LINKS BETWEEN HYDROLOGY & GLACIER DYNAMICS
GEOG 4750
Glacial processes, measurements & models
Variations in water input affect basal water volume and pressure
Accumulation zone:
slow hydrological system: slow response
Ablation zone:
fast hydrological system: immediate response
slow hydrological system: slow response
Lecture 12
Glacier erosion(1)
Erosional power
Dr. Hester Jiskoot
Basal sliding controlled by basal water pressure or volume?
Surface velocity peaks in late spring to early summer
Velocity variations too large to be explained by changes in mass-balanceinduced changes in basal stress.
Æ Surface velocity variations caused by variations in basal sliding, not ice
deformation
Æ But local variations of basal sliding do not translate over large distances
HYDRAULIC JACK EFFECT
Observations:
Nisqually glacier, Washington State (Hodge, 1974)
Max sliding early in melt season
Æ sliding speed = f(water volume)
Findelengletscher, Swiss Alps (Iken et al., 1983)
Storglaciären, Sweden (Jansson, 1995)
Max sliding coincides with rising glacier surface: UPLIFT
Æ water going into storage, rather than max storage
Æ sliding speed = f(Pw)
Columbia glacier, Alaska (Meier et al., 1994; Kamb et al., 1994)
Iken and Bindschadler, 1986
Surface-velocity fluctuations correlated over a length of 7km and with borehole
pressure upglacier
Æ variations in sliding = f(amount of water stored at the bed)
EROSIONAL POWER
Basal sliding controlled by basal water pressure or volume?
(Andrews, 1972)
Models of basal cavitation (Lliboutry, 1968; Iken, 1981; Fowler, 1986; Kamb, 1987):
Prediction: increase of basal storage with increase of pressure
Observations: Sliding speed sometimes correlated with pressure, sometimes
Total Power:
Wt=τU
Total power = shear stress x velocity (measured in watts= Joules per second)
Shear stress ~ constant at a location
Wt ∝ U
with stored volume, BUT NOT WITH BOTH!
Effective erosional power:
Solution: Sliding speed (Ub) and basal storage controlled by Pw averaged over L
(L= length scale over which basal stress is averaged by glacier dynamics)
pw measured at a point is the sum of the mean Pw and the local p’w, where p’w is
controlled by local reorganisation of the basal drainage system.
We = part of Wt
We / Wt related to the proportion of basal sliding to internal deformation
If total U = basal sliding Æ then We / Wt=1
We / Wt ~ 0.5 for temperate glaciers and We / Wt < 0.1 for cold-based glaciers
Æ Ub correlates with storage, but not with pw
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Example:
Glacier with area
= 1 km2
width
= 430 m (at firn line)
thickness = 50 m (at firn line)
U varies from 2-40 m/y
Wt = 0.25-1.5 Watts
We = 0.03-1.0 Watts
EROSIONAL PROCESSES
Abrasion:
Abrasion:
Particles at glacier base scour/drag over the rock surface: sandpaper effect.
Rock flour evacuated by meltwater.
http://gemini.oscs.montana.edu/~geol445/hyperglac/eroproc1/Erosional~process3.GIF
Plucking and quarrying:
Freeze on and dislocation of parts of bedrock by thaw/freeze
Æ T base fluctuates around pressure melting point
Makes use of pre-erosional jointing and fracturing
Plucking and quarrying:
http://gemini.oscs.montana.edu/~geol445/hyperglac/eroproc1/animated~plucking.gif
Meltwater erosion:
Sediment–laden meltwater abrades efficiently Æ suspended load
Meltwater dissolves carbonates Æ solution load
Shear strength of materials (Coulomb equation)
For any material there is a critical stress to overcome the internal strength of the
material (shear strength = resistive force that prevents permanent deformation)
Shear strength = f (cohesion + intergranular friction)
τ* = c + N tan φ
Shear strength (N m-2)
Shear strength = cohesion + (effective normal stress x coefficient of friction)
τ* =
c+
nφ
N ta
c
Normal stress (N
m-2)
Material
Cohesion
Friction
angle
Sand
Gravel
Soft clay
Stiff clay
Till
Soft sedimentary rock
Igneous rock
0
0
30-70
70-150
150-250
1000-20000
35000-55000
32-46
34-48
27-32
30-32
32-35
25-35
35-45
ABRASION MODELS (1)
Coulomb friction (Boulton, 1974; 1975; 1979)
τ* = c + N tan φ
Coulomb shear strength
N = pi − Pw
Effective pressure = ice overburden pressure – water pressure
τ ' = ( pi − Pw) tan φ
basal friction = f (Effective normal pressure x internal friction angle)
Abrasion controlled by effective normal pressure (stress) and
is inversely proportional to basal water pressure
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ABRASION MODELS (2)
Hallet friction (Hallet, 1979, 1981)
So where does the ice move fastest
towards the glacier base?
Friction independent of ice thickness (independent of N or Pw)
A = αDF n
Abrasion rate=α (Number of particles in contact with the bed x Effective force)n
α and n are constants related to the relative hardness of the bed and the particles
Abrasion controlled by rate at which ice flows towards the bed
Abrasion highest where
a) basal melting is greatest
b) below large heavy particles
c) ice flows fastest towards bed
ABRASION MODELS (3)
Sandpaper friction (Schweizer & Iken, 1992)
Debris rich basal ice layer
Leverett Glacier
W Greenland
Valid for debris rich basal ice
τ ' = ( pi − sPw) tan φ
s = the proportion of the bed occupied by cavities ( 0 < s < 1 )
Similar to Coulomb model, but friction is a function of water
pressure and area of the bed occupied by cavities
SO WHAT IS THE BEST ABRASION MODEL?
Coulomb friction
EFFECTS OF EROSION
Abrasion:
Valid for friction between two rigid bodies
Reasonable approximation:- rigid slabs of debris-rich basal ice
Smoothing of surface and striations
Elongated ridges and troughs in the flow direction
- subglacial deforming layers without interstitial ice
- particles are in direct in contact with bed (but air-or
water-filled cavities possible under particle)
Hallet friction
Valid
- when particles in glacier bed are spaced far apart
- no contact between particles
- ice ‘envelops’ particles
- ice flowing around particles not influenced
Sandpaper friction (Schweizer & Iken, 1992)
Plucking and quarrying:
Roughening of surface transverse to flow direction
Localized
Meltwater erosion:
Localized smoothing of surface
Soft sediment erosion:
Folding and squeezing out of layers
Short distance transport
Valid for debris-rich basal ice (50 % by volume)
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Striations, Haig glacier, Kananaskis
Striations in front of Columbia glacier, Columbia Icefield
Forefield of Columbia glacier, Columbia Icefield
Crescentic gouges,
West Greenland
What is the ice flow direction?
‘Whalebacks’ with till layer, West Greenland
Roche Moutonnée, Yosemite National Park, CA
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Roche Moutonnée, Switzerland
EROSION = f (GLACIER POWER & TIME GLACIATED)
Mean glacial erosion rate = 1-5 mm/year
Æ 1 km/million years
Æ Quaternary glaciations 1-2 million
years of glacial erosion
Fjords in Ryfylke region, Norway
Paternoster lakes, Carthew-Alderson, Waterton
Erosion rates
Average 1mm/year for small temperate glaciers
Variation 0.1-40 mm/year
Sediment transport for erosion rates <0.6 mm/yr
Æ 700-22000 103 kg/year
RANKING OF GLACIAL EROSIONAL MECHANISMS
Erosion mode
Cold base
Melting base
Hard rock
Soft rock
Abrasion
2
2
1
Plucking
1
1
2
Meltwater (mech)
3
3
Meltwater (chem)
4
4
ENTRAINMENT OF SEDIMENTS IN ICE
Subglacial:
Englacial/supraglacial:
Regelation (onfreezing)
Rockfall
Ice-debris accretion
Avalanches
Block incorporation
Atmospheric precipitation
Overriding
Aerosols and gases
Squeezing and thrusting
1= highest, 4 = lowest
From: Drewry, 1986
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