Observing Snow

Glacier Investigations Comparing a glacier's accumulation area with its total area gives an estimate of its health. The boundary between the accumulation zone and ablation zone is called the equilibrium line From "Glaciers of North America" by Sue Ferguson A glacier is a river of ice flowing under its own weight. It pushes large amounts of silt, gravel, and rock as it travels. Over a period of many years snow accumulations become deep enough to form glacier ice, which flows because of its own weight. Glaciers form in conditions where snow accumulates in successive years and does not completely melt during summer months. Eventually the snow crystals are subject to pressure metamorphism from the growing weight of the overlying snow and melt - freeze metamorphism from fluctuating warmer and cooler temperatures. The change from small individual ice grains to the very large, dense, and compressed glacial ice crystals forms a mass of ice through the process known as firnification. Under favorable conditions glacial ice crystals may reach a diameter of 30 centimeters. Glaciers accumulate new ice from snowfall in winter months and lose ice during melting that occurs in summer months. Accumulation of ice occurs in the higher altitudes in a region called the accumulation zone, and loss occurs at the lower latitudes, in a region called the ablation zone. The point where these two regions meet is called the equilibrium line and marks the highest level of retreat of winter snow. The equilibrium line varies from place to place and from year to year depending on the climate. If accumulation is greater than ablation, the glacier will grow or advance, and if ablation is greater than accumulation the glacier will shrink or retreat. Whether a glacier is growing or shrinking is determined at the equilibrium line, not at the terminus. The snout of an advancing glacier is often bulbous with a steep face The snout of a retreating glacier usually has a shallow slope that gradually thins. From "Glaciers of North America" by Sue Ferguson

Chapter 5 Science Content Standards A-4,5,7

It is hard to imagine a glacier moving, but they do in fact travel downhill under the force of gravity and the weight of the large ice mass. Distances from centimeters to meters per day are common. The bottom surface of the glacier slides along the bedrock aided by lubricating melt water in a process know as basal sliding. The ice mass itself also flows internally without breaking in a process called ductile deformation. Friction is greatest on the bottom and the sides because these are the surfaces contacting rock. Friction explains why glaciers move more quickly on the surface. One can visualize the layers flowing at different rates similar to the way cards slide past one another as you spread a deck.

Sometimes glaciers flow very rapidly or over steep terrain that creates ice falls. The ice crystals cannot be stretched to that degree. Brittle deformation occurs when a glacier's movement is so severe that the ice mass breaks, often forming deep crevasses.

Normal glacier movement is usually in the neighborhood of centimeters per day. A surging glacier can move 100 yards per day or more in extreme conditions. The internal " plumbing" of the glacier somehow becomes dammed . Glacial melt water cannot flow freely. A cushion of water develops and greatly reduces the friction of the large ice mass. Usually the surging motion itself breaks the melt water dam and the surging motion stops abruptly. Although the terminus of the glacier moves dramatically the overall total glacial volume is not increased. The Mapping Exercise in this chapter measures the movement of the Peter's Glacier during the 1986 -1987 surge.

 

When a glacier surges it can move ten to one hundred times faster than its normal rate of motion. This could cause a glacier, which normally moves the length of a football field in one year, to move that same distance in one day!

From "Glaciers of North America" by Sue Ferguson

The general health of the world's glaciers can be an indicator of broad global climate change. Over geological history the Earth's climate has cooled several times so that smaller systems of mountain glaciers were slowly overcome and absorbed in an advancing ice sheet that at times covered much of North America. Today the only remaining ice sheets are in Greenland and Antarctica. Scientists study glaciers to determine the rate and effects of global warming. A tremendous amount of frozen water is stored in the world's glaciers. Significant melting could have the potential to raise sea level and flood many valuable inhabited areas.

The Cordilleran Ice Sheet once covered nearly all of the mountains in southern Alaska, western Canada, and the western United States   From "Glaciers of North America" by Sue Ferguson

This map shows the approximate location of today's mountain glaciers From "Glaciers of North America" by Sue Ferguson

Geography Content Standards A- 1,3

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English / Language Arts Content Standard B-1

Glacier Slide Show In order to tap into a variety of learning styles the glacier information is organized into a slide lecture including not only photographs, but maps, graphs, and scientific illustrations. The last few slides are of the Peter's Glacier, used in the next exercise to teach map skills and to calculate its flow rate during the 1986-87 surging episode. This activity is presented with special thanks to Dennis Trabant from the USGS Water Resources Division and Keith Echelmeyer from the UAF Geophysical Institute for providing slides, aerials photos, and lots of ideas and information. All of the slides or Slides one at a time

Technology Content Standard A -2 C - 1, 2, 3

Activity: Glacier Mapping The initial introduction to mapping works well using two stations. At the first station students learn to use a stereoscope which renders the aerial photo in three dimensions. Here we are tracing ridges and drainages with our fingers. Station One: Students are introduced to a stereoscope, a tool that renders a three dimensional image by pairing two adjacent aerial photos. The class will map the Peter's Glacier surge from a series of aerial photographs. The stereoscopic image should help bridge the gap between the photographs presented in the slide show and an aerial photo showing a top or "map" view. The aerial photo is the flattened view of the stereoscopic image,and works well as a mapping tool. The class will map the Peter's Glacier surge from a series of aerial photographs.

Geography Content Standard A -1, 4

Station Two: A physical map of Denali National Park is displayed side by side with the satellite map of the Park. The teacher helps the student identify ridges, drainages, the Peter's Glacier, and its terminal moraine. It is helpful to point out and discuss the concept of map scale and contour lines. In Station Two students are guided in interpreting both a topographic map and satellite map. Open Note Review: Students who are not visiting either of the stations are directed to remain at their seats and work on an Open Note Review included at the end of this chapter. It is a comprehensive review of the material covered in the science section of the program. Students are encouraged to help each other and ask questions of the instructors in order to complete the entire review sheet.

Activity: Mapping the Surge Rate of the Peters Glacier Materials: Land Sat poster of Denali Park, features marked Topographic Map of Denali Park Stereoscope Color aerial photos for use with Stereoscope Overhead Projector Overhead Transparency of PRE surge Overhead Transparency of POST surge Four color overhead pens Mylar (one sheet per student) Colored Pencils (Four colors per student) Paper clips (Four per student) Aerial photo PRE surge (one per student) Aerial photo POST surge (one per student) Ruler Calculator A student map of ridges, drainages, and the pre-surge terminus

Geographic Content Standard A-1, 2, 3, 4, 5, 6 B -8

The instructor completes the mapping exercise on an overhead projector while the students follow along. Start with the POST Surge aerial photo first. Attach mylar with paper clips. Have each student make a scale on the mylar. Using a ruler mark a 2" line and 1/4" increments on one of the inches. Our scale is one inch equals one mile. Have students make a key that designates the colors of your choice to indicate Ridges, Drainages, PRE Surge terminus and POST Surge terminus Make sure to include a North Arrow (check on the large topographic map to be sure of the correct orientation!) Don't forget the title and to give credit to the map maker. Write your name on the map! Using the color designated in your key, trace the drainages. Using the color designated in your key, trace the ridges. Using the color designated in your key, trace the POST Surge terminus. Remove the mylar from the POST surge aerial photo Now use the ridges and drainages to key in, or register, your mylar to the PRE Surge aerial photo. This may take some turning of your map. Clip in place when the mylar is satisfactorily aligned. Using the color designated in your key, trace the PRE Surge terminus. Remove your map from the aerial photo.

Math Content Standard A - 1, 2, 3, 6 B - 3, 6, 8   Math Performance Standard age 11- 14 Measurement 4

Math Content Standards A - 2, 3   Math Performance Standards age 11-14 Measurement 1,2,4,5 Estimate and Compute age 11-14 3,4

Calculating the Surge Rate Next guide the group in calculating the surge rate of the Peters Glacier. This is an excellent opportunity to discuss the mathematical formula for a rate and determine what information is needed for calculation: Rate = Distance/Time To determine the distance the glacier traveled: using a ruler measure on your map the distance in inches. This will directly equate to miles using our scale of one inch per mile. The time between the two photographs is 212 days. Divide the distance by time to get the rate in miles per day. Guide the students to convert the answer into feet per day (one mile = 5280 ft.) Compare the result with the estimated length in feet of the classroom. It's great to make a contest out of the map project with prizes given for the most accurate, neat, and complete maps!

Observing Snow
Introduction
The Four Corners of Life
Water: the Stuff that Makes Snowflakes
Snow on the Ground Changes Through Time
Exploring Native Snow Terms
Glacier Investigations
Open Note Review
Conclusion
Bibliography & Resources

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