Geology 101 – Environmental Geology
Spring 2007
Lab #6
Introduction: This exercise develops our understanding of alpine glacial topographic
features, and explores how we can use the altitudes of key geomorphological elements in
alpine glacial terrain to estimate climate change since the last glacial epoch of the
Pleistocene. In this case, we will use examples from mountainous regions in Utah,
Wyoming and Montana. Modern alpine glaciers still persist in the Wind River Range of
Wyoming and Glacier National Park of northwestern Montana, and these glaciers act as
examples for determining the climate parameters that also controlled the distribution of
late Pleistocene glaciers. The lab also includes analysis of data on the relationship
between elevation and mean annual maximum temperature, and information regarding
modern regional patterns of precipitation. You may wish to examine pages 197-201 in
the lab manual (copies available in the lab room) to gain better familiarity with alpine
glacial features.
Altitudinal temperature gradients – adiabatic gradients: As we all know, the air
temperature decreases as altitude increases. This gradient is nearly constant over most
continental areas at mid-latitudes, although there are local variations due to differences in
average humidity levels. Likewise, average annual temperature at a given elevation
decreases with increasing latitude.
Concept of Equilibrium Line Altitude (ELA): On modern alpine glaciers that are in a
state of equilibrium (the ice front position is neither advancing nor retreating), we can
define an equilibrium line across the glacier above which there is net accumulation and
below which there is net melting (ablation). The altitude of this equilibrium line is
determined by mean summer temperature (when melting takes place) and precipitation
during the winter months. In general, ELA increases with decreasing latitude (i.e.
glaciers lie at higher altitudes closer to the equator. Glaciers can flow to sea level at high
latitudes, particularly in places where there is a lot of winter snow (e.g. Alaska). In areas
of active glaciation, the ELA is approximated by the mean elevation of the glacier,
measured as the average of the highest (cirque headwall) and lowest (glacial terminus)
elevations. The ELA of former alpine glaciers can be estimated from the elevation of
cirque floors; cirques are the bowl-shaped depressions produced by the headward erosion
as alpine glaciers accumulate and flow down slope. Cirques often contain small lakes
(tarns) that are often good indicators of the altitude of the cirque floor. In this exercise we
will determine ELA using cirque floor elevations in each of the five areas under
investigation.
Part A: Determining average ELA for the five areas.
Examine the generalized location maps and the five detail maps for Glacier, Wind River,
Wasatch, Uinta and LaSal study areas. For the Glacier and Wind River areas, determine
at least five ELAs for active glaciers and average these to produce a modern ELA for
those sites. Also determine the ELAs for at least 5 cirque glaciers that lie at the lowest
elevations within each of the map areas, including Glacier and Wind River (note that the
Wind River map area may not include lower elevation cirques, but still determine at least
5 cirque elevations. Enter the average ELA data in the table below, along with the
average northing values for the sites measured. The northing values will allow us to plot
these data along a latitudinal gradient.
Site
Parameter
Glacier
Wind River
Uinta
Wasatch
LaSal
Average
northing
Average
Modern
ELA
NA
NA
NA
Average
Late
Pleistocene
ELA
Part B: Analysis of Temperature-Altitude Data
As we will discuss in lab, the Excel Data file for this lab contains mean annual maximum
temperature and altitude data for a number of weather stations in the general areas of this
study. Use this data to plot temperature vs. altitude for the four areas. Plot temperature
on the Y axis, and sequentially add each data series to your Excel plot. Use the trendline
function to determine best-fit straight lines for each series. Use the equations of line to
calculate the modern mean annual maximum temperatures for the elevations of the ELAs
determined above, and enter this data in the table that follows. Note that the NW Utah
data is applicable to both the Uinta and Wasatch sites.
Site
Glacier
Wind River
Parameter
Mean annual
maximum modern
temperature – modern
ELA (in oF)
Mean annual
maximum modern
temperature at
Late Pleistocene ELA
Uinta
NA
Wasatch
NA
LaSal
NA
Part C: Estimation of difference in mean annual temperature. If we assume that the
modern mean annual maximum temperature at the ELA of modern Glacier and Wind
River alpine glaciers approximates the typical mean annual maximum at the ELA of the
Late Pleistocene, we can estimate a difference in mean annual temperature during the
Late Pleistocene glacial maximum in this region. Enter these temperature estimates
below:
Site
Parameter
Mean annual maximum
temperature for alpine
glacier ELA – average
of modern Glacier and
Wind River (determined
above – single value)
Mean annual maximum
modern temperature at
Late Pleistocene ELA
(determined above)
Difference in mean
annual maximum
temperature modern –
Late Pleistocene
Glacier
Wind River
Uinta
Wasatch
LaSal
Based on this analysis, the difference in mean annual maximum temperature between late
Pleistocene and modern averages
__________________
Some climate change estimates predict that the mean annual maximum temperature in the
western US may rise by 4 oF in the next 100 years. How much increase in ELA for Wind
River and Glacier alpine glacier systems would there be as a consequence of this increase
in mean annual maximum temperature? __________________ What does this mean for
these glacial systems?
Part D: Latitudinal variation in ELA: As noted above, we would expect that ELA will
increase from north to south from study area to study area in the region, because of
overall latitudinal temperature increase. Plot the average ELA for late Pleistocene alpine
glaciers vs. the northings of the five sites. Is this the trend you expect? Explain.
Examine the map (small) which illustrates the modern distribution of precipitation in the
western US. How does this data help to explain (or not) the anomalies in latitudinal
variation in ELA?
Examine the maps for the LaSal Mountains. What factors other than temperature
(altitude) and precipitation seem to have controlled the distribution of cirque glaciers?
Explain.
Be sure to also hand in:
plot of temperature vs. elevation with trendlines for each series
plot of ELA vs. northing (latitude)