Glossary of Selected Glacier Terminology

Ablation: refers to all processes by which snow, ice, or water in any form are lost from a glacier - the loss of snow or ice by evaporation and melting.

Ablation area: is the lower region of a glacier where snow ablation exceeds snowfall.

Accumulation area: is the upper region of a glacier where snow accumulation exceeds melting.

Albedo: is the percentage of the incoming radiation that is reflected off a surface. An albedo of one indicates that 100 percent of the radiation is reflected. Fresh snow has a high albedo (0.7 to 0.9), indicating that 70 to 90 percent of the radiation received is reflected; glacier ice has a lower albedo of 0.2 to 0.4.

Cirque: a glacially eroded basin shaped like half a bowl; a deep, steep-walled recess in a mountain, caused by glacial erosion.

Crevasses: are open fissures in glacier ice. Crevasses form where the speed of the ice is variable, such as in icefalls and at valley bends.

Density: is the ratio of the mass of an object to its volume. Snow has a density averaging about 0.1, firn has a density of about 0.55, and glacier ice has a density of about 0.89. The density of unmineralized fresh water is 1.

Firn: is old snow that has been recrystalized into a more dense substance. Snowflakes are compressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer's end the result is firn -- a compacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all but the most dedicated snow shovelers! Firn has a density greater than 0.55.

Glacial advance: is the net movement of glacier terminus downvalley. Advance occurs when the rate of glacier flow downvalley is greater than its rate of ablation. Advances are characterized by a convex-shaped terminus.

Glacial drift: is the loose and unsorted rock debris distributed by glaciers and glacial meltwaters. Rocks may be dropped in place by the melting ice; they may be rolled to the ice margins, or they may be deposited by meltwater streams. Collectively, these deposits are called glacial drift. Till refers to the debris deposited directly by the glacier. Rock debris rolls off the glacier edges and builds piles of loose unconsolidated rocks called glacier moraine. Lateral moraines form along the side of a glacier and curl into a terminal moraine.

Glacial flour: is the fine-grained sediment carried by glacial rivers that results from the abrasion of rock at the glacier bed. Its presence turns lake water aqua blue or brown, depending on its parent rock type.

Glacial polish: is the leveling and smoothing of rock by fine-grained debris at the glacier bed. Coarser rocks may gouge scratches called striations.

Glacial retreat: is the net movement of the glacier terminus upvalley. Retreat results when the glacier is ablating at a rate faster than its movement downvalley. Retreating termini are usually concave in shape.

Glacial till: An unsorted, unstratified mixture of fine and coarse rock debris deposited by a glacier. Also called: Till.

Glacier: is a body of ice showing evidence of movement as reported by the presence of ice flowline, crevasses, and recent geologic evidence. Glaciers exist where, over a period of years, snow remains after summer's end.

Glacier outburst flood: A sudden release of melt water from a glacier or glacier-dammed lake sometimes resulting in a catastrophic flood, formed by melting of a channel or by subglacial volcanic activity.

Hydrothermal alteration: is the alteration of rocks or minerals due to the reactions of geothermally heated water with minerals. The process weathers and weakens the rocks such that they may become unstable.

Icefalls: are somewhat analogous to waterfalls in rivers. The flow of the ice down a steep gradient often results in crevasses and seracs.

Jokulhlaup: Icelandic term for Glacial outburst floods. Jokulhlaup's are sudden outbursts of water released by a glacier. The water may be released from glacier cavities, sub-glacial lakes, and from glacier-dammed lakes in side valleys.

Kinematic waves: refer to a wave of ice moving down glacier propagated by its increased thickness. The wave of ice may move at two to six times the velocity of surrounding thinner ice.

Lateral moraine: A moraine formed at the side of a glacier. Piles of loose unsorted rocks along the side margins of the glacier. The rocks may be pushed there by the moving ice or dumped from the glacier's rounded surface.

Mass balance: describes the net gain or loss of snow and ice through a given year. It is usually expressed in terms of water gain or loss.

Medial moraines: form where two mountain glaciers bearing lateral moraines unite. They appear as dark streaks of rock along the glacier centerline.

Moraine: Rock debris deposited by a glacier.

Neoglaciation: refers to the advances made by mountain glaciers since the great Pleistocene ice age. In the Cascades the advances have occurred since 6,600 years before present.

Ogives: are arc-shaped features occasionally found across the glacier surface below icefalls. They may be ridges and swales in the ice or bands of darker or lighter ice. One theory of their formation suggests that the ice is stretched and sometimes dirtied when exposed in the icefall during the high velocities of summer; it is compressed during the winter so that bands of different ice thickness form.

Pleistocene: is the period of earth's history, roughly two million years ago to about ten thousand years ago, characterized by the advance and recession of continental ice sheets.

Roche moutonnee: is a small asymetrically-shaped hill formed by glacial erosion. The upper sides are rounded and smoothed and the lower sides are rough and broken due to quarrying by the glacier.

Seracs: are the pinnacles of ice formed where the glacier surface is torn by sets of crevasses.

Striations: are the scratches etched into the rock at the bed of a glacier. Their presence indicates grinding of sand and rock particles into the bed under considerable pressure. In some places find-grained debris polishes the bedrock to a lustrous surface finish called glacial polish.

Suncup: is a small depression on a snow or firn surface formed by melting and evaporation resulting from direct exposure to the sun.

Terminal moraine: A moraine formed at the downvalley end of a glacier. Piles of loose unconsolidated rock at the glacier's downvalley end. The rocks may be pushed there by the forward motion of the glacier or dumped from the glacier's rounded surface.

Terminus: is the downvalley end of a glacier. It is sometimes referred to as the glacier snout.

Till: is the unsorted rock debris deposited directly by the glacier without the extreme reworking by meltwater. Also called: Glacial till.

Trimlines: are the sharp vegetative boundaries delimiting the upper margin of a former glaciation. The age differences of the ground surface are often visible because of different ages of the vegetation.