Glaciers

Moreno Glacier, Argentina. Courtesy of Richard S. Williams, Jr.

A glacier is an accumulation of ice and snow, originating on land, that moves under its own weight in response to gravitational force; it transports ice from an area of accumulation to an area of disposal (Sharp, 1988). Precipitation in the form of snow adds mass to the system while mass is removed through various ablation processes including surface melting, sublimation and calving of icebergs. The motion of the glacier redistributes the mass inputs and outputs in an attempt to reach an equilibrium state (Hall and Martinec, 1985).

MODIS false color image of Greenland, 17 June 2003.

Most mountain glaciers have been retreating since the latter part of the 19th century, and sea level has risen. The following website provides some example images of receding glaciers over time, World View of Global Warming: Glaciers. The Satellite Image Atlas of Glaciers of the World is a comprehensive global baseline study of the total area of glacier ice on Earth. This effort is key to analysis of future changes in glaciers because changes cannot be assessed without baseline information on glacier extent.

Glaciers can be classified according to their size and underlying topography. Various types of glaciers exist: ice sheets, ice shelves, ice caps, outlet glaciers, piedmont glaciers, and valley glaciers. Ice sheets exist only in Antarctica and Greenland and are nearly continuous masses of ice. Ice shelves represent the ungrounded or floating part of an ice sheet that extends into the ocean. Ice caps, far more numerous than ice sheets, are dome-shaped glaciers usually covering a highland area (see, for example, Williams, 1983). Outlet glaciers flow outward from ice sheets and ice caps through valleys with distinct boundaries consisting of moraines and mountains. A piedmont glacier terminates onto flat land and its terminus spreads out into a broad lobate sheet (Sharp, 1988). A valley glacier is a stream of ice flowing down a valley from an accumulation area.

Landsat multispectral scanner (MSS) image of part of the Alaska Range, Alaska; showing glaciers flowing from Mt. McKinley - August 24, 1979.

The temperature at which a glacier melts is a function of the pressure and the overlying weight of the glacier. A glacier may be classified as cold (polar) or warm (temperate). A cold glacier has a temperature below the melting point of ice from top to bottom. Such glaciers get warmer with depth. A warm glacier is at the pressure-melting temperature throughout. Glacier ice at the melting point is in equilibrium with liquid water, so water can exist throughout warm glaciers all the way to the base.

Most glaciers move slowly, a few centimeters to less than a meter a day. However, some glaciers move very rapidly. For example, surge-type and tidewater glaciers can move very rapidly for short periods of time. A surge-type glacier experiences a relatively short-lived episode of greatly-accelerated flow followed by a relatively longer period of stagnation and retreat. Velocities of surge-type glaciers as high as 105 m per day have been recorded (Paterson, 1994). Tidewater glaciers go through a cycle (Meier and Post, 1987): the glacier will occupy a stable position with the terminus at the head of a fjord, experience a slow advance that may last 1000 years, remain relatively stable and then experience a rapid retreat that may last less than a century. This cycle is not directly related to short-term climate change. An excellent example of tidewater glacier retreat is the Muir Glacier in Glacier Bay, Alaska.

The late glaciologist William O. Field.

References

Hall, D.K. and J. Martinec, 1985: Remote Sensing of Ice and Snow, Chapman & Hall, London, 189 p.

Meier, M. and A. Post, 1987: "Fast Tidewater Glaciers," Journal of Geophysical Research, 92:9051-9058.

Paterson W.S.B., 1994: The Physics of Glaciers, 3rd Edition, Pergamon Press, Oxford, England, 480 p.

Sharp, R.P., 1988: Living Ice, Cambridge University press, 225 p.

Williams, R.S., Jr., 1983: Ch. 31: Geological applications (Williams, R.S., Jr., author-editor), Manual of Remote Sensing, 2nd Edition (Colwell, R.N., editor-in-chief), Falls Church, VA, American Society of Photogrammetry, pp. 1667-1953.