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glaciated landscapes [SG6] (glaciofluvial environments: the Holocene is…
glaciated landscapes [SG6]
glaciofluvial environments:
the Holocene is the name given to the current geological epoch which started at the end of the Pleistocene ice age [12,000 BP], and represents a warmer post glacial climate
surface melting: normally melting will increase downstream as a glacier descends into an area of warmer temperatures. aspect, orientation, and seasonal changes in temperature are also influential
geothermal melting: small amounts of heat may be transferred from the bedrock to the base of the ice
precipitation: rain and snowmelt may enter the glacier landscape system as run off down valley sides
pressure melting: heat is produced by the warm-based glaciers when pressure increases the temperature of basal ice above the PMP producing melt-water
meltwater streams:
the meltwater streams associated with the glacier are contained by the walls of the ice tunnel and so flow under hydrostatic pressure and at high velocity due o the smooth surface of the ice. the water of the streams flowing out of glaciers are often milky-coloured due to the presence of rock flour.
another special feature of melt-water streams is their variable discharge pattern [and so variable velocity] this means that the energy of the streams changes influencing erosion / deposition
what are glaciofluvial sediments like:
sorted
graded
stratified
smooth and rounded
varve:
as melt-water streams flow into a lake, deposition builds up on the lakebed. larger material [sand] forms the lighter layer in the photo, and is deposited by meltwater streams in spring and summer when discharge is higher. smaller materials [silt] forms the darker layer and is deposited by meltwater streams in autumn and winter. in this way, annual deposition builds up layers of light coloured sand, overlain by dark coloured silt on a lakebed
ice-contact features: develop under or against the ice, ex. eskers + kames [delta kame and kame terraces]
proglacial features: develop beyond the limits of the ice, ex. outwash plains such as sandurs
outwash plains also known as sandurs in Iceland:
proglacial streams transport a lot of sediment, but the highly variable discharge means that sediment can be deposited within the channel resulting in braided streams / rivers which flow over the outwash plain. summer = higher discharge + debris laden. winter = lower discharge meaning sediments are deposited within the channel to form channel bars forcing the stream to split up into smaller channels and flow around them
kettle holes: which may contain lakes can form on the outwash plain. these are produced by stagnant blocks of dead ice melting slowly. the dead ice becomes buried by the outwash plain. once the dead ice has melting, the overlying sediment forms a depression in the outwash plain which is often in filled with a lake
proglacial lakes = can form at the ice front from melt-water damned by an end moraine
eskers:
result from the deposition of material carried by subglacial and englacial melt-water streams during the glacial period. the streams are sediment laden due to flowing under hydrostatic pressure. the subglacial and englacial streams flow within tunnels.
deposition occurs when pressure is released as melt-water emerges at the glacial snout. as the glacier snout retreats, the point of deposition gradually moves backwards to leave an exposed sinuous rdige of material on the valley floor where the melt-water stream had been. some eskers are beaded with beads of greater size most likely to occur when the rate of retreat slowed
with climatic warming, in the post glacial period, there has been higher rates of melting and retreat of glaciers. this has exposed eskers in greater numbers and of greater length on the valley floor
kame hills:
mounds of sand and gravel deposited by a decaying glacier or ice sheet. post glacial slumping [a type of mass movement] common as steep sides of unconsolidated sediment easily collapse once unsupported by ice.
the sediment is more sub-rounded and less well sorted compared to outwash material beyond the snout of the glacier. this is because the sediment has been transported for relatively shorter distances
kame terraces:
flat topped ridge of sand and gravel, post glacial slumping common as step sides of unconsolidated sediment easily collapse once the glacier melts.
location: decomposition occur at the side of the valley, on the floor. the exposed valley sides heat up faster than the glacier ice, resulting in melting of ice at the valley margins. this forms small troughs with melt-water streams. the material is deposited by melt-water streams between a glacier and an adjacent valley wall
the sediment is more sub-rounded and less well sorted compared to outwash material beyond the snout of the glacier. this is because the sediment has been transported for relatively shorter distances
solifluction:
the upper layer of soil in the active layer undergoes seasonal thawing [between 1 and 5 metres in depth]. water cannot drain away due to the impermeable permafrost below, so once thawed the active layer is often saturated and may begin to move downslope even on gentle gradients of 2 degrees. this is a slow type of mass movement
ground ice leading to frost heave:
during the summer, water percolates into the ground. with the onset of winter, the ground nearest to the surface freezes first to form ice lenses between layers of the soil [or even ice needles].
the ice lenses / needles draw in water from the surrounding sediment by capillary action. frost heave causes an increase in soil volume [when water freezes, it expands by 10%] so there is an upward expansion of the soil surface.
this causes the surface to become domed. frost heave is most significant in fine grained material, ex. clays and silt which is sufficiently porous to allow capillary action and the growth of ice lenses
other periglacial processes:
the seasonal variation in discharge of the rivers in periglacial areas means that erosion, transportation and deposition become dominant at different times of the year
strong winds can also blow large amounts of fine particles, such as sand and silt, exacerbated by limited vegetation cover, and which may blanket existing relief. the wind-blown deposits are deposited as layers of loess
pingo / ognip:
there are two types of pingo - open / closed system.
open system pingos occur in valley bottoms in areas of discontinuous permafrost. water percolates through the talik within the discontinuous permafrost. groundwater from the surrounding slopes collects under gravity on the valley bottom. the water rises to the surface under artesian pressure. as the ground water rises towards the surface, it encounters colder ground and freezes. the ice lens continues to grow and becomes very large due to the continual supply of rising ground water. displacing ice lends to form a pingo
ognip: ice lens penetrates the surface and is melted by the sun. overlying sediment collapses forming a depression and is filled with rainwater.
thermokarst: is used to refer to areas of hummocky, irregular ground interposed by waterlogged depressions created as ground ice, including permafrost thaws