4 August, 2000

August 4 , 2000

Matanuska Glacier, Alaska

This morning we woke up to rain for the first time in quite awhile. Several of us had stayed up very late last night to watch for the northern lights under nice clear skies and decent temperatures. At about 1:30 am clouds moved in to block any chance of seeing it just as it was about dark enough for viewing. Overnight the clouds continued to develop into light showers. The threatening morning weather forced Bob Bigl to postpone his drilling so I went to North Vent with a couple others to retrieve the water samples from the ISCO. I’ve been asked by a number of people what ISCO stands for and no one here seems to know. We even pulled the manual out to look but to no avail. I’m guessing it is just the name of the company. After fixing lunch we all headed to Wasilla once again for showers, laundry and groceries. We also learned that the dye has come in and we plan to resume our dye tests soon using the moulin originally used in our salt tests.

It seems that the moulins we’ve watched develop tend to form where there is an existing crack in the ice. There are lots of cracks all over the glacier but some of them open up to form long deep fissures called crevasses. Crevasses can be over one hundred feet deep and several feet wide. As a glacier flows the upper 100-150 feet of ice is more brittle than that below. Below this depth the ice is under much more pressure and deforms and flows plastically. Due to the differential speeds in the flow of the ice (slower at increasing depth) this upper layer tends to crack open forming crevasses which may extend hundreds to thousands of feet across the glacier’s surface. Crevasses are one of the earliest structural features to form in a glacier and are found along the entire length from head to terminus. They often change size and shape as the glacier moves down the valley. They form perpendicular to the direction of stretching similar to cracks on a stretched rubber band. Along the length of the glacier they are generally perpendicular to the flow direction in the valley. They are more numerous in areas where the slope is steeper causing an increase in flow velocity.

Crevasses may be described in terms of their orientation to the glacier. They may also be described in terms of their location such as marginal, central or terminal. Transverse crevasses extend across a glacier and are due to the steepening of the glacier’s slope. Longitudinal crevasses form where the glacier is spreading out laterally, most commonly near the terminus. They can also form in the central part if the ice stream but they will usually curve around into an oblique orientation near the margins. I see mostly longitudinal crevasses around the terminus here on the Matanuska Glacier. On a recent hike up the glacier I could see transverse crevasses as well.

I never seem to pass up the opportunity to gaze down into crevasses. There are always many beautiful shades of blue which is especially prominent on overcast days. Usually there are sounds of running or dripping water to be heard and in some of the larger deeper ones you can listen to groanings of the glacier that may originate miles away. They are certainly interesting features to explore and enjoy.

Marvin Giesting

My partner Ben enjoys a view over a small crevasse.

Crevasses are found everywhere on the glacier including moraines which just happen to be debris covered ice.