The ComPADRE Collections

Pressure Melting of Ice: While-U-Wait

Demonstration by W.W. Locke, Department of Earth Sciences, Montana State University. Starting Point page by John McDaris.
This material was originally created for Starting Point:Introductory Geology
and is replicated here as part of the SERC Pedagogic Service.

Summary

This exercise demonstrates one of the few cases in which geologic solids visibly deform at room conditions. The phenomenon of pressure-melting is critical to understanding conditions at the glacier bed. Pressure-melting traps heat, and thus meltwater at the bed, enhancing bed deformation and abrasion. In this demonstration, a weight is suspended via a thin wire over an ice cube. Over the course of the demonstration, the wire passes through the ice, leaving undamaged ice in its wake.

Learning Goals

Students should be able to explain the melting/regelation process accurately as a result of the demonstration and the accompanying lecture/discussion.

Context for Use

This demonstration is usually used at the Junior/3XX (Geomorphology) and/or Senior/4XX (Glacial Geology) level; it can be appropriate for introductory classes if students have had the requisite physics (heat flow) and mechanics (stress/strain) or with brief introductions to these concepts. The only required introductory geology material is an introduction to glacial processes that includes the typical temperature distribution within a glacier.

Description and Teaching Materials

Materials Required

  • 2-liter soda bottle
  • 32-gauge rust-resistant craft wire
  • Ice cube
  • Fork
  • C-clamp (to mount fork to table or desk)
  • Drainage collection pan

Preparation

  • Fill bottle with water.
  • Make loop of wire to hang bottle on ice cube.
  • Arrange mount on table to suspend forl above pan.

Demonstration

  • Place ice cube (near room temperature) on fork.
  • Hang full bottle from ice cube by fine wire loop.
  • Ask students to post multiple working hypotheses as to the likely outcome.
  • Wait ~15 minutes while discussing principles of pressure melting.

Results

  • Wire cuts through ice cube (pressure melting).
  • Ice refreezes behind it (regelation).
  • Melting cube can be picked up and held by one end.
  • A septum of bubble-free regelation ice marks the healed track of the wire.

Teaching Notes and Tips

It requires about 30 minutes for the wire to cut through a standard ice cube. Finer wire would cut faster (but break even more frequently) as would a heavier load. Although the ice cube shrinks in the process, there is typically enough left to do a second pass. This might be described as testing the hypothesis that the ice heals itself because it is still cold from the freezer; the ice must be isothermal at 0°C after 30+ minutes exposed in a classroom!


The time while the demonstration is proceeding is best used in:

  1. posing multiple working hypotheses as to the outcome of the experiment,
  2. discussion of the various temperature regimes in glaciers and
  3. discussion of the mechanics of ice deformation (often demonstrated itself with Silly Putty).

Assessment

Suggested Discussion Questions:
  • Is the ice too weak to resist the wire (100 kPa sheer strength)? If so, why did the ice heal?
  • Does the ice heal as refreezing ecause of "residual cold" from the freezer? If so, why did it melt in the first place?
  • Based on the wire diameter, ice cube width, and bottle height, what is the actual stress on the ice as a result of the attached weight?

References and Resources

Locke's poster at the 2004 North Central Geological Society of America's meeting in Minneapolis, MN.