|Glaciology 4888/5888 Midterm Quiz Humphrey Spring 06
Use a separate page for answers. Open notes but not open text, since not all of you have Paterson. The last question is for the over-achievers.
25 questions. Many questions have quick, few word answers, but some require complete sentences or paragraphs. Make sure you write enough for me to understand what you are saying. Don’t spend more than a couple of minutes on any one question.
1 What is the most common defect in the crystal lattice of ice?
2 Glide deformation of an ice crystal occurs most easily on the basal plane, which is usually given a letter designation. What letter axis is perpendicular to this plane?
3 Why is the slope of the Clausius-Clapeyron line negative with increasing pressure? (don’t go into details, answer in a sentence or 2)
4 What is the name of the location on the glacier where the net summer mass balance equals the net winter mass balance?
5 How, or what are the important mechanisms, does hoar frost form? (either surface hoar or depth hoar)
6 What is the approximate pressure, or average of the normal stresses, at 100 m depth inside a slow moving, low slope, mountain glacier (in Pascals)?
7 What is the approximate value of yy in the above question, if y is the cross glacier coordinate.
8 If the slope is 5 degrees ( sin(5) is 0.087 ), what is the approximate shear stress on an internal plane parallel to the surface at 100 m depth (in Pascals)?
9 Assume the temperatures on both the east and west side of the Sierra mountains in Calif. are similar, but the snowfall is higher on the west. Are the ELAs higher or lower in the west than the east?
10 Give a couple of mass balance reasons why the glaciers in the Wind River range tend to be on the North East side of the mountains.
11 At about what density does snow/ice become impermeable to air and water movement?
12 Draw a sketch of a perfectly round rock (1cm diameter) being forced through ice, so that regelation occurs. Label with arrows the direction of rock motion, the direction of water flow, and the direction of heat flow. Also label with “M” the peak location of melting, with “F” the location of freezing and label the coldest location with a “C”.
13 What physical property of the rock most limits the stresses that are required to move the rock in the above question.
14 What happens to the internal strain heat energy in polar, and in temperate glaciers? (i.e. what is the effect on the glacier of this heat, on the two types)
15 Explain why geothermal heat does not flow into a temperate glacier.
16 During the last glaciation, glaciers covered Libby Flats in the Snowy Range.
There are terminal Moraines about 6 miles from the main peaks. Assume the area is completely flat. About how deep was the ice close to the main peaks? [Hint think of the ice as a small icesheet]
17 The famous Bull Lake moraines, were made by long valley glaciers coming out of the Wind River Mtn. Range, just north west of Lander. The average valley slope is about 2 degrees (sin 2 is 0.035). About how deep were the Bull Lake glaciers?
18 What is probably the characteristic of the bed of a glacier that most affects sliding speed?
19 The glaciers that came from the west side of the Wind Rivers carved several deep valleys that have since turned into long lakes (near Pinedale). These lakes are quite deep, and when filled with ice, the ice had to flow uphill to reach the terminal moraines. The bed slope is about -15 degrees (tan -15 is -0.27) in the direction of flow, while the surface slope of the glacier was only about 3degrees (tan 3 is 0.05). The question is: was there a trapped sub-glacial lake under the ice, or was water able to flow out of the depression to the terminal moraines?
20 What is the difference between a deviatoric normal stress and a non-deviatoric normal stress? What is the difference between a deviatoric shear stress and a non-deviatoric shear stress?
21 Does Pressure affect the flow of ice, or more precisely, does pressure affect the effective stress in ice according to Glen’s flow law?
22 What limits the depth of crevasses?
23 In mountain glaciers, give the expected states of longitudinal stress: above and below the ELA?
24 Why don’t englacial water conduits squeeze shut? [2 reasons]
25 The glacial setting is this: a glacial stream exits from a steep glacier snout. The stream is fast flowing. You want to know the pressure of a sub-glacial water stream in a glacier. You come up with a brilliant idea that will allow you to measure it without having to get underneath the glacier with a pressure meter. You assume that the water, while it was under the glacier will be at the same temperature as the ice (however you don’t know the water/ice pressure). You also assume the water is flowing fast enough that there is little heat transfer from/to the water as it comes out from under the ice. And you assume the water flow under the glacier is in steady state, so that pressures and temperatures are not changing in time.
Your brilliant plan is to measure the quantity of ice crystals in the water as it exits from the glacier.
a) Why do you expect to see ice crystals (called frazil ice) in the stream water? (Note, natural water shows virtually no super-cooling.)
b) If you find there were 0.5% ice crystals, what was the water pressure under the glacier (compared to atmospheric)? You can use 10-7 oC/Pa as the PT slope, for ease of calculations.
c) List some serious flaws with this idea.