365 Days of Climate Awareness 55 - Continental Glaciers (Ice Sheets)

Continental glaciers, or ice sheets, are thick layers of ice which cover more than 50,000 km² (9,000 mi², or 12.3M acres, or 19 Rhode Islands). They occur only where the climate is consistently below freezing, as they take thousands of years to accumulate. Currently the only two cover Antarctica and Greenland. (During the last glaciation, which ended roughly 12,000 years ago, three massive ice sheets covered a large portion of the globe: the Laurentide in North America, the Weichselian in Eurasia, and the Patagonian in southern South America, in addition to the Antarctic and Greenland sheets.)

Antarctic ice sheet.

The mid-range size of ice forms, between individual glaciers and ice sheets, are ice caps. Both large forms--caps and sheets--feed individual glaciers, or ice streams, at their margins. Like liquid water, the ice tends to flow according to gravity, following the maximum slope down to sea level. Being several kilometers thick in places--the Antarctic sheet averages about 2.1 km thick--continental glaciers might cover mountains or entire ranges. A peak sticking through the ice cover is known as a nunatak.

Janke Nunatak, Antarctica (elevation above sea level: 1308 m).

Ice transmits heat, though slowly. Once frozen, the ice can continue to drop in temperature. But heat transfer occurs extremely slowly in ice, taking 10,000 years or even more for a temperature change to penetrate to the ice's full depth. For this reason ice serves as an insulator for the ground, and geothermal heating becomes important. Geothermal heat trapped at the base of a glacier can lead to basal melting, with the resulting water building up and flowing when pressure is sufficient. Where the base of the ice is wet, flow rates of the ice increase tenfold or more.

Antarctic ice sheet flow rates.

Ice sheets terminate either on land, where the margin might (though not always) pile up an end moraine, or in the water. If the glacier extends far enough out past sea level that it begins to float, that portion is known as an ice shelf. Glaciers terminating in ocean water periodically calve icebergs. Less well known is that ocean tides will also lift the glaciers, and lead to temporary increases in the flow rate of ice!

Laurentide ice sheet, maximum extent (~20 kya).

Two maps show the nature of the Laurentide ice sheet of the past glaciation, from roughly 100 kya (thousands of years ago) to 12 kya. At its maxiumum extent, the ice sheet covered almost all of Canada and most of the northern United States. The island of Manhattan is the terminal moraine and "outwash plain"--where rivers of meltwater emerging from beneath the glacier deposited sediment in massive deltas--of this sheet. Cape Cod is partly (the purple zone) an end moraine, and mostly outwash plain, composed of sand and gravel. But it was not the terminal moraine. That was farther south, and included the modern-day islands of Martha's Vineyard and Nantucket.

Cape Cod surficial geology. Larger version

Tomorrow: ice shelves.

Be well!