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PRESSURE-MELTING OF ICE: While-U-Wait W. W. Locke, Dept of Earth Sciences, Montana State University, Bozeman, MT 59717; PowerPoint Presentation
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PRESSURE-MELTING OF ICE: While-U-Wait W. W. Locke, Dept of Earth Sciences, Montana State University, Bozeman, MT 59717; - PowerPoint PPT Presentation


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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

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Presentation Transcript
slide1
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; wlocke@montana.edu

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.