Glaciology

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Understanding crevasses:
Introduction
•A glacier is a mass of snow and ice
which has been or is in motion
•Snow falls and over time
metamorphoses to ice and flows
from higher to lower altitudes
•The ice flows over the bedrock
topography exerting different
stresses on the ice which can result
in crevasses
Glacier types
• Morphological classification
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Plateau glacier
Valley glacier
Cirque
Hanging glacier
Ice stream
Shelf ice
• Temperature types
– Warm glacier: constant around
zero degrees throughout the ice
mass (eg. Most European
glaciers)
– Cold glacier: temperature
below zero (glaciers on
Svalbard and Antarctic)
– Some glacier massifs can
consist of both types.
Glacier motion
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Can be compared with cold sirup
Movement due to gravity
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Internal movements
Gliding along the bottom
– Both processes occur together
•
Internal movements
– Deformation due to realignment and
movement of individual crystals
– Temperature dependent
– Ice thickness dependent
•
Gliding along the bottom
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Bedrock angle
Bedrock topography
Access to water
More in summer
Daily cycle
Crevasses
•
Crevasses form when the tension
formed by the flow causes some ice to
move faster than ice around it.
– Variations in terrain
– Marked roughness in the bedrock (not
always obvious in Antarctic)
– Widening or narrowing in the valley or ice
stream
– Ice stream flowing through slow moving ice
– Hinge zone (tidal motion)
– < 30m deep in European glaciers, 100m(?)
in Antarctica
•
Types of crevasses
– Marginal crevasses (at ice-rock boundary or
fast ice – slow ice boundary)
– Transverse crevasses (steeper slope /
narrowing / hinge zone)
– Longitudinal crevasses (widening /
submerged ridge
– Radial crevasses
– Bergschrund
Snow metamorphosis
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Snow is continuously subject to a process
of change from new snow to glacier ice
Mechanical degradation
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Chemical degradation
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Wind
Weight of the snow pack
Time
Evaporation from crystal points
Freezing onto flat parts of the crystal
Melt water
Density
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New snow
Firn
Glacier ice
= 0.1 – 0.3 g/cm
= 0.55 g/cm
= 0.8 g/cm
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In Europe the process takes 5-6 yrs
In Antarctica the process takes ca. 100
yrs
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Chemical construction
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Depth hoar builds up in snow bridges
weakening them
Snow bridges
•
Crevasses are often covered with snow and impossible to spot from the surface
•
Support strength is dependent on:
– Snow thickness
– Snows density
– Ice layers
– Air temperature
– Crevasse shape
•
Support strength
– Strongest : Early in the season (Antarctic spring, November)
– Weakest : End of season (February)
– Daily cycle : stronger at night(?)
•
Weakening:
– Snow evaporates /sublimes during the summer (warm days, solar radiation, wind)
– Tiny amounts of melt water percolates down the side of crevasse walls destroying the
bonds between the snow bridge and crevasse wall (hinge zone)
– Uneven driving or sledges swinging up and down over hummocks
•
Strengthening:
– Snow fall
– Wind action compacting the snow
– Freezing of melt water (?)
– Mechanical intervention and physically reinforcing the crossing point
– Repeated driving of heavy vehicles over the same tracks
Cross crevasses perpendicular
– This crevasse was approached at ca. 60°
Detecting crevasses in snow
covered terrain
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Depressions in the snow
surface
Low sun light throws shadows
on the snow
Depressions can collect dust
Often easier to spot from a
distance
Aerial reconnaissance
provides a good picture of the
surface particularly late in the
season
GPR –ground penetrating
radar
Summary
• Where are crevasses
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Hinge zone : shelf ice-inland ice boundary (also edge of ice rises)
Ice streams : fast ice-slow ice boundary
Glacier ice-rock boundary
Terrain features : steeper, direction change, narrowing, widening,
submerged ridge or peak, other irregularities, and nunataks
– “Unexpectedly” in Antarctica (hidden terrain features!)
– Easier to spot from distance / from air / GPR survey
• Snow bridges
– Antarctic snow bridges generally weak
– Strongest in November, and weakest in February (at least in the hinge
zone)
– Probing surface will not necessarily identify weaknesses due to hard crust
layers
– Inspect crevasses to find narrowest crossing point
– Always cross crevasses perpendicular
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