Notes on:
- Origin and development of glaciated landscapes.
- Erosional and depositional landforms: corries, arêtes, glacial troughs, hanging valleys, truncated spurs, roches moutonnées. Characteristic glaciated landscapes.
- Origin and development of landforms and landscapes of glacial depo...
Friction and gradient of the valley floor are key influences on crevasses.
How crevasses form:
When a glacier moves down through a valley the frictional drag between ice and rock causes
the ice within the glacier to move at a slower rate than the centre surface ice. As a result the
ice cracks, producing crevasses on the surface. Stresses and strains also cause crevasses to
form in the glacier. Stress can be caused by extensional and compressional flow, calving, or
tension between the ice attached to the valley sides and back wall and the rest of the
glacier. This is because the surface ice is unable to adjust rapidly enough to extra stresses by
basal slip or internal flow. The tension caused by the glacier pulling away from the ice
attached to the back wall produces a big semi-circular crevasse at the back of the glacier
called the bergschrund.
Types of crevasses:
• Marginal/ shear crevasses – frictional drag between the ice and the rock of the
valley sides causes the margins to move more slowly than the centre. So the
crevasses open at an angle to the margin of the glacier, pointing about 45 degrees
up-glacier from the valley wall.
• Transverse crevasses – formed when a glacier’s gradient steepens and the glacier
surface ‘extends’ itself (tensile stress) as it fits itself to the steeper profile of the
subglacial valley. They close up when the surface of the ice ceases to be extended as
the gradient becomes constant again or reduces. They are common in the
accumulation zone.
• Longitudinal/ splay crevasses – occur when a glacier spreads out laterally as the
valley widens. Here, stresses set up in the ice act across the glacier at right angles to
the margins and this crevasses open up parallel to the glacier margin. They form in
the direction of glacier flow and are commonly found at the glacier terminal.
• Radial splaying crevasses – splay out in the region of the glacier snout and result
from the spreading out of the ice into a lobe. They open at right angles to the
directions of stresses. This happens because the ice on the outside of the bend has
to travel faster than the ice on the inside corner, especially when the glacier turns a
corner. The crevasses radiate out from the inside wall.
• Bergschrund – the tension caused by the ice pulling away from the ice attached to
the back wall produces a big semi circular crevasse at the back of the glacier.
,
,Weathering and erosion
Erosion is the wearing away of the surface of the land. It includes the breakdown of rock
and its removal by wind, water and ice.
Weathering is the breakdown of rocks in situ (in their original location, without them being
moved away). This produces finer particles that can be moved by agents of erosion.
Weathering:
• Produces the debris that glaciers use in the erosive action.
• The main weathering that occurs in cold environments is physical weathering as it is
too cold for chemical or biological weathering processes.
• The most important process is frost-shattering (freeze-thaw action) – this produces
much loose material which may fall from the valley sides onto the edges of the
glacier.
Frost shattering – occurs in areas where
temperatures rise above 0* during the day but
drop below freezing at night for a substantial
part of the winter. Water that enters cracks in
the rocks freezes overnight. As it freezes the
water expands by just under 10%, meaning the
ice occupies more space and so exerts pressure
on the surrounding rock. As the process repeats
and continues, the crack widens, and eventually
pieces of rock break off. On steep slopes this
, leads to the collection of scree at the base; if the slopes are gentle, large block fields tend to
develop.
Erosion:
• Although ice lacks the turbulence and velocity of water in a river it has the advantage
of being able to melt and refreeze in order to overcome obstacles in its path and
thus has the advantage of being able to melt and refreeze in order to overcome
obstacles in its path and thus has the ability to erode the landscape ten times more
quickly than a river.
• Occurs at the ice-rock interface.
• Stationary ice has little, if an, erosive effect.
• Ice that moves may have a considerable erosive effect.
• Ice that is free of debris has little erosive effect.
• Ice that is carrying loose debris may have considerable erosive effect.
• Processes include abrasion, plucking, rotational movement and extending and
compressing flow.
Maximum erosion occurs:
• Where temperatures fluctuate around 0 * allowing frequent freeze-thaw to operate.
• In areas of jointed rocks which can be more easily frost shattered.
• Where two tributary glaciers join, or the valley narrows, giving an increased depth of
ice.
• In step mountainous regions in temperate latitudes, where the velocity of the glacier
is greatest.
Abrasion – is the sandpaper effect of angular material embedded in the glacier as it rubs
against the valley sides and floor. It usually produces smoothened, gently sloping landforms.
Large material scrapes the valley sides and floor causing striations. Fine material called ‘rock
flour’ polishes and smooths the sides of the valley.
Plucking – involves the glacier freezing onto rock outcrops, after which ice movement pulls
away masses of rock. In reality, as the strength of the bedrock is greater than that of the ice,
it would seem that only previously loosened by one of three processes:
- The relationship between local pressure and temperature (PMP) produces sufficient
meltwater for freeze-thaw.
- Water flowing down a bergschrund or smaller crevasses will later freeze onto rock
surfaces.
- Removal of layers of bedrock by the glacier causes a release in pressure and an
enlarging of joints in the underlying rocks.
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