Lab 1 exercise 2021
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117
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Geography
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Dec 6, 2023
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docx
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EPS 117: Lab Exercise 1 – Due at the beginning of lab (Sept. 8
th
/9
th
)
Fall 2021
NAME: _____________________
Lab 1: Introduction to glaciers and glacial deposits
This week you will begin working on the first field project, which requires you to create a map of the extent of glaciers and their deposits during the late Pleistocene in the southwestern corner of Lake Tahoe. For many of you this will be the first time you have examined glaciated terrain or made a map from remote sensing imagery. To help with this effort, we have created this short exercise in which you will examine currently active glaciers in the Granite Mountain area of southern Alaska using
a combination of Google Earth imagery, aerial photographs, and a topographic map. The analyses requested here are similar to that required for the first project, but in the latter case the glaciers are no longer present. Getting Started
:
Turn on Google Earth and upload the kmz files (posted on the EPS 117 bcourses). Note that in Google Earth Tools, Options, you can select “Preferences” (On a mac: GoogleEarth, Preferences) and, among, other things, choose a vertical exaggeration. Some exaggeration helps to bring out features. Upload the Granite Creek composite jpeg (also on the bcourses) to your computer. A printed copy accompanies this handout, but the electronic version allows you to zoom in on features and to switch between the photograph and Google Earth. Andrew Meigs of Oregon State University provided the photograph and the map interpretation that accompanies it. Note how each glacier in the image is numbered and the current extent of glaciers and three moraines (color coded) are mapped. Drainage divides are also mapped-except for glaciers 6 and 8. Note attached to this handout is a photocopy of a topographic map of the area. We suggest you examine the study area looking from the north towards the south. First thing to do
:
Locate the numbered glaciers on the Google Earth imagery by first noting the location “ice divide Rapids Glacier #6” pin. You will see the name “Rapids Glacier” on the topographic map, and the number 6 in the aerial photograph (the Granite Creek composite). Once you have found all that then you can look east and west and see lots of glaciers and deposits, and know exactly where you are.
A Tour
:
The fun thing to do with Google Earth is to fly around the landscape. Zoom up the glacier filled
valleys. Spin the image and see the landscape from different perspectives. This is a beautiful area filled with glaciers and glacier generated features. Look for the following: 1)
Terminal moraines
(glacier transported debris deposited at the terminus of the glacier. Note one pin has been placed on the terminal moraine of glacier #4.
2)
Lateral moraines
(glacier transported debris deposited on the side of glaciers). These are mapped on the Meigs figure and two pins have been placed to identify two examples.
3)
Equilibrium Line Altitude (ELA) (Note that all the glaciers are bounded by lateral moraines downslope of the ELA). This is the transition from the accumulation zone to the ablation zone. In
the accumulation zone the ice surface topography has concave contours (in the downslope direction). In the ablation zone the ice surface topography has convex contours in the downslope direction. A pin labeled “note shape of ice band” is located in the ablation zone where the contours are convex downslope. Look at your topographic map and see how systematically the shape of
EPS 117: Lab Exercise 1 – Due at the beginning of lab (Sept. 8
th
/9
th
)
Fall 2021
the contours change from concave to convex downslope along individual glaciers. Now use your pencil and make a zig-zag line delineating the ELA for glaciers 6, 5, and 4. On your map, write down what the ELA elevation (in feet) is for each glacier.
4)
Cirque basins
(erosional recesses, hollow, or “armchair-shaped” valley cut into ridgelines and heads of valleys). Note that pin labeled “cirque”. It corresponds to the letter “b” in the Meigs figure. All the other letters on the Meigs map also locate cirque basins, some more obvious than others. Note the small lake, the moraines and its isolation from the major glaciers occupying the landscapes. You can locate it on the topographic maps with some study. The basin is cut by the boundary of sections 27 and 28 just north of the label “Thompson”. A first map: Now perform the following tasks on the glacier labeled number 6.
1.
Using the Google Earth imagery as a guide, trace in blue pencil the maximum ice extent for the current Glacier 6 onto your topographic map. This will be relatively easy in the canyon and
ablation zone, but what do you do in the accumulation area? Note that this glacier is part of a local ice cap and spills to the south to the east and into glacier #7. If you study the topographic map carefully, you can identify where the ice surface diverges (the topographic divides). The “ice divide Rapids Glacier 6” pin locates the approximate location of where one ice divide crosses. Mark the ice divides (locations on the glacier surface where flow diverges) with dashed lines
. Draw arrows indicating direction of flow on the glacier.
Note: the topographic map also has dashed lines indicating ice extent, however, the map
and photographs may differ due to time or interpretation. Nonetheless, you can use the dashed lines as a guide. 2.
Using a dotted line
, trace all morainal crests lines bordering Glacier 6, then outline the extent of glacial deposits with your pencil and color in orange this area. Compare your crest lines with those shown in the map below the composite image. Note that there can be multiple crests.
3) . Color in green the unglaciated area bordering this glacier.
Now you have made a map—much like what you will be asked to do for the first project, except, of course, the glaciers are gone. You will use erosional and depositional features visible on the available imagery to delineate the former glaciers and even draw ice topographic lines on the recreated glaciers. This reconstruction will guide you towards an analysis of glacier dynamics and erosion. 3.
Identify the Equilibrium Line Altitude (ELA) by the following methods: a.
The highest elevation of the lateral moraines
b.
Change in shape of the topographic contours from concave downslope to convex downslope. Draw a zig-zag line
across each glacier to delineate the location of the ELA. 4.
Based on your maps and delineation of the ELA, determine the accumulation area and the ablation area and calculate the Accumulation Area Ratios (AAR) for this glacier. The AAR is the accumulation area (the area above the ELA) divided by the total area of the glacier. How do
your estimates compare to the typical value of 0.65?
EPS 117: Lab Exercise 1 – Due at the beginning of lab (Sept. 8
th
/9
th
)
Fall 2021
5.
Using your topographic map and glacial reconstruction, construct a longitudinal profile down the axis of Glacier 6, from the uppermost ice divide or headwall to the current position of the glacier snout. Do this by:
1) drawing a line normal to the contours on your topographic map
2) measuring with a ruler (noting the scale of the map) the distance between each contour line
3) summing the interval distances to get the cumulative distance associated with each contour crossing (starting your zero distance at the snout)
4) plotting on graph paper provided or using computer software (e.g. Microsoft excel, Matlab, or Python) the longitudinal profile. 5) Using the surrounding unglaciated topography, the ice surface, and any other clues sketch the
bed of the glacier onto your long profile.
6.
Use the longitudinal profile and your depth estimates to calculate the boundary shear stress in the ablation zone of the glacier, via
The shape factor (F) can be calculated from the following table:
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