Subglacial Processes chapter 5

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  T surface = f (T air )  T bed = function of: –geothermal heat –“frictional” heat –heat diffusivity  snow  ice –ice thickness)  T internal = f (all of these!)

Observed Ice Core Temps  Greenland –Shallow warm bulge –T bed < 0°C  Antarctica –Shallow warm bulge –T bed ≈ 0°C  Reflects temperature change with time –LIA, Hypsithermal

Thermal Regime  Critical to processes!  Warm = wet-based  Cold = dry-based  ρ ice < ρ water, 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 high stress

Subsole Deformation:  τ = ρ g h sin α  When τ > τ b, 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 Fig Benn and Evans, 1998, Fig  Rate of deformation = f( –stratigraphy) –coupling)  What if glacier = cold-based?

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 Island, OH –limestone 1 m

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 old- fashioned wigs (perukes) shaped with mutton fat  Essence of both plucking and abrasion

Drumlin  Ontario, Canada – relation to lakes? Copyright © Daryl Dagesse 2002

Drumlinoid ridges  Drumlinized terrain in Finnish Lapland From INQUA -

Lodg(e)ment Till  Compact  Oriented –Note a-axis alignment From Dr. T. Lowell’s Glacial Page: specifically

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