Pressure-melting of ice: demonstration
William Locke
, Earth Sciences, Montana State University
Pressure-melting of ice, which is responsible for many aspects of glacier-bed process, is demonstrable in a standard class period at effectively no cost of time or $.
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Context
Audience:
Although this can be used at all college levels, I find it most appropriate for my junior majors (geology/physical geography) course in geomorphology and/or seniot course in glacial geology.
Skills and concepts that students must have mastered:
They must understand the temperature distribution within a glacier, including sources (geothermal, frictional) and sinks (conduction upward, melt) of heat in a glacier
How the activity is situated in the course:
It is a classroom demo that "runs" while I am discussing processes at the glacier bed in the context of temperature and pressure.
National or State Education Standards addressed by this activity?:
Darned if I know!
Goals
Content/concepts goals for this activity:
They key concept is that the glacier bed is maintained at the pressure melting point (PMP) while local melt (in high pressure regions) and refreezing (in low pressure regions) takes place. This in turn shapes the glacier bed.
Higher order thinking skills goals for this activity:
This activity deals with temperature gradients, isothermal changes of state, and the counter-intuitive idea that squeezing ice drives it toward melting. I include hypothesis testing as a companion activity ("What will happen?")
Other skills goals for this activity:
none
Description of the activity/assignment
In the previous class I have developed the idea of a range of vertical temperature profiles in a glacier or ice sheet. Most of this are constrained to PMP a the bed. I suspend a weight on an ice cube by a thin (32-ga) wire, and ask what will happen. While the wire "cuts into the ice", I discuss the concept of pressure-melt and its relationship to melting, freezing, regelation ice, plucking, and abrasion. After about 20 minutes I display the ice cube apparently unscarred by the passage through it of the wire, thus demonstrating pressure melting ahead of the wire and regelation behind it.
Determining whether students have met the goals
I have never evaluated it (gulp).
Download teaching materials and tips
- Activity Description/Assignment (PowerPoint 125kB May11 05)
Other Materials
Supporting references/URLs
none