Section 8 Glaciers and the Landscape Section 8 Glaciers and the Landscape What Do You See? Soils that contain many rocks and large boulders may be expensive to dig out for development. Learning Outcomes Think About It In this section, you will Glaciers are huge masses of ice that move very slowly across the • Make a mathematical model land, changing the land’s shape as they move along. In Alaska, of an imaginary glacier. the largest glacier is the Tazlina Glacier. It is approximately • Calculate how the glacier 40 km long. Scientists who study glaciers think about questions would respond to hypothetical changes in climate. like the following: • Understand the uses and • What other materials might be in a glacier besides ice? limitations of models in Earth and space science. • How might the materials get into the glacier? • Understand the mechanics of how glaciers form and move. • How do glaciers change the landscape? • Discover that glaciers modify Record your ideas about these questions in your Geo log. Be the landscape by erosion and prepared to discuss your responses with your small group deposition. and the class. • Model the action of glacial meltwater as it drains out of a glacier. Investigate • Establish that the movement of glaciers can change stream In this Investigate, you will calculate the movement of a drainage patterns. hypothetical glacier. This will help you to gain an understanding • Model the effects of a glacier of the factors that affect the shape and movement of a glacier. You infringing on a stream. will also examine models that show the action of glaciers as they • Apply what you have observed move over land. Your teacher may run some or all parts of this to determine whether glaciers have affected your community Investigate as a demonstration for the class. in the past. 449 EarthComm EC_Natl_SE_C4.indd 449 7/12/11 9:55:16 AM Chapter 4 Surface Processes Part A: Modeling the Behavior profiles and note that the X has moved of a Glacier forward the same distance as was lost 1. In this part of the Investigate, as a to the head of the glacier). glacial geologist, you are monitoring a a) How much ice is melting each year glacier in Alaska. Assume the following from this glacier? (If the glacier about your glacier: is at equilibrium, the volume of • It is 100 km long, 5 km wide, and ice melted equals the distance of 200 m thick. glacier movement per year, times • It moves at a rate of 100 m/yr. (Note: the thickness of the glacier, times This does not mean that the glacier the width of the glacier.) gets longer by 100 m each year, but 3. To be at equilibrium, the glacier must rather that any one point in the glacier receive as much new ice each year as moves forward 100 m in a year, as it loses by melting. A lot of snow that shown in the sample profiles below.) falls on a glacier simply melts and runs • It is at equilibrium. (Note: This means off without contributing anything to the that it is receiving just enough snow glacier, especially in the warmer areas to balance what it loses through near the foot of the glacier. Assume that melting. At equilibrium, the length new ice is added only in the upper half and thickness of the glacier remain (50 km) of the glacier. about the same.) a) What volume of ice is needed to balance losses by melting? b) What thickness (depth) of ice has to be added each year to balance the melting? (Remember that the volume of ice is equal to flow per year × depth × width.) 4. On average, 1 m of snow packs down into about 10 cm of ice. a) How much snow would have to fall on the glacier each year to create the thickness of new ice that you calculated above? b) Data show that there is 7.2 m of a) How long would it take a rock that snowfall in the region in which the falls into the ice at the head of the glacier is located. Is the amount of glacier to reach the foot? (Remember snowfall required to keep the glacier that the flow rate of the glacier is in balance realistic? 100 m/yr and that the rock must travel the entire length of the glacier.) 5. Imagine that the climate in the region of the glacier changes in such a way 2. The glacier moves at a speed of that the winter snowfall is greater by a 100 m/yr. If no ice were melting from factor of two (that is, it doubles) and the glacier, it would be 100 m longer the melting rate is less by a factor of after one year. However, it was assumed two (that is, it is cut in half). that the glacier maintains a constant size. Therefore, a volume of ice must a) How much larger will the total volume be melting each year (see the sample of the glacier be after 100 years? 450 EarthComm EC_Natl_SE_C4.indd 450 7/12/11 9:55:17 AM Section 8 Glaciers and the Landscape b) What will be the percentage increase a) Describe the characteristics of in the size of the glacier? (To compute the glacier. this, subtract the original volume b) What processes might supply of the glacier from the new volume, sediment to the surface of the glacier? divide by the length of the time interval in years, and multiply by c) Describe the characteristics of the 100 to convert to a percentage.) valley-floor materials. 6. Imagine that the climate in the region of d) What processes might affect the the glacier changes in such a way that development of the valley floor? the winter snowfall is less by a factor of 2. Think about how constructive and two and the melting rate increases by a destructive surface processes affect the factor of two. landscape. Predict the following: a) How many years would it take for a) What will happen if the glacier the glacier to disappear (that is, to advances by growing larger? melt completely)? b) What will happen if the glacier Part B: Glacial Processes retreats by melting? 1. Glacial ice can grow thousands of c) Make a table with two columns meters thick and extend tens to and two rows. Label the columns hundreds of kilometers. Due to their “erosion” and “deposition.” Label the size, glaciers can have a great impact rows “processes” and “landforms.” on the landscape. You will run a model 3. You will model the movement of a that examines the destructive and valley glacier. Run the model by pressing constructive effects of a valley glacier, a the ice down into the sediments and glacier that is confined to a valley and slowly slide the ice forward so that it flows from a higher elevation to a lower advances along the valley floor. elevation. Valley glaciers are found in mountain landscapes. Work with your a) Record your observations under the group to set up your model according appropriate heading in your table. to the following diagram. Look for changes all around the glacier, including underneath. b) Compare your predictions to your results. Explain any differences. 4. You will now observe what happens when the glacier retreats by melting. a) Make another table for recording your observations. b) Predict what you will observe as the glacier retreats. 5. Run the model by letting the ice melt and record your observations. a) Compare your predictions to your results. Explain any differences. b) Discuss with your class how glaciers change the landscape. 451 EarthComm EC_Natl_SE_C4.indd 451 7/12/11 9:55:17 AM Chapter 4 Surface Processes Part C: Modeling the Action of 6. Put the pan on a surface where water Glacial Meltwater can drain from the pan without causing any damage. Prop up the end of the pan Wear goggles and a lab apron throughout this investigation. Use the hammer with care. Clean opposite the opening with a thin strip up spills. Wash your hands when you are done. of wood about 3 cm thick, or a chalkboard eraser. 1. Put an even layer of cedar bedding about 1 to 2 cm thick in the bottom of a baking pan. Put a second pan inside the pan with cedar bedding. Spread an even layer of fine sand about 0.6 cm thick on the bottom of the second baking pan. Fill the baking pan until it is almost full of water. Put the assembly into a freezer, and wait overnight until the water is a solid block of ice. Note: It will take a long time for the water to freeze all the way to the bottom because of the insulation of the cedar bedding. 2. Turn the pan upside down under warm running water until the ice block comes loose. Set the ice block aside, and cut down along two edges of the pan so that one of the narrow sides of the pan can be bent down flat, level with the bottom of the pan. 3. Replace the ice block in the pan, and wait until the block is at its melting temperature. You will know when the block has reached its melting temperature when its surface shines with a thin film of water. 4. Put the pan on the floor, place a wooden block on the ice surface, 7. Spray cold water on the upper end of and hit the board with blows from a the ice sheet.