Subglacial Processes chapter 5. Erosion @ the glacier bed

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