Consequences of Continental Glaciation
Ice Loading and Glacial Rebound
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| The concept of subsidence and rebound, due to continental glaciation and deglaciation. (Not to scale.) |
When a large ice sheet (more than 50 km in diameter) grows on a continent, its weight causes the surface of the lithosphere to sink. In other words, ice loading causes glacial subsidence. Lithosphere, the relatively rigid outer shell of the Earth, can sink because the underlying asthenosphere is soft enough to flow slowly out of the way (figure above). Because of ice loading, much of Antarctica and Greenland now lie below sea level, so if their ice were instantly to melt away, these continents would be flooded by a shallow sea.
What happens when continental ice sheets do melt away? Gradually, the surface of the underlying continent rises back up, by a process called glacial rebound, and the asthenosphere flows back underneath to fill the space. This process doesn't take place instantly, the asthenosphere flows so slowly (at rates of a few millimetres per year) that it takes thousands of years for ice-depressed continents to rebound. Thus, glacial rebound is still taking place in some regions that were covered by ice during the Pleistocene Ice Age.
Sea-Level Changes: The Glacial Reservoir
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| The link between sea level and global glaciation: glaciers store water so when glaciers grow, sea level falls, and when glaciers melt, sea level rises. |
More of the Earth’s surface and near-surface freshwater resides in glacial ice than in any other reservoir. During the Pleistocene Ice Age, glaciers covered almost three times as much land area so they held significantly more water than they do today. In effect, water from the ocean reservoir transferred to the glacial reservoir and remained trapped on land. As a consequence, sea level dropped by as much as 100 m, and extensive areas of continental shelves became exposed as the coastline migrated seaward (figure above a–c). People and animals populated the exposed coastal plains. The drop in sea level also created land bridges across the Bering Strait between North America and northeastern Asia, providing convenient migration routes for prehistoric humans.
Ice Dams, Drainage Reversals, and Lakes
When ice freezes over a sewer opening in a street, neither meltwater nor rain can enter the drain, and the street floods. Ice sheets play a similar role in glaciated regions. The ice may block the course of a river, leading to the formation of a lake. In addition, the weight of a glacier changes the tilt of the land surface and therefore the gradients of streams, and glacial sediment may fill pre-existing valleys. In sum, continental glaciation modifies or destroys pre-existing drainage networks. While the glacier exists, streams find different routes and carve out new valleys; by the time the glacier melts away, these new streams have become so well established that old river courses may remain abandoned.
Meltwater Floods
Subsidence of the land surface at the toe of a glacier locally led to the growth of large ice-margin lakes. Inevitably, the ice dams that held back these lakes melted and broke. In a matter of hours to days, the contents of the lakes drained, creating immense flood-waters that stripped the land of soil and left behind huge ripple marks. For example, glacial Lake Missoula, in Montana, filled when glaciers advanced and blocked the outlet of a large valley. When the glaciers retreated, the ice dam broke, releasing immense torrents the Great Missoula Flood that scoured eastern Washington, creating a barren, soil-free landscape called the channelled scablands.
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| Ice-age lakes in North America. |
The largest known ice-margin lake covered portions of Manitoba and Ontario, in south-central Canada, and North Dakota and Minnesota in the United States (figure above a). This body of water, Glacial Lake Agassiz, existed between 11,700 and 9,000 years ago, a time during which the most recent phase of the last ice age came to a close and the continental glacier retreated north. At its largest, the lake covered over 250,000 square km (100,000 square miles), an area greater than that of all the present Great Lakes combined. The sudden release of water from Lake Agassiz may have led to a sea-level rise of 1 to 3 m during a single year.
Pluvial Features
During the Pleistocene Ice Age, the climate in regions to the south of continental glaciers was wetter than it is today. Fed by enhanced rainfall, lakes accumulated in low-lying land at a great distance from the ice front. Many such pluvial lakes (from the Latin pluvia, rain) flooded interior basins of the Basin and Range Province in Utah and Nevada (figure above b). The largest pluvial lake, Lake Bonneville, covered almost a third of western Utah. When this lake suddenly drained after a natural dam holding it back broke, it left a bathtub ring of shoreline rimming the mountains near Salt Lake City. Today’s Great Salt Lake is but a small remnant of Lake Bonneville.
Periglacial Environments
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| Periglacial regions are not ice covered but do include substantial areas of permafrost. |
In polar latitudes today, and in regions adjacent to the fronts of continental glaciers during the last ice age, the mean annual temperature stays low enough (below 5C) that soil moisture and groundwater freeze and, except in the upper few meters, stay solid all year. Such permanently frozen ground is called permafrost. Regimes with widespread permafrost that do not have a cover of snow or ice are called periglacial environments (the Greek peri means around, or encircling; periglacial environments appear around the edges of glacial environments; figure above a).
The upper few meters of permafrost may melt during the summer months, only to refreeze again when winter comes. As a consequence of the freeze-thaw process, the ground of some permafrost areas splits into pentagonal or hexagonal shapes, creating a landscape called patterned ground (figure above b).
Permafrost presents a unique challenge to people who live in polar regions or who work to extract resources from these regions. For example, heat from a building may warm and melt underlying permafrost, creating a mire into which the building settles. For this reason, buildings in permafrost regions must be placed on stilts, so that cold air can circulate beneath them to keep the ground frozen.
Credits: Stephen Marshak (Essentials of Geology)
