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Glacial geomorphology

Glacial geomorphology. Glacier: “a natural accumulation of ice that is in motion due to its own weight and slope of its surface” Ice cores Paleoclimate archive: high-resolution records of climate change Compared to deep sea core. Ice Age. Ice Age. Ice Age. Ice Age. 22°. 12°. 17°.

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Glacial geomorphology

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  1. Glacial geomorphology • Glacier: “a natural accumulation of ice that is in motion due to its own weight and slope of its surface” • Ice cores • Paleoclimate archive: high-resolution records of climate change • Compared to deep sea core

  2. Ice Age Ice Age Ice Age Ice Age 22° 12° 17° Quaternary Ice Ages Throughout Geologic Time Pleistocene: 3Mya Figure modified after C.R. Scotese PALEOMAP Project (www.scotese.com) Karoo Saharan Sturtian: 750-700Mya Marinoan/Varangian: ended 635Mya Gowganda: 2.3Gya Average Global Temperature (0C)

  3. C.R. Scotese, PALEOMAP Project, (www.scotese.com) Last Glacial Maximum 18,000 years ago

  4. Present vs. Past Glaciation • Now – One major (Antarctica) and one minor (Greenland) ice sheets • Then - At least three major (Antarctica, Laurentide, Fennoscandian) and numerous minor (Greenland, Cordilleran, Patagonian…) ice sheets

  5. Milankovitch Cycles Eccentricity 90,000 to 100,000 years Precession 19,000 to 23,000 years ~5% ~0% Obliquity (Axial Tilt) 41,000 years Figures modified after Matt Beedle, Montana Sate University. http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

  6. How does snow become ice? • Deposition • Reworked by wind • Destructive metamorphism (orig. crystal becomes a rounded ball) • Sintering (rounded grains fuse by freezing into larger crystals) • Compaction/Cementation

  7. How long does it take? • It depends! • Antarctica – hundreds of years • Greenland – 100 years • Temperate glaciers – decades • Maritime glaciers - years

  8. Ice is derived from snow Partially compressed/compacted snow is firn (snow/ice) When a wedge of firn is thick enough to deform under its own weight and move downhill, it’s now a glacier As snow turns to ice, porosity decreases Faster in wet snow Ice density with no pore space = 917 kg/m3

  9. Snowline and Equilibrium Line Altitude

  10. Mass Balance: Net Loss or Gain of Ice H = ice thickness W= glacier width Q=ice discharge/unit width b = local mass balance, m. of water/yr mass lost or gained over an annual cycle Q represents losses due to ablation and sublimation across the width of the glacier More loss due to solar radiation than Earth’s heat

  11. Mass Balance if ablation dominates below the ELA, why are there glaciers below the ELA?

  12. At steady state, Ice in Motion • If not in motion, not a glacier but a snowfield • Motion by • Basal sliding • Internal deformation • Q = U x H • Discharge per unit width • U, mean velocity • H, thickness

  13. q q Ice Deformation: Internal Deformation (Force Balance for a Column of Ice) Body force of gravity acts upon the ice column Step 1: Resolve weight force into two components: a normal force and a shear force s W/A Step 2: Substitute for W, A, and sin q t

  14. Strain rate Fluid Deformation • Fluid deforms under its own weight • As shear is applied, the ice is strained • Strain rate is horiz vel

  15. viscosity: resistance of fluid to deformation Ice Deformation • How the ice responds to the stresses is determined by its rheology (rate and style of deformation under stress) • Ice is non-Newtonian • As shear increases, effective viscosity decreases such that ice is less “stiff” near the bed than near the surface Much more like “plug” flow

  16. Ice thickness S Ice Deformation: Highly Nonlinear “flow law parameter” in Glen’s flow law Ice discharge = f(slope and ice thickness) For a given slope, if the ice thickness increases by 15%, the ice discharge will DOUBLE

  17. Assumptions • basal shear stress t = 0.8 bars = 8x104 Pa • glacier is wide enough that walls do not support ice • contours on the map show that the ice slope is 10m/km at this location • density of ice r= 918 kg/m3 and g = 9.8 m/s2 • 1Pa = 1 • Estimate the ice thickness H at this location • The basal shear stress is given by: where ri is the ice density, g is the acceleration due to gravity, H is the thickness of the ice, and S is the ice surface slope angle • Rearrange: =890 m.

  18. Ice Deformation: Sliding • Sliding occurs because high pressure promotes melting (in water) • Down valley component of weight promotes motion • Resistance to motion due to pressure variations from bumps in the bed

  19. 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”

  20. 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

  21. Regelation Higher pressure on up-valley side of bumps than down-valley side of bumps Ice melts on the stoss (high pressure) side, consuming energy Moves around bump as water film Refreezes in the low pressure shadow (lee) Heat released by refreezing is conducted back to bump Coupling of thermal and fluid mechanics

  22. Recently deglaciated bed of Blackfoot Glacier, Glacier NP. Argillite (Belt rock) bed shows dissolution of limestone on upstream side, and reprecipitation as calcite on downstream side (white areas)

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