Week 3: Glacier flow Flashcards
Over long periods of time, glacier flow is a function of:
- CLIMATIC INPUTS
- amount snow/ice going into catchment - 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
Wedge model
Benn and Evans 1998
diagram
Steeper mass balance gradient/balance velocities =
Generally
More rapid flow
e.g. summer
Greater mass turnover
Shallow mass balance gradient/balance velocities =
Generally
Slower flow
e.g. cold polar
What else can affect the mass balance gradient following general trends
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
Faster measured mass balance than expected
Water at base/slippery bed
Slower measured mass balance than expected
Good drainage system
Glaciers are driven by…
STRESS AND STRAIN
Stress =
How much material is being pushed/pulled due to external forces
Measure of distributed force
Strain =
Amount of deformation due to imposed stress
Rate can be linear/non-linear
Normal stress =
Largely result of weight of overlying ice
= (ice density) x gravity x height
Shear stress (basal shear) =
Parallel to slope
= (ice density) x gravity x (height x sin(a))
a = surface slope
Increasing ice thickness effect on stress…
STRESS INCREASES WITH ICE THICKNESS i.e. stresses are highest at the BED
Topography effect on stress
Stress concentration on stops side of ‘bumpy’ bed and dip in stress on lee side
Graph diagram Benn and Evans 1998
Longitudinal stress effect on stress
Compressive force from ice pushing from upstream
Tensile force from ice pulling from downstream
Types of strain
Recoverable/elastic
Irrecoverable/permanent
BRITTLE/DUCTILE/shear/pure (Benn and Evans 1998)
Critical yield stress =
Stress at which permanent deformation/failure occurs e.g. ice fractures into cavities
Types of deformation
- Constant volume deformation (decreases)
- becomes squished together - Dilatancy
- material INCREASE in vol as deforms
- subglacial sediments
- shear over them
- ‘climb’ over each other at microscopic scale
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
C)
Forms of glacier flow
ICE DEFORMATION
BASAL SLIDING
SUBGLACIAL DEFORMATION
Glacier flow: ice deformation
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
Variabilities affecting ice deformation relationship of strain rate/shear stress
Ice crystal orientation (cleavage planes)
Impurities (solutes/gases/bubbles/solid debris)
Ice creep =
Movement w/in or b/w individual crystals
Ice fracture =
Brittle failure forming crevasses
Glacier flow: basal sliding
Water lubricates and smooths bed
- water P reduces frictional stress
- reduces contact of ice to bed
Requires meltwater (bed) at pressure melting point
PMP =
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
Sticky point =
Localised patch of higher basal friction on bed
Causes of sticky points
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
Overcoming bed roughness
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
Glacier flow: subglacial deformation =
Sediment BENEATH the glacier undergoes permanent strain due to applied stresses of glacier ice
Subglacial deformation experiment
Boulton 1986
Till saturated at v high porewater P
Upper till (0.5m) = 80-95% of forward motion = ductile/viscous
Lower till = brittle
Glacier surge =
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
What does clustering (location) of glacier surges suggest?
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?
Do surge glaciers measurements match balance velocity?
NO
GET STUCK
BUILD UP
SURGE QUICKLY
Active phase of surge glacier
Mass moves from up glacier to snout
Velocities 10 x quiescent phase
= thins glacier + reduces surface gdt
= stagnation glacier snout
Ice stream =
Region in grounded ice sheet where ice flows much faster than regions on either side (Paterson 1994)
Ice stream examples
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
Ice stream facts
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
Ross Ice Streams
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
