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Subglacial ProcessesSubglacial Processes
chapter 5chapter 5
Erosion @ the glacier bedErosion @ the glacier bed
Glacier Bed ProcessesGlacier Bed Processes
Most important processes happen out Most important processes happen out of sightof sight– Deformation (of the glacier & the bed)Deformation (of the glacier & the bed)– ErosionErosion– DepositionDeposition
Processes are a function of:Processes are a function of:– Thermal regimeThermal regime– Behavior of ice ± debris ± waterBehavior of ice ± debris ± water– Behavior of bed material ± waterBehavior of bed material ± water
Glacier Bed ProcessesGlacier Bed Processes
Processes are a function of:Processes are a function of:– Thermal regimeThermal regime– Behavior of ice ± debris ± waterBehavior of ice ± debris ± water– Behavior of bed material ± waterBehavior of bed material ± water
Most processes can be modeled as Most processes can be modeled as the interaction betweenthe interaction between– driving forces (shear and drag)driving forces (shear and drag)– resisting forces (strength)resisting forces (strength)
Theoretical Ice TemperatureTheoretical Ice Temperature
TTsurfacesurface = f (T = f (Tairair)) TTbedbed = function of: = function of:
– geothermal heatgeothermal heat– ““frictional” heatfrictional” heat– heat diffusivityheat diffusivity
snowsnow iceice
– ice thickness)ice thickness) TTinternalinternal = f (all of = f (all of
these!)these!)
Observed Ice Core TempsObserved Ice Core Temps
GreenlandGreenland– Shallow warm bulgeShallow warm bulge– TTbedbed < 0°C < 0°C
AntarcticaAntarctica– Shallow warm bulgeShallow warm bulge– TTbed bed ≈ 0°C≈ 0°C
Reflects Reflects temperature temperature change with timechange with time– LIA, HypsithermalLIA, Hypsithermal
Thermal RegimeThermal Regime
Critical to processes!Critical to processes! Warm = wet-basedWarm = wet-based Cold = dry-basedCold = dry-based ρρiceice < < ρρwaterwater, therefore, therefore Pressure increase Pressure increase
forces melting point forces melting point decrease – PMPdecrease – PMP– ––0.7°C / km of ice0.7°C / km of ice
Because PMP < 0°C, Because PMP < 0°C, heat is trapped at the heat is trapped at the bed of warm icebed of warm ice
Pressure MeltingPressure Melting
For ice at PMP:For ice at PMP:– Movement increases pressure, thus melting, on Movement increases pressure, thus melting, on
the up-ice side of an obstructionthe up-ice side of an obstruction– Movement away from the obstruction causes Movement away from the obstruction causes
freezing on the down-ice side – “regelation”freezing on the down-ice side – “regelation”
InterludeInterlude
For warm-based (most) glaciers, For warm-based (most) glaciers, pressure melting must be added to pressure melting must be added to other forms of deformation, and may other forms of deformation, and may be a major component of “slip” at or be a major component of “slip” at or near the bed.near the bed.
Note that high stress around Note that high stress around obstacles may also accelerate obstacles may also accelerate deformation – “enhanced basal deformation – “enhanced basal creep”.creep”.
Effects of Pressure MeltingEffects of Pressure Melting High pressure is experienced High pressure is experienced
on the up-ice side of an on the up-ice side of an obstruction.obstruction.
Pressure melt resultsPressure melt results Water migrates around/Water migrates around/
through obstaclethrough obstacle Regelation resultsRegelation results
Glacial Erosion
1. Abrasion
2. Plucking
Erosion by Erosion by PluckingPlucking
Regelation Regelation incorporates loose incorporates loose bed material into bed material into basal ice – basal ice – “plucking”“plucking”
AbrasionAbrasion
Plucked Plucked material is material is available available to wear to wear away the away the bed – bed – “abrasion”“abrasion”
Interaction at the bedInteraction at the bed
SlidingSliding
RollingRolling
CombinationCombination Eyles
Basal Clasts – “Tools”Basal Clasts – “Tools”
Void at Void at glacier bedglacier bed
Clasts in iceClasts in ice Faceting and Faceting and
striationstriation
Courtesy Tom Lowell, Univ. of Cincinnati
Abrasion Features / chattermarksAbrasion Features / chattermarks
ABRASION
Crescentic FeaturesCrescentic Features
Result of Result of pressurepressure from a from a tooltool on on the the bedbed
Crescentic FracturesCrescentic Fractures
Wind River Wind River granitegranite
Ice from L Ice from L R R
Crescentic GougesCrescentic Gouges
Wind River Wind River granitegranite
Ice from L Ice from L R R
StriationsStriations
Wind River Wind River granitegranite
Ice from L Ice from L R R Note adjacent Note adjacent
crescentic crescentic gougegouge
PolishPolish
Typical of Typical of similar similar hardness (bed hardness (bed vs. tool) and vs. tool) and fine load fine load (~sandpaper)(~sandpaper)
Plastically-molded forms (p-forms)Plastically-molded forms (p-forms)
Rocky Rocky Mountain NP - Mountain NP - granitegranite
Ice follows Ice follows weaknessesweaknesses
Requires Requires mobile ice @ mobile ice @ high stresshigh stress
Subsole Deformation:Subsole Deformation:
ττ = = ρρ g h sin g h sin αα
When When ττ > > ττbb, the bed deforms, the bed deforms The net result is erosion of (soft) bedThe net result is erosion of (soft) bed
Figure 4.12
Benn and Evans, 1998, Fig 4.12
Strain in Strain in Deformation TillDeformation Till
Fig. 10-12
Benn and Evans, 1998, Fig 10.12
Rate of Rate of deformation = f(deformation = f(– stratigraphy)stratigraphy)– coupling)coupling)
What if glacier = What if glacier = cold-based?cold-based?
Causes of variable striation directionsCauses of variable striation directions
Local topographyLocal topography Changes in ice Changes in ice
divides divides flow flow directions with directions with timetime
Changes in Changes in direction of local direction of local flow during flow during deglaciation (land deglaciation (land and water)and water)
GroovesGrooves
RMNP RMNP granitegranite– Imply Imply
multiple multiple clast passesclast passes
– Imply Imply positive positive feedbackfeedback
GroovesGrooves
RMNP graniteRMNP granite– Imply multiple Imply multiple
clast passesclast passes– Imply positive Imply positive
feedbackfeedback Kelley’s Island, Kelley’s Island,
OHOH– limestonelimestone
1 m
Relative abrasion ratesRelative abrasion rates(with pressure and ice velocity)(with pressure and ice velocity)
Like sanding Like sanding wood:wood:– Hardness Hardness
contrastcontrast– The faster the The faster the
better (belt better (belt sanders)!sanders)!
– Some pressure, Some pressure, but not too but not too much!much!
Numerical abrasion ratesNumerical abrasion rates(with pressure and ice velocity)(with pressure and ice velocity)
Effect of Pressure on AbrasionEffect of Pressure on Abrasion
““Effective” Effective” normal normal pressurepressure– implies implies
weight weight minus minus buoyancy buoyancy (from (from meltwater)meltwater)
Basal VoidsBasal Voids
““Bridging” Bridging” by thin iceby thin ice– direction?direction?
PolishPolish
Streamlined bedformsStreamlined bedforms ErosionalErosional
– Roche Roche moutoneémoutoneé
– Rock drumlinRock drumlin Depositional?Depositional?
– DrumlinDrumlin– Crag & tailCrag & tail
Fluvial???Fluvial???
RochesRochesmoutoneésmoutoneés
““Sheep-like rock”Sheep-like rock” CollectiveCollective term – term –
like waves in old-like waves in old-fashioned wigs fashioned wigs (perukes) shaped (perukes) shaped with mutton fatwith mutton fat
Essence of both Essence of both plucking and plucking and abrasionabrasion
DrumlinDrumlin Ontario, Canada – relation to lakes?Ontario, Canada – relation to lakes?
Copyright © Daryl Dagesse 2002 NY drumlins.kmz
Drumlinoid ridgesDrumlinoid ridges
Drumlinized terrain in Finnish Drumlinized terrain in Finnish LaplandLapland
From INQUA - http://www.inqua.au.dk/cog/start1.html
Lodg(e)ment TillLodg(e)ment Till
CompactCompact OrientedOriented
– Note a-axis Note a-axis alignmentalignment
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 erosionGlacial erosion Glacial erosion: removing sedimentGlacial erosion: removing sediment
– Abrasion: grinding by rocks (tools) carried Abrasion: grinding by rocks (tools) carried by the glacier onto underlying rockby the glacier onto underlying rockespecially above / near the ELA – faster flow, especially above / near the ELA – faster flow,
toward the bedtoward the bedfine = polishfine = polishcoarser = striations: scratches on bedrock coarser = striations: scratches on bedrock
– indicate the direction of ice movement indicate the direction of ice movement
– Plucking / quarryingPlucking / quarrying
Glacial erosionGlacial erosion
Glacial erosion: removing sedimentGlacial erosion: removing sediment– Plucking/quarryingPlucking/quarrying
set up by mechanical weathering, plus dislodgment set up by mechanical weathering, plus dislodgment and lifting actionand lifting action
mainly at steps – at base of temperate glaciersmainly at steps – at base of temperate glaciers– adhesion of ice is great (try to pull a stick out)adhesion of ice is great (try to pull a stick out)– need pre-existing structures in the bedrockneed pre-existing structures in the bedrock
joints, frost-wedgingjoints, frost-wedging & periodic opening of these structures to allow & periodic opening of these structures to allow
water water ice in ice inglacier moves & pries rockglacier moves & pries rock
Glacial erosionGlacial erosion Glacial erosion: removing sedimentGlacial erosion: removing sediment
– Abrasion, plucking / quarryingAbrasion, plucking / quarrying– Incision of glacier bed (~0.06–35 mm/yr) – Incision of glacier bed (~0.06–35 mm/yr) –
depends on:depends on:resistanceresistance of rock / floor to abrasion & plucking of rock / floor to abrasion & pluckingabundance & hardness of rock fragments in abundance & hardness of rock fragments in
glacierglacierspeed and duration of flowspeed and duration of flowweight (thickness) of ice (shear stress)weight (thickness) of ice (shear stress)
Glacial depositionGlacial deposition– Deposition of ice-transported material that is Deposition of ice-transported material that is
released as the ice melts released as the ice melts results: beautiful scenery, aquifers, soilresults: beautiful scenery, aquifers, soil lack organized drainage networkslack organized drainage networksglacial glacial driftdrift (generic): all material of glacial origin (generic): all material of glacial origin
– name from early theories – it had “drifted in”name from early theories – it had “drifted in”– commonly as commonly as diamictondiamicton = unsorted, unstratified = unsorted, unstratified
sedimentsediment– covers 8% of Earth’s surface, 25% of North Americacovers 8% of Earth’s surface, 25% of North America– thin to 200 m thick thin to 200 m thick
Glacial depositionGlacial deposition– Deposition of ice-transported materialDeposition of ice-transported material– TillTill = non-stratified drift, deposited ± directly = non-stratified drift, deposited ± directly
from glacial ice from glacial ice poor sorting, striations on rocks, bimodal distributionpoor sorting, striations on rocks, bimodal distribution
– (small: by abrasion – large: by plucking)(small: by abrasion – large: by plucking)– subangular unless old alluvium subangular unless old alluvium
moraine = pile or ridge of tillmoraine = pile or ridge of till– formed in zone of ablation (below snow line) – more formed in zone of ablation (below snow line) – more
time time larger moraine larger moraine– ground moraine – widespread, relatively thin deposit ground moraine – widespread, relatively thin deposit
of till in a plain as glacier advances / recedesof till in a plain as glacier advances / recedes