillside
has travelled down the hill as a huge soil mat. (CT)
Slopes affected by creep in permafrost regions can be vulnerable
to processes known as solifluction when the soil becomes
waterlogged. Solifluction is the down slope movement of water
saturated soil in a viscous or plastic state over an impermeable layer,
often permafrost. The presence of an impermeable permafrost
layer prevents the internal drainage of the soil, forcing the soil to
flow down the slope. During warm periods the surface layer thaws
and slides across the frozen layer, slowly moving down slope due to
frost heave (as for creep above). As for other processes initiated
by freeze–thaw cycles, solifluction is enhanced by the considerable
volumetric changes of water in the soil.
Solifluction can occur on slopes as gentle as 0.5 degrees and
soliflucted material can travel as much as 15 cm per year. On
steeper slopes catastrophic solifluction can occur causing the rapid
movement of soil down slope.
Periglacial
The term periglacial refers to locations at the limits of glaciated areas
characterised by intense freezing, the presence of permafrost and possibly
summer thawing of the active layer. Evidence for periglacial conditions
include mixed deposits, ice wedges and solifluction lobes.
The photograph shows the dramatic and destructive
consequences of a catastrophic solifluction event on a slope
with thixotropic soil conditions. The sudden movement of the
soil exposes the roots which causes the trees to fall. (IS)
Due to their extensive root system, trees attempt to resist creep.
This ongoing battle causes marked deformations of tree trunks.
A tree adapts to the slow movement of soil down the slope by
changing its shape so that the majority of the trunk is vertical to
combat the risk of falling. Unfortunately, trees cannot resist the
sudden or catastrophic sliding of large blocks of land. (AJ)
Thixotropy
In permafrost regions, over-moistened soil can be subjected to the
widespread phenomenon called thixotropy. Under certain moisture
conditions, particles that make up the soil mass can remain in a stable,
viscous or glue-like consistency. However, under some external
mechanical impacts (e.g. heavy vehicles) the soil body can rapidly lose
its viscosity, become unstable and rapidly slide down the slope.
It is a real challenge to dig a pit in a thixotropic soil as every strike of the
spade makes the pit walls unstable, causing the soil to flow into the pit.
In contrast to erosion, this process cannot be stopped by vegetation. Even
very old trees are twisted like matches by powerful thixotropic flows.
Solifluction is the downward movement of water-saturated soil. Solifluction occurs primarily during
the summer when the uppermost part of the soil thaws while the lower layers still remain frozen.
As the ground thaws, the underlying permafrost prevents water infiltration, increasing the moisture
content in the soil. Eventually, the ground begins to slump down the slope, to form a distinctive lobeshaped landscape. The photograph shows localised small-scale, soilfluction lobes surrounding a large
boulder in Svalbard. Solifluction can also occur over large areas and can affect entire hillsides. (HG)
Soils in Northern Latitudes | Soil Atlas of the Northern Circumpolar Region
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