27 July, 2001
WebCast and Velocity in ice
Life on ice ... almost
This morning, Greg and I woke up super early to parcticipate in a WebCast conference. While I am not certain if the "web" part worked as well as intended (remember: we are in the field!), we had a delightful conversation with some students at Lake Elementary School in Toledo, Ohio.
A couple of questions came up that I wanted to address again (because several other readers have also asked similar questions):
Is it cold?
"Cold" and "hot" are relative terms. I am extremely hot compared to an ice cube, but extremely cold compared to the sun.
Compared to Columbus, my time at the Matanuska has been "cold" to "slightly warm". In the mornings, it is usually cloudy, cool and humid, making me feel very chilly. If the sun comes out and the wind is pretty calm or if we are doing something active, it can feel quite warm (t-shirt and shorts). Most times, I only wear t-shirt and shorts for a brief while before the wind picks up again and cools us right down.
Temperatures during the day range from 50 to high 60 degrees F. Nighttime temperatures fall to high 40 or low 50 degrees F.
How do you stay warm?
Lots of different layers of clothes!
Each layer of clothing acts as another layer of insulation between our body heat and the cold. Most times, the clothing I brought does a pretty good job keeping me warm (but there have certainly been times when I have wished I had more layers!)
See my usual morning dress up in the picture below.
How do crevasses form?
Glaciers are giant slabs of ice (sometimes miles long) that are getting pulled slowly downhill by gravity. As the block of ice travels over the ground, it interacts with the ground at its sides and underneath it, and may also flow slowly internally (a bit like a slowed down lava flow).
When the ice has to travel uphill, a small "traffic jam" is caused (ice doesn't like to go up hill). As pressure builds, the ice cracks. Since it is getting pushed from behind, the only way for the cracks to open (at the top of the ice) are parallel to the ice flow.
When the ice passes over the top of a hill, the ice on top is "stretched"; the ice on the surface cracks and creates crevasses perpendicular to the direction of ice flow.
At today's site, the crevasses formed in long lines from one side of the glacier to the other. What do you think is happening at the ground level underneath the glacier?
More news: tomorrow Chris Donovan (a TEA who just finished her research in the tundra) will be coming down for a brief visit... Most excellent!
Science at work
Just when you thought it was over... velocity is back.
I reported yesterday that the velocity of seismic waves in ice is about 3,800 m/s. This is the velocity at the surface of the ice. The situation gets a bit more complicated (and is much cooler!)
Snow falls and begins to get buried. As it is buried, pressure builds, converting snow to firn (granulated snow, consistency of sand). As it undergoes more burial, the firn begins to compress and begins to transform into ice. The more it compresses, the more the air squeezes out and the more the ice grows together. As it grows, the ice crystals tend to grow in an organized way (with their orientation related to direction of stress they are experiencing.)
Just as you can swim faster down a stream than across a stream, it turns out that seismic waves are able to travel faster in certain directions through the ice. Yesterday, we found that seismic waves at the surface travel a fast 3,800 m/s. Looking at additional data, today we found that the seismic waves travel 4,300 m/s vertically! Using this information, Greg converted the seismic data to "distance", and by a similar process, did the same with the radar data.
Tomorrow morning, the task will be to compare from the two methods! But now, it's bed time... Whew!
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