Theory Ice is weird! δ Ice < δ Water Pressure drives solid ice liquid water
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PRESSURE-MELTING OF ICE: While-U-Wait W. W. Locke, Dept of Earth Sciences, Montana State University, Bozeman, MT 59717; [email protected] • Theory – Ice is weird! • δ Ice < δ Water • Pressure drives solid ice liquid water • Pressure-melting point (PMP) drops 0.7 ºC/ vertical km of ice – Melting point under 5-km thick Antarctic Ice Sheet ≈ -3.5 ºC – Most basal ice is at PMP • Can’t cool further because of thermal gradient • Can’t warm further because of ice; basal melting • Maximizes melt @ bed! – Pressure-melting strongly affects basal ice processes • Plucking = f(pressure-melt and regelation • Abrasion = f(basal melt) downflow, returning clasts to the bed • Practice – Requires: • 2-liter soda bottle • 32-guage rust-resistant craft wire • Ice cube • Mount (fork and C-clamp) • Drainage collection pan – Preparation: • Fill bottle with water • Make loop of wire to hang bottle on ice cube • Arrange mount on table to suspend fork above pan – Demo: • Place ice cube (near room temperature) on fork • Hang full bottle from ice cube by fine wire loop • Ask students to pose multiple working hypotheses as to the likely outcome. • Wait ~15 minutes while discussing principles (listed to the left) – Result: • Wire cuts through ice cube (pressure melt) • Ice refreezes behind it (regelation) • Melting cube can be picked up and held by its end! • A septum of bubble-free regelation ice marks the healed track. – Discussion: • Is ice too weak (100 kPa shear strength) to resist wire; if so, why did the ice heal? • Does ice heal as refreezing because of residual “cold” from the freezer; if so, why did it melt at first? • The actual stress can be calculated from wire diameter, ice cube width, and bottle weight. – Significance: • Pressure-melt is a phenomenon that occurs at scales of time, space, and force that can readily be duplicated in the classroom – it is of widespread importance beneath glaciers and ice sheets. Demo setup Demo detail. Note wire resting on top of ice cube on fork. Demo half-complete. Note wire disappearing into middle of ice cube. Demo nearing completion. Note wire resting on fork edge, but not yet to the base of the (melting) ice cube. Demo Grand Finale! Note that the ice cube, although smaller, is intact. Healed septum is barely visible. Demo finale. Note wire resting on fork and the ability to pick up the shrinking ice cube. http://www.chemistry.uvic.ca/codding/outline0124_files/image034.gif http://gemini.oscs.montana.edu/~geol445/hyperglac/morphology1/ http://gemini.oscs.montana.edu/~geol445/hyperglac/systems1/
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Theory Ice is weird! δ Ice < δ Water Pressure drives solid ice liquid water Pressure-melting point (PMP) drops 0.7 º C/ vertical km of ice Melting point under 5-km thick Antarctic Ice Sheet ≈ -3.5 º C Most basal ice is at PMP Can’t cool further because of thermal gradient - PowerPoint PPT Presentation
Transcript of Theory Ice is weird! δ Ice < δ Water Pressure drives solid ice liquid water
PRESSURE-MELTING OF ICE: While-U-WaitW. W. Locke, Dept of Earth Sciences, Montana State University, Bozeman, MT 59717; [email protected]
• Theory
– Ice is weird!
• δIce < δWater
• Pressure drives solid ice liquid water
• Pressure-melting point(PMP) drops 0.7 ºC/vertical km of ice
– Melting point under5-km thick AntarcticIce Sheet ≈ -3.5 ºC
– Most basal ice is at PMP
• Can’t cool further becauseof thermal gradient
• Can’t warm further becauseof ice; basal melting
• Maximizes melt @ bed!
– Pressure-melting strongly affects basal ice processes
• Plucking =f(pressure-melt and regelation
• Abrasion =f(basal melt)downflow, returning claststo the bed
• Practice– Requires:
• 2-liter soda bottle
• 32-guage rust-resistant craft wire
• Ice cube
• Mount (fork and C-clamp)
• Drainage collection pan
– Preparation:• Fill bottle with water
• Make loop of wire to hang bottle on ice cube
• Arrange mount on table to suspend fork above pan
– Demo:• Place ice cube (near room temperature) on fork
• Hang full bottle from ice cube by fine wire loop
• Ask students to pose multiple working hypotheses as to the likely outcome.
• Wait ~15 minutes while discussing principles (listed to the left)
– Result:• Wire cuts through ice cube (pressure melt)
• Ice refreezes behind it (regelation)
• Melting cube can be picked up and held by its end!
• A septum of bubble-free regelation ice marks the healed track.
– Discussion:• Is ice too weak (100 kPa shear strength) to resist wire; if so,
why did the ice heal?
• Does ice heal as refreezing because of residual “cold” from the freezer; if so, why did it melt at first?
• The actual stress can be calculated from wire diameter, ice cube width, and bottle weight.
– Significance:• Pressure-melt is a phenomenon that occurs at scales of time,
space, and force that can readily be duplicated in the classroom – it is of widespread importance beneath glaciers and ice sheets.
Demo setup
Demo detail. Note wire resting on top of ice cube on fork.
Demo half-complete. Note wire disappearing into middle of ice cube.
Demo nearing completion. Note wire resting on fork edge, but not yet to the base of the (melting) ice cube.
Demo Grand Finale! Note that the ice cube, although smaller, is intact. Healed septum is barely visible.
Demo finale. Note wire resting on fork and the ability to pickup the shrinkingice cube.
http://www.chemistry.uvic.ca/codding/outline0124_files/image034.gif
http://gemini.oscs.montana.edu/~geol445/hyperglac/morphology1/
http://gemini.oscs.montana.edu/~geol445/hyperglac/systems1/
