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Glaciers and Glaciation > Week 3: Glacier flow > Flashcards

Week 3: Glacier flow Flashcards

1
Q

Over long periods of time, glacier flow is a function of:

A
  1. CLIMATIC INPUTS
    - amount snow/ice going into catchment
  2. SIZE/GEOMETRY
    - with constant shape/size of catchment through cross section of glacier, ice flow through the cross section must balance acc up-glacier and abl down-glacier to maintain steady state
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2
Q

Wedge model

A

Benn and Evans 1998

diagram

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3
Q

Steeper mass balance gradient/balance velocities =

Generally

A

More rapid flow

e.g. summer

Greater mass turnover

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4
Q

Shallow mass balance gradient/balance velocities =

Generally

A

Slower flow

e.g. cold polar

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5
Q

What else can affect the mass balance gradient following general trends

A

Topography
- e.g. convergent funnelling = increased velocity

Glacier driving/resistive forces
- may not be in equilibrium with climate
E.G. AIS ice tributaries (Bamber et al 2000)

= measured differ from balance values

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6
Q

Faster measured mass balance than expected

A

Water at base/slippery bed

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7
Q

Slower measured mass balance than expected

A

Good drainage system

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8
Q

Glaciers are driven by…

A

STRESS AND STRAIN

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9
Q

Stress =

A

How much material is being pushed/pulled due to external forces

Measure of distributed force

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10
Q

Strain =

A

Amount of deformation due to imposed stress

Rate can be linear/non-linear

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11
Q

Normal stress =

A

Largely result of weight of overlying ice

= (ice density) x gravity x height

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12
Q

Shear stress (basal shear) =

A

Parallel to slope

= (ice density) x gravity x (height x sin(a))

a = surface slope

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13
Q

Increasing ice thickness effect on stress…

A

STRESS INCREASES WITH ICE THICKNESS i.e. stresses are highest at the BED

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14
Q

Topography effect on stress

A

Stress concentration on stops side of ‘bumpy’ bed and dip in stress on lee side

Graph diagram Benn and Evans 1998

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15
Q

Longitudinal stress effect on stress

A

Compressive force from ice pushing from upstream

Tensile force from ice pulling from downstream

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16
Q

Types of strain

A

Recoverable/elastic

Irrecoverable/permanent

BRITTLE/DUCTILE/shear/pure (Benn and Evans 1998)

17
Q

Critical yield stress =

A

Stress at which permanent deformation/failure occurs e.g. ice fractures into cavities

18
Q

Types of deformation

A
  1. Constant volume deformation (decreases)
    - becomes squished together
  2. Dilatancy
    - material INCREASE in vol as deforms
    - subglacial sediments
    - shear over them
    - ‘climb’ over each other at microscopic scale
19
Q

Is ice:

a) perfectly plastic which eventually reaches a yield and deforms
b) newtonian viscous material with strain rate proportional to shear stress
c) non-linear viscous material

A

C)

20
Q

Forms of glacier flow

A

ICE DEFORMATION

BASAL SLIDING

SUBGLACIAL DEFORMATION

21
Q

Glacier flow: ice deformation

A

Ice creep

Ice fracture

Glen’s flow law = exponential relationship b/w strain rate and shear stress (often to the power of 3) but in practice v different due to variabilities

22
Q

Variabilities affecting ice deformation relationship of strain rate/shear stress

A

Ice crystal orientation (cleavage planes)

Impurities (solutes/gases/bubbles/solid debris)

23
Q

Ice creep =

A

Movement w/in or b/w individual crystals

24
Q

Ice fracture =

A

Brittle failure forming crevasses

25
Q

Glacier flow: basal sliding

A

Water lubricates and smooths bed

  • water P reduces frictional stress
  • reduces contact of ice to bed

Requires meltwater (bed) at pressure melting point

26
Q

PMP =

A

Pressure Melting Point

Not just 0’C due to increasing P with depth, therefore PMP also increases

e.g. beneath 2000m of ice = -1.27’C

27
Q

Sticky point =

A

Localised patch of higher basal friction on bed

28
Q

Causes of sticky points

A

ADHESION DUE TO FREEZING

  • cold ice
  • since basal sliding requires meltwater at PMP, low T = low P = not achieved
  • (low T also retards creep/strain rates)

BED ROUGHNESS
- drag

LACK OF LUBRICATING WATER AT BED

  • can smooth bed
  • represents efficient removal

DEBRIS AT BASE OF BED

  • frictional drag
  • N.B. difficult to model
29
Q

Overcoming bed roughness

A

Regelation sliding
- water melting/refreezing on bumps

Enhanced creep

  • (Glens flow law)
  • stress concs locally enhance strain rates = ice accelerates around obstacles
  • larger obstacle = greater strain = more effective
30
Q

Glacier flow: subglacial deformation =

A

Sediment BENEATH the glacier undergoes permanent strain due to applied stresses of glacier ice

31
Q

Subglacial deformation experiment

A

Boulton 1986

Till saturated at v high porewater P

Upper till (0.5m) = 80-95% of forward motion = ductile/viscous

Lower till = brittle

32
Q

Glacier surge =

A

Period of rapid advance (months/yrs) followed by quiescent phase (yrs/decades) of much longer duration

Linked to re-organisation of drainage system
- more organised/efficient = stops

33
Q

What does clustering (location) of glacier surges suggest?

A

Climatic influence (Semester and Benn 2015)

  • glacier requires balance b/w mass gains/losses via heat/meltwater runoff
  • this is achieved in cold/dry and warm/humid environments
  • what about in between?
34
Q

Do surge glaciers measurements match balance velocity?

A

NO

GET STUCK

BUILD UP

SURGE QUICKLY

35
Q

Active phase of surge glacier

A

Mass moves from up glacier to snout

Velocities 10 x quiescent phase

= thins glacier + reduces surface gdt
= stagnation glacier snout

36
Q

Ice stream =

A

Region in grounded ice sheet where ice flows much faster than regions on either side (Paterson 1994)

37
Q

Ice stream examples

A

Antarctica = too cold for surface melt = ice streams!!!

Losing 10 GT through them draining into oceans
- 96%, even though only takes up 13% of SA

Greenland = surface melt AND ice streams

38
Q

Ice stream facts

A

Fastest flowing ice in world up to 12,000m/a

Large; 300km long 30km wide

Dominate ice discharge

Contribution to SL rise

Complex behaviour

Always above soft, saturated, slipper sediments
- controlled by conditions at bed

39
Q

Ross Ice Streams

A

Antarctica

Low driving stresses (flat) but rapid velocity
Extremely low basal shear stresses 2kPa = slippery bed
?Subglacial deformation +/ basal sliding???
Relatively rapid switches in velocity/location

Glaciers and Glaciation (29 decks)

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