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
