Subglacial Processes
chapter 5
Erosion @ the glacier bed
Glacier Bed Processes
 Most
important processes happen out
of sight
– Deformation (of the glacier & the bed)
– Erosion
– Deposition
 Processes
are a function of:
– Thermal regime
– Behavior of ice ± debris ± water
– Behavior of bed material ± water
Glacier Bed Processes
 Processes
are a function of:
– Thermal regime
– Behavior of ice ± debris ± water
– Behavior of bed material ± water
 Most
processes can be modeled as
the interaction between
– driving forces (shear and drag)
– resisting forces (strength)
Theoretical Ice Temperature
Tsurface = f (Tair)
 Tbed = function
of:

– geothermal heat
– “frictional” heat
– heat diffusivity
 snow
 ice
– ice thickness)

Tinternal = f (all of
these!)
Observed Ice Core Temps

Greenland
– Shallow warm bulge
– Tbed < 0°C

Antarctica
– Shallow warm bulge
– Tbed ≈ 0°C

Reflects
temperature
change with time
– LIA, Hypsithermal
Thermal Regime





Critical to processes!
Warm = wet-based
Cold = dry-based
ρice < ρwater, therefore
Pressure increase
forces melting point
decrease – PMP
– –0.7°C / km of ice

Because PMP < 0°C,
heat is trapped at the
bed of warm ice
Pressure Melting

For ice at PMP:
– Movement increases pressure, thus melting,
on the up-ice side of an obstruction
– Movement away from the obstruction causes
freezing on the down-ice side – “regelation”
Interlude
 For
warm-based (most) glaciers,
pressure melting must be added to
other forms of deformation, and may
be a major component of “slip” at or
near the bed.
 Note that high stress around
obstacles may also accelerate
deformation – “enhanced basal
creep”.
Effects of Pressure Melting
High pressure is experienced
on the up-ice side of an
obstruction.
 Pressure melt results
 Water migrates around/
through obstacle
 Regelation results

Glacial Erosion
1. Abrasion
2. Plucking
Erosion by
Plucking

Regelation
incorporates loose
bed material into
basal ice –
“plucking”
Abrasion

Plucked
material is
available
to wear
away the
bed –
“abrasion”
Interaction at the bed
 Sliding
 Rolling
 Combination
Eyles
Basal Clasts – “Tools”
 Void
at
glacier bed
 Clasts in ice
 Faceting and
striation
Courtesy Tom Lowell, Univ. of Cincinnati
Abrasion Features / chattermarks
ABRASION
Crescentic Features

Result of
pressure
from a
tool on
the bed
Crescentic Fractures
Wind River
granite
 Ice from L  R

Crescentic Gouges
Wind River
granite
 Ice from L  R

Striations
Wind River
granite
 Ice from L  R
 Note adjacent
crescentic
gouge

Polish

Typical of
similar
hardness (bed
vs. tool) and
fine load
(~sandpaper)
Plastically-molded forms (p-forms)
Rocky
Mountain NP granite
 Ice follows
weaknesses
 Requires
mobile ice @
high stress

Subsole Deformation:
τ = ρ g h sin α
 When τ > τb, the bed deforms
 The net result is erosion of (soft) bed

Figure 4.12
Benn and Evans, 1998, Fig 4.12
Strain in
Deformation Till
 Rate
Fig. 10-12
of
deformation = f(
– stratigraphy)
– coupling)
 What
if glacier =
cold-based?
Benn and Evans, 1998, Fig 10.12
Causes of variable striation
directions
Local topography
 Changes in ice
divides  flow
directions with
time
 Changes in
direction of local
flow during
deglaciation (land
and water)

Grooves
 RMNP
granite
– Imply
multiple
clast passes
– Imply
positive
feedback
Grooves
 RMNP
granite
– Imply multiple
clast passes
– Imply positive
feedback
 Kelley’s
OH
Island,
– limestone
1m
Relative abrasion rates
(with pressure and ice velocity)

Like sanding
wood:
– Hardness
contrast
– The faster the
better (belt
sanders)!
– Some pressure,
but not too
much!
Numerical abrasion rates
(with pressure and ice velocity)
Effect of Pressure on Abrasion
 “Effective”
normal
pressure
– implies
weight
minus
buoyancy
(from
meltwater)
Basal Voids
 “Bridging”
by thin ice
–direction?
 Polish
Streamlined bedforms
 Erosional
– Roche
moutoneé
– Rock drumlin
 Depositional?
– Drumlin
– Crag & tail
 Fluvial???
Roches
moutoneés
“Sheep-like rock”
 Collective term –
like waves in oldfashioned wigs
(perukes)
shaped with
mutton fat
 Essence of both
plucking and
abrasion

Drumlin
 Ontario,
Canada – relation to lakes?
Copyright © Daryl Dagesse 2002
NY drumlins.kmz
Drumlinoid ridges
 Drumlinized
Lapland
terrain in Finnish
From INQUA - http://www.inqua.au.dk/cog/start1.html
Lodg(e)ment Till
 Compact
 Oriented
– Note a-axis
alignment
From Dr. T. Lowell’s Glacial Page: http://tvl1.geo.uc.edu/ , specifically
http://tvl1.geo.uc.edu/ice/projects/lab99/25Oct99/index.html
Till – Juneau, Alaska
Glacial erosion
 Glacial
erosion: removing sediment
– Abrasion: grinding by rocks (tools)
carried by the glacier onto underlying
rock
 especially
above / near the ELA – faster flow,
toward the bed
 fine = polish
 coarser = striations: scratches on bedrock
– indicate the direction of ice movement
– Plucking / quarrying
Glacial erosion
 Glacial
erosion: removing sediment
– Plucking/quarrying
 set
up by mechanical weathering, plus
dislodgment and lifting action
 mainly at steps – at base of temperate
glaciers
– adhesion of ice is great (try to pull a stick out)
– need pre-existing structures in the bedrock
 joints, frost-wedging
 & periodic opening of these structures to
allow water  ice in
 glacier
moves & pries rock
Glacial erosion
 Glacial
erosion: removing sediment
– Abrasion, plucking / quarrying
– Incision of glacier bed (~0.06–35 mm/yr)
– depends on:
 resistance
of rock / floor to abrasion &
plucking
 abundance & hardness of rock fragments in
glacier
 speed and duration of flow
 weight (thickness) of ice (shear stress)
Glacial deposition
– Deposition of ice-transported material
that is released as the ice melts
 results:
beautiful scenery, aquifers, soil
 lack organized drainage networks
 glacial drift (generic): all material of glacial
origin
– name from early theories – it had “drifted in”
– commonly as diamicton = unsorted,
unstratified sediment
– covers 8% of Earth’s surface, 25% of North
America
– thin to 200 m thick
Glacial deposition
– Deposition of ice-transported material
– Till = non-stratified drift, deposited ±
directly from glacial ice
 poor
sorting, striations on rocks, bimodal
distribution
– (small: by abrasion – large: by plucking)
– subangular unless old alluvium
 moraine
= pile or ridge of till
– formed in zone of ablation (below snow line) –
more time  larger moraine
– ground moraine – widespread, relatively thin
deposit of till in a plain as glacier advances /
recedes