14.1: Glacier Formation

Glaciers form when repeated annual snowfall accumulates deep layers of snow that are not completely melted in the summer. Thus there is an accumulation of snow that builds up into deep layers. Perennial snow is a snow accumulation that lasts all year. A thin accumulation of perennial snow is a snow field. Over repeated seasons of perennial snow, the snow settles, compacts, and bonds with underlying layers. The amount of void space between the snow grains diminishes. As the old snow gets buried by more new snow, the older snow layers compact into firn, or névé, a granular mass of ice crystals. As the firn continues to be buried, it compresses and recrystallizes, the void spaces become smaller, and the ice becomes less porous, eventually turning into glacial ice. Solid glacial ice still retains a fair amount of void space that traps air. These small air pockets provide records of the past atmospheric composition.

There are three general types of glaciers: alpine or valley glaciers, ice sheets and ice caps. Most alpine glaciers are located in the world’s major mountain ranges such as the Andes, Rockies, Alps, and Himalayas, usually occupying long, narrow valleys. Alpine glaciers may also form at lower elevations in areas that receive high annual precipitation such as the Olympic Peninsula in Washington state.

Figure \(\PageIndex{1}\): Greenland ice sheet.

Ice sheets (also called continental glaciers) form across millions of square kilometers of land and are thousands of meters thick. These are thick accumulations of ice that occupy a large geographical area. Earth's largest ice sheets are located on Greenland and Antarctica. The Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere with an extensive surface area over 2 million square kilometers (1,242,700 square miles) and average thickness up to 1500 meters (5000 feet or almost a mile) and has a volume estimated at nearly 3 million cubic kilometers (~102 billion cubic feet) [1].

Figure \(\PageIndex{2}\): Thickness of Greenland ice sheet in meters (Source: Eric Gaba).

The Antarctic Ice Sheet is much larger and covers almost the entire continent. The thickest parts of the ice sheet are over 4,000 meters thick (>13,000 feet or 2.5 miles) and its weight depresses the Antarctic bedrock to below sea level in many places beneath the ice [2]. The Antarctic Ice Sheet contains the most ice as illustrated by the figure below comparing cross-sectional views of both ice sheets.

Figure \(\PageIndex{2}\): Cross-sectional view of both Greenland and Antarctic ice sheets drawn to scale for size comparison (Source: Steve Earle)

Ice cap glaciers are smaller versions of ice sheets that cover less than 50,000 km², usually occupy higher elevations and may cover tops of mountains. There are several ice caps on Iceland. A small ice cap called Snow Dome is near Mt. Olympus on the Olympic Peninsula in the state of Washington.

Figure \(\PageIndex{3}\): Left: Snow Dome ice cap near Mt. Olympus, Washington. (By NPS; public domain via Wikimedia Commons.) Right: Vatnajokull ice cap in Iceland. (By NASA; public domain via Wikimedia Commons.)

References

1. Bamber, J. L., Layberry, R. L. & Gogineni, S. P. A new ice thickness and bed data set for the Greenland ice sheet: 1. Measurement, data reduction, and errors. J. Geophys. Res. 106, 33773–33780 (2001).

2. Lythe, M. B. & Vaughan, D. G. BEDMAP: A new ice thickness and subglacial topographic model of Antarctica. J. Geophys. Res. 106, 11335–11351 (2001).