Glacier Mass Budget
Accumulation:
How?
Addition of ice to the glacier
Snowfall, Freezing Rain, Avalanches
Ablation:
How?
Removal of ice from the glacier
Surface or basal melting, evaporation,
sublimation calving or wind blow)
Snowline: Altitude above which there is net snow accumulation
Latitudinal Variation in Snowline
Equator: Snow only at highest altitudes
0 to 25°N or S: Snowline lowers with latitude
25 to 30°N or S: Snowline rises again due to dryness
30 to 50°N or S: Snowline drops regularly
50 to 70°N or S: Snowline drops rapidly (cold and humid)
Polar Regions: Snowline at sea level but glaciers inactive (dry)
Positive Budget
(Accumulation)
Negative Budget
(Ablation)
North America:
nucleation in Labrador
(high snowfall/cool summers)
and Keewatin
(cold reasonable snowfall).
No glaciation in Alaska
due to dryness.
Pleistocene glaciation occurred further north in Europe than in NA
Why ? Warming influence of the Gulf Stream
Glacier Formation and Morphology
Glacier
Stratigraphy
(See Lecture 1)
Glacier Mass
Budget and
Morphology
Thermal Classification of Glaciers:
A
A. Temperate Glaciers:
- Warm ice throughout
- Upper active layer is thick
- Meltwater percolates and refreezes
- Percolation maintains ice at pressure
melting point (latent heat of phase change)
- Deeper ice colder than shallower ice
- Heat from below cannot be conducted:
melts at base
B
B. Polar Glaciers:
- Cold ice in inner layer
- Thin active layer
- Thick firn in accumulation zone (high pressure
metamorphism only)
- Frozen to the bedrock unless thick enough for
basal melting (adjacent zones still frozen; see C)
C
Morphological Classification of Glaciers
Valley Glaciers
Ice Streams
Reticular Glaciers
Outlet Glaciers
Alpine Glaciers
Cliff and Reconstituted
Wall-Side Glaciers
Cirques and Aretes
Apron Glaciers
Ice Sheets
Continental Ice Sheets
Ice Caps
Plateau Glaciers
Highland Ice-sheets
Lowland Glaciers
Piedmont, Expanded Foot, Fringing and Stagnant Glaciers
ICE SHEETS:
Large, unconfined masses
Flow in irregular radial fashion from central ice domes
Multiple domes are possible
Only somewhat affected by underlying bedrock
CONTINENTAL ICE SHEET
Continental-sized dome with few nunataks (eg. Antarctica/Greenland)
Greenland
Ice Sheet
Nunataks
ICE CAPS: Dome-shaped or flat ice-sheets with nunataks.
Ice Cap
(Vatnajokull, Iceland)
Plateau Glaciers:
Flat ice-sheets on highland plateaus
Tongue-like ice cascades often along edges
Nunataks at margins
(Eg. Iceland and mountains of Norway)
Highland Ice-sheets:
Broad ice-sheets at high altitude
Undulating surfaces due to bedrock morphology
Many isolated nunataks
(Eg. Canadian Rocky Mountains)
Ice Stream:
Ice stream
(Greenland)
Long, slender, fast-moving ice mass
within a continental ice sheet
VALLEY GLACIERS:
Elongated and streamlike glaciers in valleys
Flow is confined by the valley walls
Reticulated Glacier:
- A valley glacier almost like an ice sheet but
with flow channeled by the underlying bedrock
- Can be thought of as a step toward a valley glacier
from an ice stream
Outlet Glacier:
- An emegence of a glacier downward from a higher
ice sheet or through valleys
Outlet Glacier
(Vatanajokull, Iceland)
Alpine Glacier
Mountain glaciers
confined within
bedrock valleys
1. Simple
(one valley)
2. Dendritic
(main glacier
joined by
secondaries)
3. Hanging
(tributaries reach
valley/glacier at
higher elevation)
4. Composite (various
glaciers join bur keep
identity)
Simple Alpine Glacier
Hanging Glacier
Cliff and Reconstituted Glacier:
Forms on a slope too steep to hold ice
Ice falls to the bottom of the cliff and is reconstituted
Cirque:
Masses of ice in small, confined bedrock alcove
Name derived from the fact that they sometimes take on
circular or oval shapes
Cirques
(Grand Tetons)
Apron Glaciers:
Thin masses of snow and ice on mountainsides
Apron Glaciers
(Mt. Adams,
Washington)
Lowland Glaciers
Piedmont and Expanded Foot Glaciers:
- Form where valley glaciers spread out onto flatter areas
- Piedmont glaciers are gently-sloping and multilobate
Fringing Glaciers:
- Remnants of shelf-ice forming a belt along sea coasts
Stagnant Glaciers:
- Hummocky, debris-laden terminal zones of glaciers
- May support vegetation
Piedmont Glacier
(Malaspina Glacier, Alaska)
Piedmont Glacier
Stagnant Glacier
Glacier Movement
“Glaciers move on their own, flowing to reach a
gravitational equilibrium form, with material moving
from the accumulation to the ablation zone”
SLOW:
MID-SPEED:
FAST:
FASTEST:
Polar or debris-laden temperate glaciers
Temperate glaciers move a few
centimetres per day
Glaciers on steep slopes
Outlet glaciers can reach 80m/day
Cliff fall
Summer: Fastest movement in ablation zone
Winter: Fastest movement in accumulation zone
Movement is non-uniform, occurring when
pressure overcomes frictional resistance of ice
and obstacles further along path of movement
River-like movement
- valley glaciers slowest at flanks and along the base
- fastest at centre and along the surface
- box-shape flow profile during surges with shearing
near (but not at) the confining wall
Flow in regular,
U-shaped valley
Block flow of a
surging glacier
Flow in an irregularlyshaped valley
Forces in Glacier Movement
1. Internal stresses from weight of own material
(Most important)
2. Expansion and contraction due to internal
temperature fluctuation (bending/breakage)
Freeze-thaw processes enhance glacier movement
3. External forces where a glacier meets the sea
(winds, waves, tides and currents)
Brittle deformation dominant in surface layers
Plastic deformation dominant at greater depth
Glacier Movement
A.
Intergranular adjustments in snow and firn
B.
Downward water movement followed by refreezing
(within glacier or downslope)
C.
Slippage in glaciers at pressure melting point.
(Movement enhanced by soft, wet sediment at base)
Slippage perpendicular to c-axis
D.
Internal slippage along fractures, especially near
terminus (thrust faulting)
Mechanisms of Glacier Movement
Movement of Ice Sheets
Melting at base reduces friction allowing movement
1. Movement slow at centre due to low precipitation
and the lack of melting
2. Margins are most active since there is higher
precipitation and melting
3. Polar glaciers can be frozen to the bed (move through
internal deformation)
4. Portions of glaciers move faster than others (ice streams)
Extending and Compressive Flow
Glaciers move faster on steep slopes
This causes tension in upper reaches (pulling) and
compression in flatter, lower portions
Extending flow on slopes helps to pull upstream portions
Compressive flow may cause thrust faults if tension
exceeds brittle strength of ice
Thrust faults at terminus affects composition of moraines
(sediments thrust upward from bottom of glacier)
Thrust faults may form against stagnant parts of ice sheet or valley glacier
(inset) due to compressive flow. Crevasses may form in extending flow.
Crevasses
Bergschrund crevasse: Forms at the head of a valley
glacier as it slides from the headwall
Transverse Crevasse:
Faster part of glacier pulls away from slower part
due to a change in slope
Marginal Crevasse:
Tensional stresses develop due to the difference in speed
between the centre and the sides of a glacier
Splaying Crevasse:
Tensional stresses develop due to radial flow as a glacier
expands into a wider part of a valley or an outlet
In all cases, poorly-sorted sediment may accumulate
in crevasses (crevasse fillings), forming small, elongated
hills after melt.