GEOLOGY 445 - GLACIAL GEOLOGY

LAB 5 - Glaciers across Space

"Conventional wisdom" has it that glaciers are excellent integrators of climate.  If that is the case, then present glaciers should be sensitive to relatively minor spatial variation in present climate.  Today's exercise involves the assessment of modern climate in a glaciated region through two methods - estimation of the lowest equilibrium line altitude and of the "glaciation limit" (or "glaciation threshold").

Definitions: Glacierized means "presently occupied or affected by glacial ice".  The term is distinctly different from "glaciated", which can be interpreted as "affected by glaciers in the past" (e.g., "a glaciated mountain range"). The equilibrium line altitude (ELA) of existing glaciers can be estimated as the median altitude of small glaciers or the altitude above which lies 2/3 of a larger glacier's area (e.g., Meierding, 1982).  The ELA cannot be reliably estimated for ice caps and outlet glaciers, for which the accumulation area is poorly defined.  In any given area, the lowest ELA (usually N to NE-facing in the Northern Hemisphere) is consistently representative of optimal conditions for glacierization. The glaciation limit (GL) is the height which a mountain can achieve without becoming glacierized.  In practice it is estimated as midway between the altitudes of the lowest glacierized. peaks and the highest non-glacierized. peaks (Figure 1; from Flint, 1971).  Local topography is not supposed to be a factor, so horns are not considered non-glacierized. if they have no nearly flat area near their summits to support an ice cover.

Exercise:

As a solo exercise (but help others if you finish early!):

• Select a 1:250,000 quadrangle from the available collection (coastal Alaska),
  • Lightly tape a sheet of tracing paper (provided) to cover (as well as possible) the glacierized region of the map.
  • Label your tracing paper with reference points from the quadrangle margin, a title, your name, yada, yada, yada...
• Define point values for as many lowest ELAs (usually on NE-facing glaciers) as possible, write the values IN RED on your tracing paper (in hundreds of feet; e.g., 3400 = 3.4), and contour the values at increments of 500'. 
• Define the Glaciation Limit (GL) in as many localities as possible and write the values IN GREEN on your tracing paper (in hundreds of feet; e.g., 3600' - 3.6) and contour the values at increments of 500'
• Communicate with students working on adjacent quadrangles to constrain contour trends at the margins of your maps.

Answer the following questions:

• What local (tens of km) and regional (100's of km) trends are evident in your (and your neighbors') ELA contour map?
• What is th average difference between your spot GL values and the ELA surface at that point?

Discuss your results and assumptions!

References: Meierding, T. C., 1982, Late-Pleistocene glacial equilibrium-line altitudes in the Colorado Front Range: A comparison of methods.  Quaternary Research, 18, 289-310.