GEOLOGY 445 - GLACIAL GEOLOGY

LAB 1 - Mass and Energy Balance

Note:  The lab is due by Friday at 5 PM.  As my computer no longer readily accepts diskettes, I require that submissions be submitted as e-mail attachments.

I) MASS BALANCE CALCULATION  A glacial survey team is studying a simple valley glacier of uniform slope. They use a line of measurements equally spaced up the centerline of the glacier to evaluate the glacier's mass balance. Each station is in the center of the area it represents, and the stations are 1 km apart. The data below are the results of their measurements over a balance year. The measurements taken include snow depth and density (end of winter) and change of surface elevation (end of summer). End of summer snow density is 550 kg/m³; assume ablated ice to have a density of 900 kg/m³.  Note:  also accessible as mbdata.xls. [HINT: right-click the link and save the file to your own directory, then open in Excel.]

Calculate each of the following:

1. Average specific (at the average point) winter mass balance, average specific summer mass balance, and average specific net mass balance, and
2. Elevation of the highest firn line.
   HINT: Do not forget to consider the effects of varying area of each increment! Draw up the results as a graph of summer, winter, net balance, and area versus elevation, and multiply.  I strongly suggest that you set up a spreadsheet in which rows represent elevation and columns represent length, width (thus area), winter snow depth, snow density, net annual change, firn density, and ice density.  You can then add columns in which the incremental changes are calculated and accumulate those values for the entire glacier.  Drawing a sketch of the glacier may help you!

What is the net mass balance (Bn) for the glacier, for the balance year?

How far would the terminus have to retreat to maintain a long-term equilibrium line altitude (ELA) at the measured firn line?

Discuss your results and assumptions.

II) ENERGY BALANCE.  The accompanying table summarizes research into energy available for ablation on glaciers. Evaluate and explain the relative significance of radiation, convection, and condensation in ablation of snow and ice.  Does that significance vary relative to geographical setting, such as latitude, elevation, or continentality?  Note: also available as ebdata.xls - see hint above.  Sort by various criteria and see if any relationships are evident.

III) MASS BALANCE RECONSTRUCTION (Longs Peak, CO, 1/24,000 Quad) - Optional - discuss the process even if you don't do the work!

1. On the photocopied map, outline the "Pinedale" moraine crests of the Longs Peak glacier.  A color version is attached.  (HINT: Look for "Mills Moraine", undrained depressions and ice-marginal channels.  Also - consider the Geologic Map of Rocky Mountain National Park; 1990, USGS MI-1973, by W. A. Braddock and J. C. Cole, with which I do not necessarily agree, in detail.)
2. Outline the approximate ice extent above the moraines. (HINT: Maintain a nearly constant surface slope.)
3. Estimate the appropriate ELA. (HINT: The highest lateral moraine is a fair estimate - why?)
4. Adjust the mass balance curve of Pierce (1979) to your estimated ELA. (HINT: Change the "Altitude" scale by the difference between Pierce's ELA - for the northern Yellowstone outlet glacier - and yours.)
5. Calculate the "Best estimate" net mass balance for the Longs Peak glacier under those conditions.
   1. Estimate the area of the glacier surface (from the topo map) for increments of 500 vertical feet
   2. Multiply each area by the "Best estimate" specific mass balance
   3. Accumulate these values to yield a net mass balance.
6. Discuss your results and assumptions.