Geomorphology. Glacial Flow and Reconstruction

Transcription

1 Geomorphology Glacial Flow and Reconstruction We will use simple mathematical models to understand ice dynamics, recreate a profile of the Laurentide ice sheet, and determine the climate change of the little ice age as indicated by the moraines of a now shrunken glacier. By the end of this lab, you should understand that the behavior and climate sensitivity of complex ice sheets can be modeled by simple equations based on physical parameters. Using map, air photo, and field evidence, you should be able to recreate plausible versions of former ice masses and understand what they imply for climate change. There are three parts to today s lab. Best to do them in any order. Please complete them and hand in this sheet before leaving lab today. This material will be on the exam next week. A. Ice Flow and Glen s Flow Law ICE DEFORMATION Ice flow (strain) can be described by a simple flow law that has only a few terms. Today, we will explore the importance of A ( the temperature sensitive term) and n (the exponential term that describes ice resistance to flow as a function of changing stress). Both of these terms describe the material properties of ice. DOWNLOAD the Excel spreadsheet entitled flowlaw.xls from the class web page. Similar to last week, we will be doing sensitivity testing to understand how ice responds to changes in A and n. 1. Examine the graphs of strain rate versus stress carefully noting that the large graph is a log/normal graph with fixed axes scales and the small graph is a normal graph with auto scaling axes. Do not change n and K from the nominal

2 values of 3 and 6.8 * DESCRIBE what happens to strain rate of ice when the stress on the ice increases. IDENTIFY the type of material behavior the stress/strain relationship of ice approximates. 2. Now, vary the exponent term n between the extreme value of 1.5 and 4.2 and DESCRIBE what happens to the stress/strain relationship. Note that 3 is the generally accepted value of n. 3. Keeping n constant at 3, vary K from the expected value at 0 o C to the expected value at -35 o C. PLOT a graph of the strain rate at a stress of 100 kpa (about the most a glacial bed can support). 4. Using the graph above, EXPLAIN whether warm ice ( o C) or cold ice (35 o C) is likely to flow faster? In your answer, GENERALIZE about the deformation rate of ice as a function of temperature.

3 B. The now-vanished Laurentide Ice Sheet Examining its morphology In the 1950s, Nye came up with a model for ice sheet behavior that incorporated the shear stress term, of which are all familiar, as well Glen s flow law. You can check out his original paper on our lab web site (but beware, it s a bit dense): Nye s model allows you to approximate the ice sheet profile (at steady state) along a flow line from one point on the margin back to the interior of the ice sheet. His model requires that you estimate the shear resistance (or friction) of the material at the base of the ice sheet, that would be a mixture of water, till and ice most probably. The model can be applied to any variety of situations. We will apply it to the Laurentide ice sheet stretching along two flow parallel flow lines from Long Island north. One line runs up the Hudson and Champlain lowlands; the other line runs from Cape Cod north to the White Mountains. The goal here is to understand how basal friction values affect the surface profiles of ice sheets. To run this model, download the spreadsheet: nye.xls There is a single cell that you can alter. This cell is highlighted green and allows you to change the shear strength of the material over which the ice sheet is moving. The nominal value is 1 bar but glaciologists working around the world have fit values ranging from 0.1 for ice moving over easily deformed, saturated clay-rich lake sediments to 1.5 for ice moving over crystalline rock to values >5 when ice moves over well-drained gravels or karst (fractured limestone) so that the base is dry.

4 Note that each degree of latitude corresponds to 111 km on the ground.

5 1. Using the nye.xls spread sheet, DESCRIBE the shape of the Laurentide ice sheet (in profile) when the nominal basal shear stress value of 1.0 bar is used in the model. 2. CHANGE the basal shear strength of the underlying material over a range from 0.1 (soft, saturated clay) to 1.5 (till derived from crystalline rocks). DESCRIBE the changes that result in the shape and thickness of the ice sheet as a result of changing the material over which the ice flows. 3. Because we have reliable field evidence indicating that ice overran both Mt. Washington and Mt. Mansfield (young soils, fresh erratics) you can use the model to constrain a minimum value for basal shear strength of the material underlying the distal Laurentide ice sheet. GIVE your value (in bars) and EXPLAIN below what you did to arrive at your value.

6 C. Glacial Reconstruction and Climate Change Glacial geologists study the deposits and features left behind by glaciers in order to understand their affects on the landscape, and the climates that formed the glaciers. Here we will examine the Easton Glacier, map its recent moraines, and recreate it as it may have looked at the height of the Little Ice Age (about 150 yrs ago). That reconstruction will allow us to compare the climate conditions that existed then compared to those that exist now. The Easton Glacier flows down the southwest side of Mt. Baker. 1. Using the topographic maps and aerial photographs of Mt. Baker, LOCATE the glacier, and OUTLINE the margins of the glacier as it appears on the map today. (NOTE: in the upper parts of the glacier, the ice becomes a nearly continuous sheet across the volcano; in this area, you ll need to estimate the margins of the glacier as ice divides; these are essentially the same as drainage divides: if you start at the upper outer edges of the Easton glacier tongue, use your pencil to work your way upslope from that point, always keeping your line perpendicular to the contours on the ice; this will tell you which ice will eventually flow into the tongue, and which will not). 2. Using the air photos and the topographic map of the glacier, MAP the moraines that are associated with the maximum advance of the Easton Glacier during the Little Ice Age, or LIA. The LIA was a global cool period that started about years ago, and ended only in the last 100 years. During the LIA, glaciers around the world advanced substantially. The moraines are the sharp crested ridges beyond today s ice margin. The lateral moraines are most clearly shown. 3. Knowing that the crest of lateral moraines record the top of the outer edges of the glacier when it was at its LIA maximum, SKETCH in topographic lines across the LIA paleoglacier to show what it would have looked like 150 years ago. Remember that on a glacier, topographic contours tend to bow downstream below the equilibrium line and upstream above the equilibrium line 4. The Equilibrium Line is the line that separates the accumulation area from the ablation area of a glacier. Although we can measure the equilibrium line directly on modern glaciers, glacial geologists often want to calculate where the Equilibrium Line Altitude (ELA) was in past times, as a measure of past climate conditions. From studies of modern glaciers, glaciologists have found that, in general, roughly 2/3 (or 65%) of the area of most mountain glaciers is above the ELA (the accumulation area), and 1/3 (or 35%) is below (the ablation area). Knowing this, USE box counting on transparent graph paper to DETERMINE the ELA of the glacier, currently, and during the LIA (determine the total area of both glaciers, then the lower 35% of each; the line connecting the left- and right-lateral moraines across the top of the lower 35% will be the ELA). Remember to draw the ELA as a line at a single altitude, that is, if it starts at 6000 on one side of the glacier, it should stay at 6000 all the way across the glacier and hit the 6000 contour at the moraine on the far side of the glacier.

7 LIST the ELA for the modern glacier LIST the ELA for the Little Ice Age glacier CALCULATE the ELA difference m m m 5. We know that temperature and elevation are related by a variable termed the lapse rate. Depending on atmospheric saturation with water, the lapse rate can vary between 0.9 o C/100m and 0.4 o C/100m. Using this range of lapse rates and assuming that snowfall rates did not change during the little ice age, ESTIMATE the change in average temperature based on the ELA lowering. SHOW both your answer and your work below.

8 Area of top air photo

9 Close up of end moraine area