Today glaciers cover nearly 10 percent of Earth’s land surface; however, in the recent geologic past ice sheets were three times more extensive, covering vast areas with ice thousands of meters thick. Many regions still bear the mark of these glaciers. The basic character of such diverse places as the Alps, Cape Cod, and Yosemite Valley was fashioned bynow-vanished masses of glacial ice. Moreover, Long Island, the Great Lakes, and the fiords of Norway and Alaska all owe their existence to glaciers. Glaciers, o course, are not just a phenomenon of the geologic past. As you will see, they are still sculpting and depositing debris in many regions today.
GLACIERS: A PART OF TWO BASIC CYCLES
Glaciers are a part of two fundamental cycles in the Earthsystem, the hydrologic cycle and the rock cycle. Earlier you learned that the water of the hydrosphere is constantly cycled through the atmosphere, biosphere, and solid Earth. Time and time again, the same water evaporates from the ocean into the atmosphere, precipitates upon the land, and flows in rivers and underground back to the sea. However, when precipitation falls at high elevations or highlatitudes, the water may not immediately make its way toward the sea. Instead, it may become part of a glacier. Although the ice will eventually melt, allowing the water to continue is path to the sea, water can be stored as glacial ice for many tens, hundreds, or even thousands of years. During the time that the water is locked up in a glacier, it can be a powerful erosional force. Erosionalprocesses are an important part of the rock cycle. Like rivers and other erosional processes, the moving ice modifies the landscape as it accumulates, transports, and deposits sediment.
TYPES OF GLACIERS
A glacier is a thick ice mass that originates on land from the accumulation, compaction, and recrystalization of snow. Because glaciers are agents of erosion, they mustalso flow. Although glaciers are found in many parts of the world today, most are located in remote areas.
VALLEY (ALPINE) GLACIERS
Literally thousands of relatively small glaciers exist in lofty mountain areas, where they usually follow valleys that were originally occupied by streams. Unlike the rivers that previously flowed in these valleys, the glaciers advance slowly, perhaps only a fewcentimeters per day. Because of their setting, these moving ice masses are termed valley glaciers or alpine glaciers. Each glacier actually is a stream of ice, bounded by precipitous rock walls, that flows downvalley from an accumulation center near is head. Like rivers, single or with branching tributaries. Generally, the widths of alpine glaciers are small compared to their lengths. Some extend forjust a fraction of a kilometer, whereas others go on for many tens of kilometers. The west branch of the Hubbard Glacier, for example, runs through 112 kilometers of mountains terrain in Alaska and The Yukon Territory.
In contrast to valley glaciers, ice sheets exist on a much larger scale. The low total annual solar radiation reaching the poles makes these regions eligible for greatice accumulations. Although many ice sheets have existed in the past, just two achieve this status at present. In the area of the North Pole, Greenland is covered by an imposing ice sheet that occupies 1.7 million square kilometers, or about 80 percent o this large island. Averaging nearly 1500 meters (5,000 feet) thick, the ice extends 3000 meters (10,000 feet) above the island’s bedrock floorin some places.
In the South Polar realm, the huge Antarctic Ice Sheet attains a maximum thickness of nearly 43000 meters (14,000 feet) and covers an area of more than 13.9 million square kilometers (5.4 million squane miles). Because of the proportions of these huge features, they often are called continental ice sheets. Indeed, the combined areas of present-day continental ice sheets represent...