Homework #4               Glaciology, GEOL 4888,  Spring 2022 Humphrey

1. Consider the Bench Glacier in coastal Alaska. (It is the ‘potato’ glacier I show pictures of in class). It is about 200m deep and about 1km wide at the ELA. The glacier is about 10km long and the ELA is about halfway down the glacier. The accumulation region is a bit wider than the ELA, it averages about 1.5km wide. So the accumulation area is about 5km long and 1.5km wide. The net accumulation in the accumulation zone is about 2m ice equivalent per year. What is the approximate Balance Velocity in the region of the ELA?

This next question is straightforward, but you need to follow directions. I expect it will be a challenge for some of you, give yourself enough time to beat your computer into submission, feel free to collaborate on this question if that will help..

2. Plot (using a computer so it is neat! [if you really don’t have access to a printer, email me the plots, but I would prefer hard copy]) 3 curves related to the simple ‘slab’ Bench glacier from question #1 that is 200m deep with a surface and bed slope of 2 degrees near the ELA.

a Plot the shear stress vs depth.

b Plot the deformation velocity vs depth.

c Plot the shear strain rate vs depth.

You must plot the vertical axis as depth(!) with the surface at the top of the plot and the bed at the bottom. You also need to put a title and axis labels on your plots!

Notes: For the first plot you will need (from your notes) t_xy = r_ice g tan( a )y, where y is the depth from the surface down(!), not from the bed up, and alpha is the surface slope.

For the second plot you will have to use our fourth power velocity curve we derived from the NYE-GLEN flow law, but remember the ‘y’ coordinate is from the surface down. For the ‘A’ softness parameter you can use 7x10^-24 per sec per Pascal cubed, which is appropriate for warm ice. Plot the velocity in m/year. Note, you will need to offset the velocity so the curve goes to zero at the bed, not at the surface.

Finally Plot c may be conceptually hard for some of you. Feel free to ask questions.