Continental glaciers

Continental glaciers once covered much of northern North America and Europe, but now only Greenland and Antarctica have such huge masses of permanent ice and snow. Because continental glaciers are so large, they affect the climate of large regions outside their boundaries by cooling air and water temperatures. Continental glaciers of past ice ages have produced various erosional and depositional features in northern latitudes.

Formation, Structure, and Location

Continental glaciers are ice sheets of huge extent. These continent-sized masses of ice overwhelm nearly all the land surface at their margins. Modern continental ice sheets occur only in Greenland and Antarctica and account for nearly 96 percent of all glacier ice on the Earth and 68 percent of the freshwater. From about 2.58 million and 11,700 years ago, during the Pleistocene glacial period of the world's geological history, continental glaciers spread over much of northern North America and Europe. The most recent glacial advance, called the Last Glacial Maximum (LGM), occurred during this period, forming North America’s Great Lakes. Since the latest glacial period, Earth has been in a period called the Holocene, or interglacial period.

Continental ice sheets tend to create their own weather that is naturally favorable for glaciers. A huge ice sheet can even have worldwide climatic effects. The greatest part of the world's ice in Greenland and the Antarctic occurs in high latitudes characterized by very low winter temperatures, low summer temperatures, small annual precipitation, and minimal ablation. It does not snow much, but it is so cold that what falls tends to remain for a very long time. As a result of all these factors, such ice masses are relatively inactive and stable. Greater movement activity is noticeable in areas of less extreme cold and moderate precipitation.

Large ice sheets are complex and consist of several domes from which the ice flows radially to the ice margin or to broad interdome saddles, where the ice flow diverges downslope. The location of ice domes and ice saddles determines the flow path of ice on an ice sheet, but such features can change location over time as an ice sheet grows or shrinks in size. Continental glaciers average two kilometers thick. Ice does not have the strength to support the weight of an appreciably thicker accumulation. If more ice is added by increased precipitation, the glacier simply flows out from the domal centers of accumulation more swiftly. Also, as the pressure at the base increases sufficiently, basal melting occurs, further decreasing ice thickness.

Although snow accumulates over much of a continental ice sheet and gradually transforms into granular firn (snow that has survived past warming seasons and recrystallized) and finally to glacial ice, it is from the highest interior domal areas that the main ice streams flow. The ice surface is built up at the interior and slopes outward on all sides. The glacier moves down and out in all directions. At the edge of the glacier, if the area is mountainous, such ice sheets will break up into narrow tongues resembling valley glaciers that wind through the mountains to the sea. Otherwise, the ice may end in giant ice ramparts that calve, or break off, icebergs into the ocean, or as floating ice shelves over large continental embayments.

Ice shelves occur at several places along the margins of the Greenland and Antarctic ice sheets, as well as locally in the Canadian Arctic islands. They are nourished by ice streams flowing off the land, as well as by direct snowfall on their surface, and perhaps by freezing of the sea ice on their undersides as well. The largest ice shelves extend hundreds of kilometers seaward from the coastline and can reach a thickness of 3,000 meters. The Ross Ice Shelf in Antarctica, for example, is about as large as the state of Texas.

The continental ice sheet of Greenland covers an area of about 1.8 million cubic kilometers or about 80 percent of the country's land area. The volume of the ice is about 2.8 million cubic kilometers. In cross section, the ice has the shape of an extremely wide lens, convex on the smooth upper surface and the rough lower boundary with the ground. The center of the ice sheet is more than 3,200 meters thick. The greatest area, 18 percent, occurs between 2,440 and 2,740 meters, and 6.5 percent lies between 3,050 and 3,390 meters. The equilibrium line, or boundary between the upper accumulation area and the lower wastage area, is at about 1,400 meters, and 83 percent of the total area lies in the accumulation zone. Measurements of ice velocity in Greenland show that the main ice cap advances at approximately ten to thirty centimeters per day, but the outlet glaciers near the coast can move as fast as one meter per hour. In some places, the ice can actually be seen to move.

The continental glaciers that once covered much of North America, Europe, and elsewhere during the Pleistocene ice age rivaled Antarctica in size and also exerted a widespread effect when they were at their maximum extent. The Laurentide ice sheet of North America, at its largest, covered an area similar in size to the present Antarctic ice sheet, but the Scandinavian ice sheet of Europe covered only about one-half of this area during the maximum of glaciation.

The Laurentide and Scandinavian ice sheets did not extend to the seas in their southern limits as do the ice caps of Greenland and Antarctica. Instead, these continental glacial systems covered a large part of the northern continents and caused several significant peripheral changes of the regional physical setting outside their limits. The weight of the ice depressed the ground surface isostatically (in a process called isostatic adjustment whereby the crust of the Earth maintained equilibrium by subsiding when loaded and uplifting when unloaded), just as it does in Greenland and Antarctica, so the land sloped toward the glacier. Consequently, glacial lakes formed in the depressions along the ice margins, or arms of the ocean invaded the depressions. The preglacial drainage systems were greatly modified, as the streams that flowed toward the ice margins were impounded to form lakes. Later, as the ice dams melted away and the land again rose isostatically after the weight of the ice was removed, many such lakes and arms of the sea drained away, leaving behind extensive lake and marine clays and silts.

Landforms

Glaciers produce many different erosional and depositional features due to their interaction with the ground beneath or at the front of the ice. During the Pleistocene ice age, the great thick continental ice sheets moved over the flat, low-lying areas of the northlands, removing the existing soils and eroding up to several meters into the bedrock. As a result, many thousands of square kilometers of northern North America and Europe have little or no soil cover, and the effects of the former glaciation are seen everywhere in the polished and grooved fresh bedrock. Continental glaciers are so large they can produce widespread or massive abrasion and streamlined forms, and erode huge lake basins, which are exposed after the ice melts away. Large parts of the north-central United States and much of central and eastern Canada have such landforms plentifully displayed.

In many areas near the outer edges of former continental ice sheets, the land surface has been molded into smooth, nearly parallel ridges that range up to many kilometers in length. These forms resemble the streamlined bodies of supersonic aircraft and offer minimum resistance to glacier ice flowing over and around them. The most common is the drumlin, which is a smooth, stream-lined hill or ridge consisting of glacially deposited sediment that is elongated parallel with the direction of ice flow. Some drumlins are composed of contemporaneously deposited and smoothed sediment; others are of older sediment that was eroded long after its initial deposition. Bedrock can also be eroded in this fashion. In some places with steeply rising mountains that were overridden by continental ice, the up-ice side of such mountains will be streamlined and smoothed by the ice abrasion, whereas the down-ice side will be plucked and quarried into a rough and jagged cliff. Such asymmetrical mountains are called flyggberg (“flying mountains”) topography. Smaller such forms a few meters in height are referred to as roches moutonnées, or “wig-shaped” rocks, after their fancied resemblance to the curls of the smooth and powdered periwigs of the eighteenth century.

Continental glaciers erode by the incorporation of blocks of rocks in the base of the ice and by abrasion with these blocks against the bedrock farther along in the ice stream. The process produces large quantities of sedimentary debris. Sediments deposited by continental glaciers can be more than 300 meters thick so they blanket most of the preglacial topography upon which they rest, thus modifying, disrupting, and obliterating previously established drainage systems.

Most of the rock debris that is transported by glaciers is deposited near the terminus, where melting dominates. The material accumulates as a moraine ridge marking the former front edge of the glacier. As a glacier retreats from an area by backwasting, it may deposit a series of recessional moraines in loops or ridges one behind the other. The sediment of such recessional and terminal moraines is made up of a jumbled mixture of all sorts of rock materials, ranging from clay to boulders, collectively called till.

As continental glaciers develop large quantities of meltwater from wastage of the ice, streams of meltwater begin to flow in tunnels within and beneath stagnant ice and carry a large load of sediment in their ice-walled beds. When the ice melts away, such bed loads can be deposited beneath the ice to form a long, sinuous ridge called an esker.

The plentiful meltwater at the terminus of a continental ice sheet will also flow through the terminal moraine landforms and erode away much of the till. This debris will be transported and reworked by the meltwater before being sorted into different sizes and deposited by rivers beyond the ice margins. The resulting layers or strata of sorted sediment can be spread out in broad outwash plains pockmarked with kettle holes where blocks of ice have later melted away. Both the unsorted, unstratified, or unlayered till and the sorted and stratified outwash materials are collectively called drift, the name being a heritage of the time when such materials were thought to have drifted to their present locations during Noah's flood.

Principal Terms

ablation: the result of processes, primarily melting (evaporation is also involved), that remove ice and snow from a glacier

equilibrium line: the line or zone that divides a glacier into the upper zone of accumulation and the lower zone of wastage

isostatic adjustment: the adjustment of the crust of the Earth to maintain equilibrium by subsiding when loaded and uplifting when unloaded

Pleistocene ice age: the time from about two million years ago to about 10,000 years ago, during which large continental glaciers covered much of northern North America, Europe, and other parts of the world

pressure melting temperature: the temperature at which ice will melt under a specified pressure; under pressure, water can exist even at temperatures below freezing

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