16 - Glaciers as Landforms
Unconfined Ice Sheet
• 2% of all water
• 88% of FW
• Covers
Antarctica and
Greenland avg
2.5 km thick
• Max 4 km thick
• During
Pleistocene 20%
of water on land
Accumulation and Ablation
Geomorphology of Alpine Glaciers
Valley Glaciers
Equilibrium line ~snow line, maximum flow in
x-sec. Glaciers much higher average surface gradient
than streams
• http://www2.wwnorton.com/college/geo/egeo/animations/ch18.htm
A Cirque Glacier
End of Glacier Debris Flow
Importance of glaciers
• Influence earth’s
energy balance albedo
• Major component of
(fresh) water storage
– affects sea level
• Important agent of
landform evolution
Center moves faster than margins. Note ogives, annual waves
Obstacles
• Upglacier, the
movement of ice
closes joints in the
rock. Only abrasion
occurs.
• Downglacier, over the
crest of the bedrock,
ice flow opens joints.
Meltwater gets into
cracks and freezes,
enlarging cracks.
Plucking removes
rock.
Ledges (obstacles) beneath ice reflected by Crevasse Fields and Ice Pinnacles
(Seracs)
Accumulation zone extending, ablation zone and buried obstacle compressing
Terminus
Davidson Glacier near
Haines, Alaska
An Alpine glacier system
Ice Caps and Continental Ice Sheets
http://www.nmm.ac.uk/server/show/conWebDoc.1221
White, reflective, high latitude, positive feedback
Nunatak
Sentinal Range, Antarctica
•Antarctica is the broadest high place on Earth, the ice cap is up to 4km thick
and covers the continent
•Antarctica is a desert, with only 15 cm (6 inches) of snowfall a year around
the South Pole
•The lowest recorded temperature is -89.2 °C.
•There is no life in Antarctica except near the coast
Nunatak: exposed rocky area not covered with ice or snow
Byrd Glacier, Antarctica, an outlet glacier
Ablation
Vatnajokull
http://pubs.usgs.gov/fs/2005/3056/
Zone of Accumulation
Ablation
Outlet Glaciers
Formation of Glacier Ice
Firn
BTW: note leading edges thrust over terminal debris
Ice Deformation
Wet Glaciers
•
•
•
Warm = wet-based
Cold = dry-based
ρice < ρwater, therefore
Pressure increase on ice increases
density to that of liquid water.
• Causes "melting point decrease“.
Ice than would normally melt at
0oC = 32 o F now melts at a cooler
temp. High pressure melting point
called PMP - pressure melting
point
– 0.7°C/ km of ice above you
• “Warm” ice = thick, fast
• Moves mostly by basal sliding on
wet base
Pressure
Melting
Point
Cold, High Polar, Dry Based
Base ice is frozen
These glaciers do not slide,
they only move by internal
deformation
Plucking
Intermediate (subpolar)
Temperate, Wet Bed
Striations, transport by subice flows
Rapidly Flowing Glaciers
Observed Ice Core Temps
• Greenland
– Tbed < 0°C
• Antarctica
– Tbed ≈ 0°C
Pressure Melting and Regelation
melt
melt
• For ice near Pressure Melting Point:
– Movement increases pressure, thus melting,
on the up-ice side of an obstruction
– Movement away from the obstruction releases
pressure - causes freezing on the down-ice
side – “regelation”
Effects of Pressure Melting
• High pressure is
experienced on the
up-ice side of an
MELT
obstruction.
• Pressure melt
results
• Water migrates
around obstacle
• Regelation occurs in
low pressure zone
REFREEZE =
"Regelation"
• Melt then regelation
Erosion
mix loose bed
material and
refreezing ice at base
– gets into rock
cracks, expands w/
freeze and pushes:
“plucking”
by Plucking
Abrasion
• Plucked
material is
available
to wear
away the
bed –
“abrasion”
Abrasion Features / chattermarks
Polish
• Typical of similar
hardness (bed vs.
tool) and fine load
(~ sandpaper)
Glacial Abrasion in Bedrock
Source: Tom Bean
Glacier Hydrology
Liquid water
flows on, within,
and beneath
glaciers
Subglacial Channels
• Eroded into
bedrock
• Polar
Glaciers
– Pressure
– Tools
Subglacial - Ice Caves
Ice Cave
Mendenhall
Glacier, AK
Photographer:
John Bortniak, 1991
NOAA Corps
Note Ice contact debris
terminus
outwash
To be continued