Glacier Mass Budget

Snowline: Altitude above which there is net snow accumulation

Latitudinal Variation in SnowlineEquator: Snow only at highest altitudes0 to 25°N or S: Snowline lowers with latitude25 to 30°N or S: Snowline rises again due to dryness30 to 50°N or S: Snowline drops regularly50 to 70°N or S: Snowline drops rapidly (cold and humid)Polar Regions: Snowline at sea level but glaciers inactive (dry)

Accumulation: Addition of ice to the glacierHow? Snowfall, Freezing Rain, Avalanches

Ablation: Removal of ice from the glacierHow? Surface or basal melting, evaporation,

sublimation calving or wind blow)

Positive Budget(Accumulation)

Negative Budget(Ablation)

North America:nucleation in Labrador (high snowfall/cool summers) and Keewatin (cold reasonable snowfall).

Pleistocene glaciation occurred further north in Europe than in NAWhy ? Warming influence of the Gulf Stream

No glaciation in Alaska due to dryness.

Glacier Formation and Morphology

Glacier Stratigraphy(See Lecture 1)

Glacier MassBudget and Morphology

Thermal Classification of Glaciers:

A. Temperate Glaciers:- Warm ice throughout- Upper active layer is thick- Meltwater percolates and refreezes- Percolation maintains ice at pressuremelting point (latent heat of phase change)- Deeper ice colder than shallower ice- Heat from below cannot be conducted:melts at base

A

B

B. Polar Glaciers:- Cold ice in inner layer- Thin active layer- Thick firn in accumulation zone (high pressuremetamorphism only)- Frozen to the bedrock unless thick enough for basal melting (adjacent zones still frozen; see C)

C

Morphological Classification of Glaciers

Ice SheetsContinental Ice SheetsIce CapsPlateau GlaciersHighland Ice-sheets

Valley GlaciersIce StreamsReticular GlaciersOutlet GlaciersAlpine GlaciersCliff and ReconstitutedWall-Side GlaciersCirques and AretesApron Glaciers

Lowland GlaciersPiedmont, Expanded Foot, Fringing and Stagnant Glaciers

ICE SHEETS:

Large, unconfined massesFlow in irregular radial fashion from central ice domesMultiple domes are possibleOnly somewhat affected by underlying bedrock

CONTINENTAL ICE SHEETContinental-sized dome with few nunataks (eg. Antarctica/Greenland)

GreenlandIce Sheet

Nunataks

Ice Cap (Vatnajokull, Iceland)

ICE CAPS: Dome-shaped or flat ice-sheets with nunataks.

Plateau Glaciers:Flat ice-sheets on highland plateausTongue-like ice cascades often along edgesNunataks at margins(Eg. Iceland and mountains of Norway)

Highland Ice-sheets:Broad ice-sheets at high altitudeUndulating surfaces due to bedrock morphologyMany isolated nunataks(Eg. Canadian Rocky Mountains)

Ice Stream: Long, slender, fast-moving ice masswithin a continental ice sheet

Ice stream(Greenland)

VALLEY GLACIERS:Elongated and streamlike glaciers in valleysFlow is confined by the valley walls

Reticulated Glacier:- A valley glacier almost like an ice sheet butwith 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 higherice sheet or through valleys

Outlet Glacier(Vatanajokull, Iceland)

Alpine GlacierMountain glaciersconfined within bedrock valleys1. 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 iceIce falls to the bottom of the cliff and is reconstituted

Cirque:Masses of ice in small, confined bedrock alcoveName derived from the fact that they sometimes take oncircular or oval shapes

Cirques(Grand Tetons)

Apron Glaciers(Mt. Adams, Washington)

Apron Glaciers:Thin masses of snow and ice on mountainsides

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 agravitational equilibrium form, with material movingfrom the accumulation to the ablation zone”

SLOW: Polar or debris-laden temperate glaciersMID-SPEED: Temperate glaciers move a few

centimetres per dayFAST: Glaciers on steep slopes

Outlet glaciers can reach 80m/dayFASTEST: Cliff fall

Summer: Fastest movement in ablation zoneWinter: 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 asurging glacier

Flow in an irregularly-shaped 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 movement3. External forces where a glacier meets the sea (winds, waves, tides and currents)

Brittle deformation dominant in surface layersPlastic 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 nearterminus (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 melting2. Margins are most active since there is higher precipitation and melting3. 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 portionsCompressive 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 partdue 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 glacierexpands into a wider part of a valley or an outlet

In all cases, poorly-sorted sediment may accumulatein crevasses (crevasse fillings), forming small, elongatedhills after melt.