28 July, 2000

July 28, 2000

Matanuska Glacier, Alaska

Temperatures were mild this morning as we got out early to collect last night’s samples and reprogram the ISCOs to take our 9:00 am experiment. We experienced some problems with them again however. We lost power to the one at Mega Vent last night after it was half finished and we really had no power at all at M-1 all night. With all the ISCOs being used, not only by us but by the CRREL staff, there was a mix up on which batteries were charged completely. Fortunately we believe we will get sufficient samples to see the results at Mega Vent. We did manage to get two hours of samples and in our previous tests the dye seemed to flush out in that time. The other data at M-1 will be sorely missed. The batteries are always strong enough that you can view the program screen and reprogram the ISCO. But there is no way of knowing that it lacks the energy to pump the samples into the rack. There used to be a voltmeter that could verify the charge in the batteries but that broke a week or so ago and was unavailable. To compound the problem we also lost some samples this morning while we went back for fresh batteries. Those lost samples are not too critical because we started the sampling at the same time we poured the dye. It takes more than 30 minutes for the dye to appear at the vents so we may miss a small part of the dye curve at the start. We had fresh batteries installed at all three stations just in time for the 9:50 sample so we may miss just one sample containing a little dye. We’ll have a look at bthem tomorrow.

In my July 21 journal I mentioned that we must filter all the glacial flour from the samples before taking a reading with the fluorometer. This material which results from rocks scraping against each other is just one example of the erosive action of glaciers. Glaciers may be the most efficient mechanism for erosion on earth. Whenever they move the landscape is changed. As they move over bedrock they carry and drag a load of sediment and rock called the “basal till layer”. It is this layer which is frozen into the basal ice that polishes rocks and scrapes away glacial flour by “abrasion”, much like sandpaper . Abrasion in glaciers can produce materials up to several inches in size. In another process called “glacier plucking” large blocks of bedrock can be lifted and carried away once they have been loosened from the bedrock by freezing and thawing of water in the cracks and fractures in the rock. Some of these blocks can have dimensions as large as twenty-five feet.

As glaciers move through valleys they tend to change them into a characteristic U-shaped through abrasion and plucking. Stream valleys are generally V-shaped. The effect of all this is to widen and deepen the valley. When this happens in coastal valleys the glacier often erodes the valley floor to below sea level and can extend into the bays. When the glacier eventually retreats through melting a deep water-filled channel called a fjord is formed. Alaska has hundreds of fjords with the best known being in the Inside Passage, Glacier Bay and Prince William Sound.

There is evidence of this erosive power everywhere on and around the Matanuska Glacier. These same processes shaped much of the landscape near where I live in Indiana. It is extremely interesting and educational to see these processes at work here. It is truly a hands-on lab experience.

Marvin Giesting

This is an exposed piece of basal ice from the terminus. The darker layer of ice is the basal till layer and it is this layer that polishes rocks and scrapes away glacial flour.

This is an area of rock debris near the terminus and shows material of all sorts that has been transported by the glacier. You can see flows of small grain sediments as well as some larger boulders, all eroded and carried from the valley above by the glacier.