Glaciology 4888 Midterm Quiz Humphrey Spring 2022

Open book and notes, and you can surf the web. However, please don’t talk to anyone else. There are 30 questions. Many questions have quick, few word answers, but some require complete sentences or paragraphs or (gasp) calculations. Make sure you write enough for me to understand and mark!

Don’t spend more than a couple of minutes on any one question. Save questions (such as with calculations) that take you too much time and come back to them. Over ½ the questions should be very quick answers

Neatness Counts! (I am much more likely to award points if I don’t have to struggle to read your answer). Feel free to ask me about problems or unclear questions.

1 In words: what determines the depth of a dry crevasse?

2 Again in words: why does water in a crevasse increase the depth of cracking.

3 What is the most common defect in the molecular crystal lattice of ice? Give a name and describe it in one sentence.

4 Is the c-axis of an ice crystal perpendicular to the puckered or boat shaped oxygen hexagonal rings.

5 Why is the slope of the Clausius-Clapeyron line negative with increasing pressure? (don’t go into details, a concise answer requires one sentence)

6 What physical characteristic of snow or ice determines how much shortwave (visual) radiation is not absorbed.

7 Where do the water molecules come from to form hoar frost on the surface of snow on a cold, clear night?

8 What is the approximate pressure, (or average of the normal stresses), at 2000 m depth inside the flat ice of central Greenland (in Pascals)?

9 At this very central part of the ice cap (called an Ice Divide, since the ice has no surface slope) what is the approximate value of the deviatoric stress t_xx near the bed, if the ice is 2000m thick.

10 A region of a glacier (call it a) is steeper than another region (b). As a result, at some depth region a has twice the shear stress ( t_xz ) than region b. Based on Glen’s flow law, and assuming the shear is the dominant stress, how much faster is region a shearing than b?

11 If glacier slope is 5 degrees ( sin(5) is 0.087 ), what is the approximate shear stress on an internal plane parallel to the surface at 200 m depth (in Pascals)? What is the deviatoric shear stress?

12 Why is a typical mass balance vs elevation curve change much faster in the ablation zone, than in the accumulation zone. In other words, why doesn’t snow accumulate above the ELA as much as it melts below the ELA?

13 At about what density does snow/firn become impermeable to air movement?

14 Does Pressure affect the flow of ice, or more precisely, does pressure affect the effective stress in ice according to Nye-Glen’s flow law?

15 A large storm deposits 0.5m of snow at -5C and a density of 300 kg/m3 at the Snowy Range ski area. The next day is warm and melts 3cm water equivalent. Did the new layer of snow ‘ripen’ (get to 0C)? (must show your work)

16 Sketch a typical approximate down-valley ‘deformation velocity’ profile in a simple valley glacier. (draw a side view of a glacier) You need to label your sketch to show surface and bed, and your velocity curve has to be sufficiently precise to illustrate where the velocity v_xz(z) is largest and smallest.

17 During the last glaciation, glaciers covered Libby Flats in the Snowy Range. There are terminal Moraines showing the terminus was 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, since it was]

18 (hard) I made a big deal about the fact that only deviatoric stresses cause ice deformation. Why didn’t I stress the concept of deviatoric strains in ice deformation? You can answer this merely by mentioning an important rheological property of ice.

19 A glacier terminates in a fresh water lake (temperature approximately +4C) with a calving front, does the water in front of the glacier circulate upwards or downwards due to temperature/density gradients in the water.

20 Which is likely to have a larger effective water pressure; an englacial water conduit with small or large water discharge flux?

21 The Bull Lake moraines, were made by long valley glaciers coming out of the east side of the Wind River Mtn. Range, just north west of Lander. The average valley slope is about 1.25 degrees (tan 1.25 is ~0.02). About how deep were the Bull Lake glaciers? (hint, you know a typical basal shear stress ( t_xz ), which also applies to these glaciers).

22 For any water flowing inside the ice of a glacier there is always a balance between pathway/conduit closure and opening. What are the main controls on these 2 processes? (list at least 2)

23 The glaciers that came from the other (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 basal ice had to flow uphill to reach the terminal moraines. The bed slope is about -12 degrees (tan -12 is -0.2 upward) in the direction of flow, while the surface slope of the glacier was only about 2 degrees (downward). The question is: was there a trapped sub-glacial lake under the ice, or was water able to flow uphill out of the depression to the terminal moraines?

24 Why does the discharge of a Jokulhlaup typically stop very quickly?

25 If a region of ice has been undergoing shearing for an extended time, what can you say about the development of a fabric and its direction? A sketch of the fabric would be best.

26 A flat tabular iceberg is floating with the flat surface about 50m out of the water, how deep is the flat bottom under the water. Assume an ice density of 920kg/m^3.

27 On google earth, you see a glacier with the imagery taken in Sept. You see bare ice exposed on the lower 2/3rds of the glacier. What do you conclude about the health of the glacier?

28 (hard) Why do you often see ice crystals (called frazil ice) in the outlet stream water coming from under deep glaciers? (Note, natural waters show virtually no super-cooling.)

29 What is the melting temperature of ice under the glacier in question 8? You can use -10^{-7 oC}/Pa as the PressureTemperature slope, for ease of calculations.

30 (calculation question) Based on your answer to 21, what was a typical total deformation velocity of the Bull Lake glacier. You can assume the ice was temperate.