Glacial
erosion requires the mechanical and chemical breakup
and removal of rock.
Processes
- Abrasion
(microfracturing)
- Polishing (removal of fine
asperities)
- Fracturing of rock
- Chipping
- Crushing
- Jointing
- Crack propagation and
formation
- Debris
entrainment
- Meltwater (corrasion and
corrosion)
Abrasion
Abrasion is the wearing down
of rock through small scale fracturing (crushing).
- Evidence: striations,
grooves and polish
- Fundamental requirements of
abrasion are:
- basal debris
(controversy: can clean ice erode solid rock? Is the ice
actually fracturing the rock or simply removing loose
grains)
- sliding of basal
ice
- transport of debris toward
bedrock
- Other factors affecting rate and
type of abrasion
- Factors affecting basal
contact pressure (summary)
- Ice
thickness
- Basal water
- High basal water pressure
reduces effective pressure
- Water-filled cavities
beneath debris-rich ice can locally increase contact
pressure between a particle and the bed (discussed
more later).
- Particle characteristics:
- Relative hardness of
rock particles and bedrock
- size: the larger the
particle the greater the contact force
- sharpness
- shape: less frictional
drag is exerted by rolling particles than sliding
particles
- Efficient removal of rock
flour
- basal sliding
velocity
- Bedrock topography,
asperities and strength.
-
Abrasion Models
- Boulton's
(1974) model (Also
called Coulomb Friction Model): Abrasion is controlled by
effective normal pressure and sliding velocity.
- Conditions where
applicable: During temporary stages when particles are not
fully encased by ice or where debris-rich slab is being dragged
along the bed.
- Problems:
- may not be applicable in most
situations because
- Doesn't taken into account
that ice deforms around clasts
- basal sliding typically
occurs when basal water pressure is high and effective
normal pressure is low
- Note: Boulton's curve
(Boulton,
1974 ,
fig. 7, p. 52)
- Hallet
(1981) model:
Because ice deforms around particles abrasion is independent of
ice thickness. The force exerted by a particle on the bed is a
function of the size, density, and contact area of the particle
and the rate at which the ice is flowing towards the bed.
- F=Fb+Fi
- Buoyant weight (Fb) =
(4/3)r3(pr-pi)g (size and
buoyant weight of particle)
- Fi=Drag force of
particle resulting from flow towards the bed ( velocity
towards bed)
- Conditions where
applicable: Low debris concentration (<50% by volume),
particles are suspended in the the ice, particles are
transported to the bed by basal melting or extending
flow
- Sandpaper
friction model of Schweizer
and Iken (1992):
Similar to Boulton's model but takes into account the area of
the bed containing water-filled cavities. Because a high
concentration of debris prevents the ice from flowing around the
particles the debris held in the ice behaves much like sand held
in sand paper. Stresses between debris particles and the bed
increase as the percentage of water-filled cavities
increases.
- Conditions where
applicable: A high concentration of water filled cavities
beneath undeformable debris-rich (>50% by volume)
ice.
Summary of factors
governing particle-bed friction (contact force)
(modified from Benn
and Evans, 1998)
- particle-bed friction increases with particle size and contact
area
- basal melting increases particle drag by bringing the particle
closer to the bed
- friction is high against upglacial (stoss) side of
obstacles
- low pressure cavities around a particle will increase the
normal stress exerted by the particle.
- friction increases with debris concentration
- friction is decreases by particle rolling
- high basal water pressure decreases friction
Obstacle related abrasion
In all models abrasion is greatest on the up-ice (stoss) side of obstacles because:
- Stoss side experiences the greatest
effective pressure (Boulton's theory)
- Pressure melting and enhance basal
creep force particles into contact with the stoss slope (Hallet
theory)
- Debris encased in ice through
lee-side regelation attacks the stoss side of obtacles down
ice.
Chipping and fracturing of
bedrock
Chipping
Evidence: percussion
marks (friction cracks)
Fracturing: Processes
of joint formation
- Preglacial processes:
- tectonic deformation: tectonic
(shear) joints
- preglacial dilation: dilation
joints (e.g. sheeting, exfoliation, etc.)
- Subglacial processes:
- Subglacial unloading (dilation
joints)
- Subglacial shattering
along preexisting fractures (discontinuous rock-mass failure)
- Stresses beneath the glacier
work to extend and open preexisting fractures
- growth of subcritical
and critical cracks
- Subglacial frost splitting
- frost splitting is accomplished
by
- ice grown: water drawn
toward a freezing from
- expansion
- Periglacial processes:
Related Terms
- Quarrying: the fracturing
and removal of rock by ice.
- Plucking: The removal of
fractured rock and debris.
- Joint exploitation: The removal
of rock along preexisting joints.
Mechanisms of debris entrainment
- These processes ultimately result
in the suspension of material into the body of the
ice.
- Ice pressure--exerts a
tractive forces on particles
- Deformation of ice around
particle
- Regelation--refreezing of
meltwater
- Obstacle-related
regelation
- Net adfreezing--freezing
of meltwater to the glacier's sole.
- Saturated debris and sediment
can become frozen to the sole.
- Large-scale mechanisms of debris
entrainment
- Ice-debris
accretion
- Dependent on
- a changing thermal regime
beneath a glacier
- non-equilibrium
conditions
- Interior thawed bed:
Beneath thick interior regions geothermal heat and heat
produced by sliding may not be easily conducted to the
surface. Basal ice is melted and the water is forced toward
the margin under high hydrostatic pressure.
- Marginal frozen bed:
Along the thinner margins of the glacier the ice is frozen
to its bed. These two regions are divided by the location of
the 0° isotherm where regelation processes
dominant.
- Movement of the
isotherm in response to changes in ice thickness results
in large-scale incorporation of debris into the
ice
- Block incorporation
(Similar to above theory)
- Dependent on:
- variable thermal
regime
- variable basal water
pressure
- Theory attempting to explain
large thrust blocks and numerous depressions in the Midwest.
(Review Clayton
and Moran 1974, Moores
1990)
- Similar changing thermal
regime required
- Water under high hydrostatic
pressure is forced into unfrozen permeable layers beneath
the impermeable frozen bed. Pore-water pressure increases
reducing the shear strength of the sediment and large blocks
of ground are rafted.
- Produces a variety of
glaciotectonic features
- Mass-wasting and lateral
abrasion
- Debris is incorporated from
topographically high regions either through mass-wasting or
erosion of material from the flanks of an obstacle
- TERM: Nunatak
- Overriding: Incorporation of
the frontal debris apron into an advancing glacier
Meltwater
erosion
- Occurrence:
- subglacial channels and in areas
where there is a thin film of water at the ice-rock
interface
- typically restricted to
warm-based, or temperate, glaciers
- Erosion related to corrasion
(abrasion) is governed by bedload
- Erosion related to corrosion
(solution) is dependent on the solubility of the
bedrock
Factors controlling
the amount of erosion
- Thermal
regime and
meltwater
production
- controls: basal sliding, basal
water, regelation processes, etc.
- Erodibility of
substrate
- Mass balance: controls flow
velocity and the erosive power of a
glacier
- Degree of topographic
inundation
Erosion and Thermal regime
Abrasion and thermal regime
The greatest amount of abrasion
occurs beneath warm-based glaciers. Abrasion beneath cold-based
glaciers occurs as ice flows around obstacles by enhanced basal
creep.
Summary of erosional
process related to thermal regime
(modified from Sugden
and John, 1976, Table 8.2)
-
-
|
Process
|
warm
|
variable
|
cold
|
|
Abrasion due to basal
melting
|
x
|
x
|
--
|
|
Obstacle related abrasion
|
x
|
x
|
x
|
|
Fracturing of fresh rock
|
x
|
x
|
x
|
|
Joint exploitation --
freeze-thaw
|
x
|
x
|
--
|
|
Joint exploitation --
dilation
|
x
|
x
|
x
|
|
Debris entrainment --regelation
|
x
|
x
|
--
|
|
Meltwater erosion
|
x
|
x
|
--
|
|
Meltwater--evacuation
of debris
|
x
|
x
|
--
|
|
Block incorporation
|
|
x
|
|
Transportation of
debris in a glacier
- Review terms: competence and
capacity
Position of debris within the
glacier-source of debris
- Supraglacial
- debris source: rock fall or
material repositioned by convergent flow
- Englacial
- debris source: Later abrasion,
upward transportation from the bed, etc.
- Subglacial
- debris source: bed abrasion,
basal melting, extensional flow, etc.
Transportation paths of glacial
debris
- Supraglacial > englacial;
subglacial > englacial > supraglacial:
- Question: How might the
transportation path affect clast characteristics?
-
-
Topography produced
by glacial erosion
- Alpine regions: erosion
sharpen preexisting relief, valleys are carved deeper.
- Large obstacles (nunataks) and
interfluves are oversteepened.
- Small obstacles are
streamlined
- Continental glaciers
- Areal Scouring
- Overall relief is reduced
- Preglacial overburden is removed
and underlying structure is etched out (e.g. Canadian Shield)
- Obstacles are commonly
streamlined (roche moutonnees, whale backs, etc.) or rounded
- Selective linear erosion in regions
of former ice stream
- valleys and elongate depressions
are carved out or overdeepened
[Glacial
and Quaternary Geology]
[extended GeoIndex][QkRef][Geological
Sciences]
[Degree
Programs]
[Salem
State College]
Lindley
Hanson
(email)
Last Modified
3/25/03