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Mendenhall terminus July 2008


Glaciers are rivers of ice that typically persist year-round and move under their own weight. Glacier ice (which is the same as ordinary ice) is largely incompressible but can flow and deform in response to stress. This page describes some further aspects of the glaciers in our region of study, Southeast Alaska centered on Juneau, with particular emphasis on the relationship between the solid ice and the liquid water that flows on, around, through, and under it.

Densification of Snow

Snow is made of ice but it is not particularly dense. Unless I have to shovel it. On glaciers we have a sedimentary compression process that converts snow (the snow that falls sufficiently high up on the glacier) into ice over the course of typically a few years. As this snow is compressed under the weight of other snow it becomes more dense, so occupies less volume. The ice itself is not compressible, so compressing snow is like crumpling egg cartons: You are really getting rid of the air. The air surrounding the snowflakes may be partially squeezed out but a lot of it is trapped and compressed. Eventually it gets dissolved into the crystalline structure of the ice.

Density of water

Ice floats, is significantly less dense than liquid water, even after it has been compressed under a bunch more ice. The buoyancy of ice is easy to forget when looking face on at a glacier coming at you, where the message is Big Heavy Mass. But water flows down into and through glaciers to the bed (rock/ice interface) where it pools and also flows downhill. Will Harrison characterizes the situation in two words: "Bad plumbing". We ask:

IS THERE MORE to be said about glacial water flow beyond "bad plumbing"?

If the answer is Yes (which we hope is the case) then:

How can we figure that out in detail?

Deformational versus sliding ice flow

Experimental methods

What sorts of experiments might we carry out to trace the behavior of water and ice and the stuff trapped in the water and ice?

You might immediately suggest: Why don't you find some water on top of the glacier that you think is going down under and then along the bottom, dump a bunch of environmentally friendly dye into this water, and wait for the dye to show up in the stream or lake down at the bottom of the glacier. This is a fine idea for an experiment.

Remarks From A Glaciologist

The more you know about what you measure, the better. Or the other way around: if you don't know what you measure it might be worthless. For example, temperature: Everybody is interested in temperature records. If you throw a linked thermometer onto a melting ice surface (glacier in summer), it is going to show something very close to 0 C. If the sensor is mounted at 1 m above the ice it will show something different and at 2 m something different again. The lesson is, it is not enough to go out and measure as much as possible, it is equally important to know what exactly is being measured.

Having written that, there are many interesting measurements from a glacier that would be useful. Some of the more obvious ones are climate related: temperature, wind, relative humidity, radiation budget. Some of these variables are easier measured over bedrock and perhaps more relevant there (ex. Temperature over a melting snow/ice surface does not tell you all that much about what's really going on, except that there is melt).

Very useful: Any kind of ice ablation and snow accumulation measurement. Very useful: Anything about motion. Having the displacement of a sensor sitting on top of the ice is interesting, but having one tracking actual ice particles (attached to a stake drilled into ice) is better.

Any kind of seismic data, number of events is relatively straight forward, recording wave forms is a good deal harder.

Imagery: you can use it to track snow lines, terminus positions, perhaps motion fields, instrument set-ups, etc. Seeing is believing.

Subsurface: Ice internal deformation (tilt+orientation), englacial temperature, ice fabric (How to measure??) Subglacial water pressure, motion (sliding, till deformation, ...)

A dream experiment would be to track something as it progresses through a glacier: Deploy many sensors in the accumulation area and let them become buried. Track them with orientation information over many years / decades. Keep supplying new sensors, so as they get buried they have a comm network to talk to the surface with relatively low power.


These notes have been transplanted to the Terminus Profiling page.