GEOLOGY 445 - GLACIAL GEOLOGY

LAB 3 - Glacier Deformation

I)  The accompanying map (original available above) shows a large portion of the Blue Glacier, Olympic Mountains, Washington, at a contour interval of 5 m. The original of this map (at a scale of 1:10,000) is also posted. You are asked to interpret some aspects of the flow in and deformation of this glacier. Note that flow is driven by the gravitational gradient displayed as slope of the glacier surface, and that the flow velocity at any point is a complex function of mass balance, cross-sectional area, ice temperature, shape of the channel, and ice surface slope.

1. The line A-A' represents a line of stakes across the glacier and the dashed line represents the same stakes two years later.
   1. Calculate the average daily velocity of the glacier surface at that point.  In your discussion, consider the variables hidden in that calculation.
   2. How representative is that calculation of the average velocity of the glacier?

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2. Crevasses are open fractures in glacial ice.  Crevassing occurs where stress application locally exceeds the strength of ice, causing fracture.  Most crevasses are the result of differential flow velocities, resulting in tension, compression, and shear. Ice is generally incompressible, although flow velocities may change under compression, but fractures under tension and shear. Generally, only tension fractures will open to form crevasses. See Fig. 6.3, p. 213, in Benn and Evans.
   1. Using arrow symbols, show in blue the horizontal stress fields (compression and tension) at each of the crevasse areas shown. Give a name to the crevasse type below.
      1. w', w'''' ____________________
      2. w'', w''' ____________________
      3. x         ____________________
      4. y         ____________________
      5. z                   (tic-tac-toe)
   2. Show the direction of ice flow in the neighborhood of each of these crevasse fields with red arrows, and indicate relative velocities by varying the size of the arrow.
   3. If a layer of new firn or ice, initially "cropping out" in a line across the glacier (B-B'), were to be deformed by the relative velocities interpreted in the first part for several (5) years, what would it look like? Draw a new line B-B'. This will describe the generation of foliation on the glacier.
   4. If the crevasses at w' to w'''' were rotated as described in the previous question, what would they look like?  why are no such crevasses evident?
   5. Briefly explain the causes for the observed crevasses and the inferred relative velocities.

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3. Discuss the assumptions and results of each of the exercises above.

II)  Examine the map of the McCall Glacier and suggest why crevasses are such a minor part of the flow regime there, relative to the Blue or Lemon Creek glaciers.