PRINS OF GLACIOLOGY
PRINS OF GLACIOLOGY ESS 431
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Date Created: 09/09/15
EARTH AND SPACE SCIENCES 431 Principles of Glaciology MID TERM October 27 2008 Instructions Please answer the following 5 questions The actual 5 questions will be selected from these 13 questions below or some minor variants of these questions Each question has equal value of 10 points The numbers in square braces with each subquestion indicate the points available for that subquestion This is a closedbook test however you are free to use a calculator and a dictionary if your rst language is not Enlish Different instructors may grade the different questions To facilitate grading please start each question on a new piece of paper and write your name on each page You have 1 hour and 20 minutes 130 250 1 Terminolo Give short definitions of the following 10 terms There may be different terms on the actual test flm accumulation area ratio equilibrium line destructive metamorphism depth hoar sastrugi closeoff depth thermal conductivity rime albedo ice nucleus 2 The Cryosphere The cryosphere is composed of ice stored in a variety of reservoirs and forms Some components may be more responsive to shortlived climate changes than others a The climate event is 3 successive unusually cold and wet winters Identify a component of the cryosphere that you would expect to change signi cantly in response to this climate event 3 Describe how this cryospheric component would change and why Describe how this change in the cryosphere might in turn affect the climate 1 Identify a component of the cryosphere that would not change signi cantly in response to the above climate event Why not b The climate event is 3 successive unusually warm sunny spring seasons 1 Identify a component of the cryosphere that you would expect to change significantly in response to this climate event 3 Describe how this cryospheric component would change and why Describe how this change in the cryosphere might in turn affect the climate 1 Identify a component of the cryosphere that would not change signi cantly in response to the above climate event Why not 3 Physics of the phase diagram Explain in physical terms related to molecular and crystal structure and Hydrogen bonds why a 3 Increasing pressure in water decreases the melting point temperature b 3 Increasing curvature of ice surfaces decreases the melting temperature c 4 Water is most dense at 4 C rather than at 0 C 4 Ice in clouds a 5 Describe the primary differences in characteristics and relative abundances of cloud condensation nuclei CCN and iceforming nuclei IN b 5 In a cloud at a temperature of 15 C explain how the difference in abundance of CCN and IN allows rapid growth of ice crystals to sizes large enough to precipitate 5 Avalanches Three skiers have arrived at the top of a slope and they are trying to decide whether to ski it They have dug a snow pit and found a weak layer of buried surface hoar l m below the surface They have measured the average density of the snow slab above the weak layer p150 kg m393 the slope angle 938 degrees and calculated the average downslope shear stress at the base of the weak layer p g H cos9gtltsin9750 Pa They have also used a shear frame to measure the shear strength of the weak layer They find that it fails at 950 Pa a 6 Give reasons to support why they should or should not ski the slope b 4 Are there more tests that would help in their decisionmaking 6 Deformation of Glacier Ice a 4 Describe how the structure of ice crystals atomic arrangement and bonding allows ice to ow when a stress is applied b 3 Why do imperfect ice crystals deform more easily than perfect crystals c 3 Why is colder ice harder to deform than warmer ice 7 Advance and Retreat 0f Glaciers on Mt Baker Six major glaciers on Mt Baker WA including Easton Glacier shown below receded until the early 1950 s then advanced by several hundred meters from the 1960s through the early 1980 s The glaciers have subsequently retreated The accompanying figures show winter precipitation and summer temperature in the Puget Lowland 2965 ml measurement point 0 bottom reflector 600 cross Section 1956 l terminus 39 A 400 A E terminus E z o 500 m 39 E t m l I i d39quot n 2 times vertical exaggeration 1545 rn 0 4 North South g u cross section 3 200 39 Gasman v Roosevelt WQSt ziz 25215115 EaStq A Deming 5quotquot quot39Z39F39quot 5 0 Boulder 400 9 Rainbow r I EaSton O BO 100 m 50 m 39 600 I s r r 1 1 I l l I 1 l l u l I u no vertical exaggeration 4o 45 50 55 60 65 7o 75 so 85 90 Year 1900 quot 18 I 175 l 4 155 le A 15 W NilT 145 I V i i I i 1940 1960 1980 2000 1940 1960 1980 2000 Year Year a 4 Explain Why the advance and retreat pattern ts scienti c expectations Be sure to discuss assumptions behind expectations A 39l A G A 9 01 A 4 A N A O OD Summertemperature deg C 8 Winter monthly precipitation cm H20 O5 b 3 Estimate the net ablation rate at the terminus of Easton Glacier based on its response to climate changes c 3 Do you expect Easton Glacier to advance again soon Why or Why not 8 Glacier Flow The shear stress at the base of a glacier generally falls into fairly narrow range near 1 bar 105 Pa With variations of typically less than a factor of 2 Glacier speeds on the other hand span a relatively Wide range from 100 to 103 meters per year a 6 What are the physical causes for these two behaviors b 4 Why are their percentage ranges of variation so dramatically different 9 Deformation in a Glacier We select a coordinate system on a glacier with the X aXis running downslope along the surface and the z aXis pointing downward and normal to the glacier surface The shear strain rate 3 in a glacier force directed in the x direction on planes whose normal vector points in the z direction is related to the shear stress by the relation Glen s Flow Law a 1 M Apg zsin03 2 dz where z is depth below the surface uz is horizontal velocity p is ice density g is acceleration due to gravity and 9 40 is surface slope of the glacier The softness parameterA in Glen s Flow Law for ice near 0 C isA 2gtlt103916 Pa393 yr39l The glacier is 200 m thick An unfortunate mountain goat of a rare subspecies that stood 1 meter tall at the shoulder was buried upright in a crevasse in the glacier 50 years ago The Burke Museum is interested in recovering this rare specimen however the curator is concerned that the specimen may have been distorted too much by shear ow The goat s depth of burial in the glacier probably varied over the 50year interval however to make calculations simpler you can assume that it was buried for the entire 50 years at its average depth a 3 Calculate the current height of this specimen at the shoulder ie distance from hoof to shoulder if its average depth of burial over the 50year period was 10 In You can estimate this by following the change in separation over time for 2 points eg hoof and shoulder that were originally on a vertical line and separated by 1 meter Show your work and list and explain your assumptions b 2 Calculate the current heigh of this specimen if it was buried on average at a depth of 100 m c 2 Is the specimen museumworthy after this time Why or why not d 3 Ifthe goat was on average 10 meters below the surface how far has it moved from the location where it fell into the crevasse Be sure to mention your assumptions about basal sliding 10 Glacier Surge a 3 Describe how a surgetype glacier changes through a complete surge cycle Include changes to the glacier profile and the timing of those changes b 2 Does the fast ow in a surge result from sliding or from internal deformation Brie y outline 2 lines of evidence to support your answer c 3 The 198283 surge of Variegated Glacier stopped on July 4 Why do surging glaciers tend to pick up speed slowly but stop abruptly Be sure to consider the basal water system d 2 How does the tidewater glacier cycle differ from the surging glacier cycle 11 Glacier Flow a 2 Sketch the ow pattern on the surface of a simple no tributaries steady state glacier by making a map outline and using ow vectors use at least 12 vectors spread over the glacier surface to represent the ow direction and relative speed at different points on the surface Your map should show 0 How the alongvalley component of the ow changes along the glacier and from margin to centerline How the crossvalley ow component differs in the accumulation area compared to the ablation area b 2 Make sketches showing vertical crosssections across the glacier in the accumulation area and in the ablation area paying particular attention to how the shapes of the glacier surface differ in the 2 regions Add ow vectors showing the directions and relative magnitudes of ice ow in the plane of your cross sections c 2 Make a sketch showing a vertical crosssection along the center line of the glacier from the headwall to the terminus Use vectors to show 0 how ice ow velocity varies with distance and depth in this section 0 where the speed is maXimim d 4 Explain these patterns in terms of accumulation ablation ice ux andor frictional drag 12 Avalanche Hazard During the summer of 1996 the slope above a ski chalet at Alpental village near Snoqualmie Pass was logged On March 1 1997 a slab avalanche released naturally from near the top of the clear cut The avalanche caused minor structural damage to one side of the chalet before stopping on the access road below the deposit on the road was up to 5m deep In subsequent litigation the residents argued that the logging had created an avalanche hazard there is no evidence to indicate that the slope had avalanched prior to logging The judge ordered the logging company to install avalanche protection fences and to start reforestation a 4 Discuss how logging of a slope would increase the potential for avalanche initiation and subsequent ow b 3 Give at least three essential design requirements for the avalanche fences c 3 What else could be done to mitigate the concerns of the residents Background Information and Measurements for study purposes A typical snowpack at Snoqualmie Pass consists of a relatively homogeneous base 23 meters deep Storms deposit up to 1m of new snow and natural avalanches during storms or subsequent rain often causes all or some of the new snow to avalanche Measurements after logging indicate 0 slope angle 35 degrees 0 length of slope 150 meters 0 height of stumps 15 meters 0 distance between stumps 210 meters Additional useful background material is contained in The Avalanche Handbook pp 91 113 13 Energy balance The surface energy budget components for Pionerskaya Station at 2700 m elevation in East Antarctica are as follows with the positive sign meaning a gain of heat for the surface in units of watts per square meter June December solar shortwave radiation downward 0 372 upward 0 3 12 net 0 60 infrared longwave radiation downward 106 173 upward l34 209 net 28 36 sum of radiation terms 28 24 sensible heat 23 l6 latent heat 1 2 sum 4 6 a 2 From these data what is the snow albedo in December Show the formula that you use explain the terms in it and describe what it does b 2 From these data what is the snow surface temperature in June Show the formula that you use explain the terms in it and describe what it does c 2 The sensible heat ux is positive in June and negative in December What does this imply about the relative temperatures of air and snow in these two months Explain your answer d 2 Which one of the following four processes is responsible for the latent heat ux in June evaporation liquid gt vapor sublimation solid gt vapor condensation of dew deposition of frost Explain your answers e 2 The energybalance terms do not add up to zero Where does the extra 6 W m392 in December go What process is indicated by the 4 W m392 in June Explain your answers For a and b give the formula you would use to obtain these values It is optional to actually calculate the numerical result Some possibly useful or useless numbers Solar constant 1370 W m392 StefanBoltzmann constant 567X10398 W m39zK391 Latent heat of freezing 334 Jg Latent heat of sublimation 2835 J g Dow Jones index 9265 20081020 1700 EDT
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