Chapter 17-Glaciers Section 1: Glaciers – Moving Ice Section 2: Glacial Erosion and Deposition Section 3: Ice Ages

Section 1: Glaciers – Moving Ice Natural forces compact snow to make a large mass of moving ice called a glacier. At high elevations and in polar regions where ice and snow remain year round, the motionless mass is called a snowfield. The snowline is the elevation above which ice and snow remain throughout the year. The size of a glacier depends on the amount of snow received and the amount of ice lost. When new snow is added faster than it can melt, the glacier gets bigger. Changes in the size of a glacier may indicate climatic change.

Alpine glaciers – a narrow, wedge-shaped mass of ice that forms in a mountainous region and is confined to a small area by surrounding topography. Alpine glaciers are located in Alaska, the Himalaya Mountains, the Andes, the Alps, and New Zealand. Continental glaciers – massive sheets of ice that may cover millions of square miles and may be thousands of meters thick. Today continental glaciers exist only in Greenland and Antarctica. If these ice sheets melted, the water they contain would raise the worldwide sea level by more than 80 meters.

Glaciers are sometimes called “rivers of ice” because both glaciers and rivers flow downward. Glacial ice cannot move rapidly or flow easily around barriers. Glaciers move by two basic processes – basal slip or internal plastic flow. They can move as little as a few centimeters in a year or as much as a kilometer or more. Basal slip is the process that lubricates a glacier’s base and causes the glacier to slide forward on melted ice and sediment from the ground. Internal plastic flow occurs when the pressure deforms gains of ice and cause the glacier to move slowly. The rate of the flow is different in different parts of the glacier – moving more quickly in the center and more slowly on the edges.

Section 2: Glacial Erosion and Deposition Many landforms in Canada and northern United States were formed by the movement of glaciers. Glaciers are agents of erosion. Deep depressions form when a moving glacier plucks a rock from the bedrock and then is dragged across the bedrock forming deep depression. Cirque - a bowl-shaped depression Arete – sharp, jagged ridges formed between cirques Horn – several arete join and form a sharp, pyramid-like peak U-shaped valleys and hanging valleys

Alpine glaciers form sharp, rugged features. Continental glaciers erode by leveling landforms into smooth, rounded landscape. Large rocks that a glacier transports from a distant source are called erratics. Glacial drift is a term for all sediments deposited by a glacier. Moraines are ridges of unsorted sediment on the ground or on the glacier itself. Kettles are depressions formed on the ground when the ice melts in the drift. Eskers are long, winding ridges of gravel and sand left behind when the continental glaciers recede.

Section 3 – Ice Ages An ice age is a long period of climatic cooling during which the continents are glaciated repeatedly. Several major ice ages have occurred on the Earth. Ice ages probably begin with a long, slow decrease in the Earth’s average temperatures. A drop in average global temperature of only about 5°C may be enough to start an ice age.

What causes ice ages? The Milankovitch Theory – the theory that cyclical changes in Earth’s orbit and in the tilt of Earth’s axis occur over thousands of years and cause climatic changes. Other explanations are reduced sunlight reaching the earth due to varying amounts of sunlight produced by the sun, volcanic dust blocking the sun’s rays, and plate tectonics because changes in the positions of continents cause changes in global pattern of warm and cold air and ocean circulation.