PRESSURE-MELTING OF ICE: While-U-Wait Bill Locke, Dept ...

PRESSURE-MELTING OF ICE: While-U-Wait Bill Locke, Dept ...

PRESSURE-MELTING OF ICE: While-U-Wait
W. W. Locke, Dept of Earth Sciences, Montana State University, Bozeman, MT 59717; [email protected]
Theory

Practice
Requires:

Ice is weird!

Ice < Water Pressure drives solid ice liquid water Pressure-melting point (PMP) drops 0.7 C/ vertical km of ice Preparation: Melting point under 5-km thick Antarctic Ice Sheet -3.5 C http://www.chemistry.uvic.ca/codding/outline0124_files/image034.gif Fill bottle with water Make loop of wire to hang bottle on ice cube Arrange mount on table to suspend fork above pan Demo detail. Note wire resting on top of ice cube on fork. Demo setup 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) Most basal ice is at PMP Cant cool further because of thermal gradient Cant warm further because of ice; basal melting Result: Maximizes melt @ bed! Pressure-melting strongly affects basal ice processes http://gemini.oscs.montana.edu/~geol445/hyperglac/morphology1/ 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. 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. Discussion: Plucking = f(pressure-melt and regelation Abrasion = f(basal melt) downflow, returning clasts to the bed 2-liter soda bottle 32-guage rust-resistant craft wire Ice cube Mount (fork and C-clamp) Drainage collection pan 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. http://gemini.oscs.montana.edu/~geol445/hyperglac/systems1/ Demo finale. Note wire resting on fork and the ability to pick up the shrinking ice cube. 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 Grand Finale! Note that the ice cube, although smaller, is intact. Healed septum is barely visible.

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