Chapter 18: Amazing Ice: Glaciers and Ice Ages
Study Plan
Mendenhall Glacier, in Juneau, Alaska. During an ice age, glaciers like this-and larger-covered vast areas of continents.
Credit: Anthony J. Spuria
Guide to Reading
You might think by now there would be nothing left to say about water. Not so. This chapter is all about water, but in its solid state: ice. More specifically, the chapter deals with glaciers, which are accumulations of snow that recrystallize into ice and begin to flow. They create an awesome but hostile environment. If they are widespread, they can dominate conditions over a large percentage of Earth for thousands of years. If they occupy smaller areas, they can rework the scenery, by erosion and by deposition, and create unique landscapes that were first correctly interpreted by Louis Agassiz in the mid-1800s. The chapter begins by recognizing his work and continues with a discussion of the nature and characteristics of ice, its albedo, crystal form, reactions to temperature and pressure changes, and similarities to metamorphic rocks.
A discussion of ice quite naturally leads to the many types of glaciers (mountain or alpine glaciers, including cirques, valley glaciers, mountain ice caps, piedmont glaciers, and the other major groups, continental glaciers). The author makes the point that while pieces of glaciers may end up in the sea by the process of calving off icebergs, all true glaciers originate and move on land. They begin as accumulations of snow that change to firn and eventually become ice. (Masses of ice that originate as frozen seawater are called ice shelves.) Once formed, glaciers move. They may be wet-bottom or temperate glaciers and move by basal sliding, or they may be dry-bottom or polar glaciers and move by internal flow. They may break apart in their upper brittle zone as they move over uneven ground and create crevasses. Their average speeds may vary from ten to a few hundred meters per year, and they may show occasional periods of exceptionally fast movement, called surging. The ultimate cause of glacial movement is the pull of gravity, which may create lateral movement, called gravitational spreading, or simple down-valley motion. To complicate the picture, even though glacial ice always moves in response to gravity, it doesn't always appear to keep advancing in the same direction. If it loses enough ice by melting, sublimation, or calving, its terminus (toe) may retreat, and its zones of ablation and accumulation may grow or diminish. Lest the reader feel all this is irrelevant to human experience, the author gives a detailed account of the Titanic's infamous encounter with ice in the sea.
Glaciers are powerful agents of erosion. They plow through the landscape, incorporate and pluck fragments from the bedrock, abrade the land surface, gouge out striations, and grind off rock flour to leave glacially polished surfaces. In their head (upper) regions, valley glaciers erode mountain peaks and valleys to create spectacular, jagged scenery composed of cirques, tarns, arêstes, horns, U-shaped valleys, hanging valleys, truncated spurs, roche moutonnées, and fjords.
Glaciers also cause major changes in the landscape by their depositional activities. First-time observers in glacial areas are often amazed at the amount of sediment and rock associated with glaciers, at times more obvious than the ice. Sometimes rocks even take over, and the glaciers become slowly moving jumbles of rock, impregnated with ice, called rock glaciers. There are always moraines-lateral, medial, end, terminal, ground, and recessional. Many other depositional features may occur, including kames, glacial drift (stratified and unstratified), till, lodgment till, erratics, drop stones, glacial marine sediment, outwash, glacial lake-bed sediment, varves, drumlins, kettle holes, knob and kettle topography, and eskers. Not surprisingly, glaciers greatly modify the climate. Average temperature near a glacier can be up to 13ºC cooler than elsewhere, and strong glacial winds (catabatic winds) pick up and transport and deposit fine-grained sediment called loess, creating immediate dusty conditions and future fertile farmland.
Glaciation leaves its mark on the landscape. Some previously glaciated areas are spectacular enough that they have been made national parks (Glacier in Montana, Yosemite in California, Voyageurs in Minnesota, Acadia in Maine, and Glacier Bay in Alaska). Other notable glacial scenery is found in the rugged mountains of the European Alps, the Rockies of western North America, the Andes of South America, and the fjord country of New Zealand, Alaska, and Scandinavia. The thousands of small lakes of northern Minnesota and the Great Lakes and the Finger Lakes in central New York owe their existence to the Pleistocene ice sheet.
Ice loading causes glacial subsidence, and melting causes glacial rebound. Sea level changes drastically when glaciers tie up great quantities of water, and this affects life in the area. Lowered sea levels in Pleistocene times exposed land bridges that allowed animals (including humans) to migrate extensively. Humans crossed the Bering Strait from Asia to Alaska and eventually spread throughout North America. Sea level is higher now than it was in the Pleistocene, and it will get much higher if all current ice sheets melt. If that happens, coastal areas will be flooded, numerous new land lakes will form, and stream systems will be altered. You read of warming times toward the end of the Pleistocene that resulted in meltwater lakes, pluvial lakes, oversized valleys, and catastrophic floods like the Great Missoula Flood. Even areas around but not under the ice (periglacial areas) showed distinctive features like permafrost, patterned ground, and stone rings.
Roughly the last third of the chapter is devoted to ice ages, particularly the most recent, Pleistocene Ice Age, which began about 3 million years ago and ended about 11,000 years ago. You read about the Laurentide and Cordilleran ice sheets that covered northern North America, and how ice sheets covered roughly 30% of all land and greatly affected life on Earth. Homo sapiens was only one of the life forms that had to cope with the harsh environment.
Why have ice ages happened? Milankovitch's ideas (which involve cyclical changes in Earth's orbit); plate tectonics phenomena such as shifting continents, mountain building, and continental rifting; and changes in the amounts of atmospheric carbon dioxide are all major factors to consider. Will another great ice age come in the near future? How do scientists study these issues? The subject is very complex, and new research has produced more questions than answers about ice age issues. Equally qualified experts predict totally different glacial futures for planet Earth, and neither side can absolutely prove its viewpoint. Only time will tell.
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