Paper No. 4
Presentation Time: 1:20 PM-5:20 PM
PRESSURE-MELTING OF ICE: WHILE-U-WAIT
The base of a glacier is a dynamic environment in part because so much can happen so rapidly to and by ice near its pressure melting point. Ice is particularly unusual because its solid state is less dense than its liquid state, thus increasing pressure has a tendency to drive ice towards the molten state and pressure reduction enables refreezing, all with negligible energy exchange. A two-liter soda bottle, some fine craft wire, a fork, a C-clamp, a pan and an ice cube enable a demonstration of this bizarre behavior in real-time, during a lecture. The fork is clamped to a table and the pan centered beneath it (to catch inevitable meltwater). The ice cube is rested on the fork and the full soda bottle suspended on the ice by a loop of craft wire. I ask the class what they expect to occur; responses typically include nothing (ice is a solid) and the wire will cut the ice cube in half. While the lecture continues, the wire cuts through the ice cube. After about 15 minutes the loop rests on the fork and the ice cube (a little melted around the edges!) can be picked up, apparently entire and without damage! Closer inspection shows a bubble-free seam in the ice where ice has melted under pressure, water has moved around the wire, and refreezing has occurred atop the wire. Although I have not done so, it is possible to calculate rates of heat transfer into and through the wire and the strength of ice based on the diameter of the wire, the weight of the bottle and the time required to cut through. This demonstration clarifies the major mechanism by which ice is able to pluck clasts from its bed.