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Glacial II – Flow and Process

Glacial II – Flow and Process. Specific Mass Budget. Point mass budgets measured with ablation stakes End of summer to EOS Uses old snow/firn/ice as a marker. Specific Mass Budget trends. Accumulation often increases slightly with increasing altitude. @ ELA, b n = 0

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Glacial II – Flow and Process

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  1. Glacial II – Flow and Process

  2. Specific Mass Budget • Point mass budgets measured with ablation stakes • End of summer to EOS • Uses old snow/firn/ice as a marker

  3. Specific Mass Budget trends • Accumulation often increases slightly with increasing altitude. • @ ELA, bn = 0 • Ablation increases rapidly with decreasing altitude below the ELA. • Why? EQUILIBRIUM LINE ALTITUDE

  4. Snowlines • Indicator of mass balance • Short-term • Firn line • Highest firn line • Long-term • ELA

  5. Snowlines • Glaciation Threshold • Cirque floors • Highest lateral moraines • Toe-headwall altitude ratio • Accumulationarea ratio

  6. Controls on Snowlines I - Latitude • Latitude ≈ temperature (treeline) • Highest near equator • Latitude ≠ precipitation (snowlines) • Saddle near equator • Weak gradients (<1 m/km)

  7. Controls II - Continentality • Lowest near moisture source • Higher inland • Strong gradients • Up to 10 m/km

  8. Spatial Resolution • Humlum (1985) • West Greenland • Local data are consistent • Needs no smoothing • High resolution!

  9. MT/ID Paleoclimate • Complex pattern! • More detailed than modern weather stations and SNOWTEL sites! • Explain?

  10. Observed Flow: Plan and Profile • Plan View • Parabolic • Septum (ice streams) • Profile • Exponential • Non-zero at the bed • τ = ρgh·sin(α)

  11. Modes of Profile Flow • Total Velocity = • Internal Velocity • Laminar • Sum of processes • + Basal Slip • Not if frozen to bed • + Bed Deformation • If not rock

  12. Observed Bed Deformation Shear Plane? • Inferred from structures in till • Measured from markers emplaced in basal sediment and recovered

  13. Brittle Deformation - Crevasses • Long observed • Results from rapidly-applied stress • Form many distinctive patterns

  14. Crevasse Examples • Depth <10m • Tensional and marginal • Terminal splays • Complex systems

  15. Unsteady Flow - Surges • Many glaciers (~10%) surge • Stagnant for years • Increase in thickness • Surge! • Decouple from the bed? • Surface fracturing • Thrusting?

  16. Surging Terminus

  17. Summary of Flow Process I

  18. Summary of Flow Process II τ≈1 bar

  19. Glacier Bed Processes • Most important processes happen out of sight • Deformation (internal, bed) • Erosion • Deposition • Processes are a function of: • Thermal regime • Behavior of ice • Behavior of bed material

  20. 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

  21. 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 • “Warm” ice = thick, fast, temperate or heated Pressure Melting Point

  22. Pressure Melting melt melt • 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”

  23. Effects of Pressure Melting • High pressure is experienced on the upice side of an obstruction. • Pressure melt results • Water migrates around/through obstacle • Regelation occurs in low pressure zone MELT REFREEZE

  24. Erosion by Plucking • Regelation incorporates loose bed material into basal ice – “plucking”

  25. Abrasion • Plucked material is available to wear away the bed – “abrasion”

  26. Abrasion Features / chattermarks

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

  28. Erosion and Deposition • Erosion • Plucking vs. abrasion • Effect of pressure • Effect of velocity • Deposition • Lodgment vs. meltout

  29. The Subglacial System • f(distance) downice • Water increases to the terminus • Debris decreases below the terminus

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