Part 1. The basics
A glacier is long living body of ice that moves under the influence of gravity and its own weight. It develops on land as compacted snow is transformed into ice. There are two main types of glaciers. Glaciers that are confined to valleys are called valley or alpine glaciers. Such glaciers are relatively common in the NW United States and in Alaska.
Those glaciers that are not restricted to a valley but cover large areas of continents are called continental glaciers. On a global basis, glaciers cover 1/10th of the Earth's surface, and store over 75% of the world's freshwater supply. If all of the global supply of land ice locked up in glaciers were to melt, then sea level would rise ~70 meters.
All glaciers consist of two parts. The upper part is perennially covered with snow, and is referred to as the zone of accumulation. In contrast, in the lower part calving, melting, and evaporation occur. The lower part is called the zone of wastage. The boundary between the zone of accumulation and the zone of wastage is the snow line, a line marking the highest point at which the glacier's winter snow cover is lost during a melt season.
If, over a period of time, the amount of snow a glacier gains is greater than the amount of water and ice it loses, then the glacier will expand.
If the amount of water and ice a glacier looses is greater than the amount of snow it gains, then the glacier will shrink.
Question #1: What do you think will happen to the snow line under these two different sets of conditions?
An expanding glacier is said to have a positive budget and, as a result, is said to be advancing. A shrinking glacier is said to have a negative budget and, and as a result, is said to be retreating. Throughout the 20th century most glaciers on a global basis have been retreating. Movement varies from a few millimeters a day to more than 15 meters a day.
Question #2: Why do you think the glaciers are retreating?
Basal sliding refers to the sliding motion of the whole glacier over underlying rock.
Such sliding is facilitated by a thin film of melt water. Plastic flow occurs in a zone of variable thickness lying immediately above the interface between the glacier and the underlying rock. Here flow takes place by slippage along internal planes of individual crystals. The rigid zone overlies the zone of plastic flow and extends upward to the surface of the glacier. Ice within the rigid zone is brittle; it rides passively on the underlying zone of plastic flow.
Question #3: Cracks in glaciers occur only in the rigid zone and commonly form crevasses. Why would you not expect to find such structures in the zone of plastic flow?
Part 2. Landforms created by ancient glaciers
In the remaining parts of this lab we will learn that glaciers produce some of the most beautiful landscapes on the globe, and will focus on the glacial features of Yosemite National Park. If you have not visited the park, then you are encouraged to do so. I can almost guarantee you that you will not be disappointed!
Glacial Features of Yosemite National Park
The Valley of Tenaya Creek
Valley of Tenaya Creek, Yosemite National Park. Half Dome is on the right, and North Dome is on the left.
The topographic map and grid for Question 4 and 5 can be found at the end of this lab.
Draw the topographic profile along the transverse shown in the map labeled Valley of Tenaya Creek. A grid for drawing the profile is provided on the last page.
Question #4: How would you describe the shape of the profile valley?
Question #5: How do you think this profile differs from one of a valley carved entirely by a river?
Bridalveil Falls
View of Bridalveil Falls, Yosemite National Park. The falls emanate from Bridalveil Creek.
The topographic map and grid for question 6 and 7 can be found at the end of this lab.
Draw the topographic file for the transverse shown on them map labeled Bridalveil Falls.
Question #6: How far does the water in Bridalveil Creek fall in reaching Yosemite Valley (the distance from top to bottom of the fall)?
Question #7: Given that Yosemite Valley is an ancient glacial valley, describe how Bridalveil Falls formed?
Question #8: What name would you apply to the valley that Bridalveil Creek is in?
Snow Creek Glacier
Photo of debris deposited to the side of the glacier that moved down Snow Creek. The road in the foreground is Tioga Road (see illustration below).
Topographic map showing Tioga Road and Snow Creek.
Review the topographic map showing the location of Tioga Road and Snow Creek.
Question #9: After reviewing the above topographic map, which way do you think the glacier was flowing that once filled up Snow Creek?
Question #10: What is a more technical term for the glacial material that was deposited laterally relative to the ancient glacier in Snow Creek?
Olmsted Point lies off of Tioga Road and just before Tenaya Lake. It contains a variety of glacial features a few of which are shown in the next two photographs.
Topographic map showing the location of Olmsted Point and photos A and B.
Photo A. At the base of glaciers a fine powder composed of ground up rock material acts as a polishing agent. As the overlying ice moves slowly up or down a valley the fine powder under the pressure exerted by the overlying ice polishes the rock beneath the ice. When this polished surface is later exposed after the glacier has melted it is slowly worn away via abrasion and weathering. Can you pick out the polished versus the weathered surface? The bedrock in the photo is granite.
Photo B. The ridge above Olmsted Point is in places highly polished and sitting above this surface are large boulders of rock that appear to be completely out of place. These are believed to be boulders that were carried in the glacial ice, and then when it melted the boulders came to rest on the polished surface.
Review the map showing the location of Photos A and B, and then study the figures and their captions.
Question # 11: In Photo A what part of the photograph is glacial polish and what part is exposed granite? Briefly explain your answer.
Question # 12: In Photo B how do we know that the boulders were deposited after the glacier melted and before the modern day topography was developed?