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 ≈ -3.5ºC 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. PRESSURE-MELTING OF ICE: While-U-WaitW. W. Locke, Dept of Earth Sciences, Montana State University, Bozeman, MT 59717; firstname.lastname@example.org Demo detail. Note wire resting on top of ice cube on fork. Demo setup http://www.chemistry.uvic.ca/codding/outline0124_files/image034.gif 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. http://gemini.oscs.montana.edu/~geol445/hyperglac/morphology1/ Demo finale. Note wire resting on fork and the ability to pick up the shrinking ice cube. http://gemini.oscs.montana.edu/~geol445/hyperglac/systems1/ Demo Grand Finale! Note that the ice cube, although smaller, is intact. Healed septum is barely visible.