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23 July, 2000

July 23, 2000

Matanuska Glacier, Alaska

One of the most vivid memories that I will have of this glacier is how dynamic it is, at least during the warm summer months. In my journal on July 12 I mention that a day visitor would have no clue as to how much it changes from day to day. And for my part I similarly have no direct observations about the big changes that occur in the glacier as it moves, changes that may be incrementally small but occurring over long periods of time. I know that glaciers move along rather slowly most of the time. This cannot be observed casually but the type of motion and the changes caused by that motion are extremely interesting. As I move about the area I can only observe some of the many changes that the glacier has on the landscape. It would be very interesting to actually see the motion of the ice in fast forward motion.

For any glacier the terminus may be stationary, retreating uphill or advancing downhill. In any case the ice within the glacier is constantly moving downhill. Within the glacier there are differences in the speed with which the ice moves. If the temperature of the ice at the base of the glacier is much below the freezing point the ice bonds tightly to the rock and cannot move. This occurs in polar regions such as Antarctica and Greenland as well as during winter months for others and even high altitudes in the summer months. But even when the bottom is motionless, the ice in the deepest layer, near the basal layer, deforms as ice crystals warp and slip past one another. This deep ice is subjected to the greatest stress due to the weight and resulting pressure from all the ice above. The greater the pressure the more the glacial ice will be deformed. As you move nearer to the surface this stress is continually decreasing and less deformation occurs. This deformation is most easily accomplished in warm ice. During the winter months when the ice near the surface is colder it is not as easily deformed but is much more brittle that the deeper ice. As a result many crevasses form whenever changes in stress are too great for the deformation to keep up with.

During summer, meltwater seeping down into the cracks and moulins warms any subfreezing temperature ice and eventually the entire glacier is at roughly the same temperature. The deep basal layer warms up near its melting point.Thawing and refreezing allows water at the interface between ice and rock to act as a lubricant and the glacier can now slide on its bed. With the ice near its melting point the high pressure created when ice pushes on a rock surface serves to lower the melting point of the ice. This liquid water will creep along until it reaches an area of low pressure where it will refreeze again. In addition to this slipping the warmer summer ice is more quickly deformed and “flows” more quickly. As a result the forward motion of the glacier in the summer occurs at a quicker pace than during the cold winter months. In either season the ice at the top of the glacier will move faster than the lower ice, resulting in the many cracks and crevasses on the surface of the glacier.

I mentioned on July 17 that glaciers at one time extended to my hometown in southern Indiana. The best way to picture a glacier moving that far without a significant drop in elevation is to consider that glaciers flow much more slowly, but in a similar fashion to poured honey. If you were to pour honey in one spot on a table it would well up and move slowly outward from the center. If you used refrigerated honey it would behave the same way but much more slowly. The glacier that advanced across most of Indiana and the upper Midwest did so like ultra-slow honey. As more and more snow accumulated in the polar regions it produced the pressures necessary to cause the ice to deform slowly and begin to “flow” and move southward. As the ice advanced it scraped and scoured the land just as the Matanuska Glacier is doing now, leaving behind landforms and deposits that look similar to those I can observe here.

Marvin Giesting

A wall of seracs with debris filled cracks.

Continuous cracks running perpendicular through adjacent parallel ridges.

Highly fractured, impassable portions of the glacier.

I'm standing on one of several narrow, parallel ridges produced in the upper layers of glacial ice near the terminus. This upper ice gets much colder in winter and is subject to fracturing more easily under stress.

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