Subglacial Processes chapter 5 Erosion the glacier bed

Subglacial Processes chapter 5

Erosion @ the glacier bed

Glacier Bed Processes u Most important processes happen out of sight – Deformation (of the glacier & the bed) – Erosion – Deposition u Processes are a function of: – Thermal regime – Behavior of ice ± debris ± water – Behavior of bed material ± water

Glacier Bed Processes u Processes are a function of: – Thermal regime – Behavior of ice ± debris ± water – Behavior of bed material ± water u Most processes can be modeled as the interaction between – driving forces (shear and drag) – resisting forces (strength)

Theoretical Ice Temperature Tsurface = f (Tair) u Tbed = function of: u – geothermal heat – “frictional” heat – heat diffusivity u snow u ice – ice thickness) u Tinternal = f (all of these!)

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

Thermal Regime u u u 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 u Because PMP < 0°C, heat is trapped at the bed of warm ice

Pressure Melting u 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 u 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. u 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. u Pressure melt results u Water migrates around/ through obstacle u Regelation results u

Glacial Erosion 1. Abrasion 2. Plucking

Erosion by Plucking u Regelation incorporates loose bed material into basal ice – “plucking”

Abrasion u Plucked material is available to wear away the bed – “abrasion”

Interaction at the bed u Sliding u Rolling u Combination Eyles

Basal Clasts – “Tools” u Void at glacier bed u Clasts in ice u Faceting and striation Courtesy Tom Lowell, Univ. of Cincinnati

Abrasion Features / chattermarks

ABRASION

Crescentic Features u Result of pressure from a tool on the bed

Crescentic Fractures Wind River granite u Ice from L R u

Crescentic Gouges Wind River granite u Ice from L R u

Striations Wind River granite u Ice from L R u Note adjacent crescentic gouge u

Polish u Typical of similar hardness (bed vs. tool) and fine load (~sandpaper)

Plastically-molded forms (p-forms) Rocky Mountain NP granite u Ice follows weaknesses u Requires mobile ice @ high stress u

Subsole Deformation: τ = ρ g h sin α u When τ > τb, the bed deforms u The net result is erosion of (soft) bed u Figure 4. 12 Benn and Evans, 1998, Fig 4. 12

Strain in Deformation Till u Rate Fig. 10 -12 of deformation = f( – stratigraphy) – coupling) u What if glacier = cold-based? Benn and Evans, 1998, Fig 10. 12

Causes of variable striation directions Local topography u Changes in ice divides flow directions with time u Changes in direction of local flow during deglaciation (land water) u

Grooves u RMNP granite – Imply multiple clast passes – Imply positive feedback

Grooves u RMNP granite – Imply multiple clast passes – Imply positive feedback u Kelley’s OH Island, – limestone 1 m

Relative abrasion rates (with pressure and ice velocity) u 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 u “Effective” normal pressure – implies weight minus buoyancy (from meltwater)

Basal Voids u “Bridging” by thin ice – direction? u Polish

Streamlined bedforms u Erosional – Roche moutoneé – Rock drumlin u Depositional? – Drumlin – Crag & tail u Fluvial? ? ?

Roches moutoneés “Sheep-like rock” u Collective term – like waves in oldfashioned wigs (perukes) shaped with mutton fat u Essence of both plucking and abrasion u

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

Drumlinoid ridges u Drumlinized Lapland terrain in Finnish From INQUA - http: //www. inqua. au. dk/cog/start 1. html

Lodg(e)ment Till u Compact u Oriented – Note a-axis alignment From Dr. T. Lowell’s Glacial Page: http: //tvl 1. geo. uc. edu/ , specifically http: //tvl 1. geo. uc. edu/ice/projects/lab 99/25 Oct 99/index. html

Till – Juneau, Alaska

Glacial erosion u Glacial erosion: removing sediment – Abrasion: grinding by rocks (tools) carried by the glacier onto underlying rock u especially above / near the ELA – faster flow, toward the bed u fine = polish u coarser = striations: scratches on bedrock – indicate the direction of ice movement – Plucking / quarrying

Glacial erosion u Glacial erosion: removing sediment – Plucking/quarrying u set up by mechanical weathering, plus dislodgment and lifting action u 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 u joints, frost-wedging u & periodic opening of these structures to allow water ice in u glacier moves & pries rock

Glacial erosion u Glacial erosion: removing sediment – Abrasion, plucking / quarrying – Incision of glacier bed (~0. 06– 35 mm/yr) – depends on: u resistance of rock / floor to abrasion & plucking u abundance & hardness of rock fragments in glacier u speed and duration of flow u weight (thickness) of ice (shear stress)

Glacial deposition – Deposition of ice-transported material that is released as the ice melts u results: beautiful scenery, aquifers, soil u lack organized drainage networks u 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 u poor sorting, striations on rocks, bimodal distribution – (small: by abrasion – large: by plucking) – subangular unless old alluvium u 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