Homework #5               Glaciology, GEOL 4888,  Spring 2017                        Humphrey

Since nobody got it the first time around, please redo question #1, using the notes from thursday’ lecture.

Here are some calculations to back up some of our thoughts from the lectures.  Note question 1 takes you through the complexity of the Nye Glen flow law. This may be hard for some of you, so feel free to ask me questions.

 1. A section of glacier is being stretched, the longitudinal stress (t_xx) is 200KPa. The only stresses in the glacier are this extensional stress and the basal shear stress (t_xz) which is about 100KPa, and of course the overburden stress or pressure. The glacier is about 600m deep.

1. What is the overburden pressure? (ignore the longitudinal stress)

2. The longitudinal stress is a deviatoric stress, so what is the approximate total (non-deviatoric) stress (s_xx) near the bed?

3. Calculate the effective stress (t_effective) for use in the Nye-Glen flow law?

4. Use the effective stress from part c to calculate the shear strain rate (e_xz with a dot over it) near the bed using the full Nye-Glen flow law [ e_xz with a dot over it = A _* (t_effective)² _* t_xz ]. Assume the ice is -5C.

5. Compare the result from part d with the shear strain rate that you would calculate with the Glen flow law approximation, that doesn’t use the effective stress, but only raises the shear stress to the 3^rd power?