Ablation:

• 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:

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

Accumulation area:

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

Albedo:

• 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.

Cirque lake:

• A small body of water occupying a cirque depression, dammed by a rock lip, small morain, or both. See Tarn".

Crevasses:

• 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:

• 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.

Equilibrium line:

• Equilibrium line is the boundary between the accumulation area and the ablation area.

Firn:

• 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:

• 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:

• 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:

• 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:

• 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:

• 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:

• A 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.

Great Ice Age:

• The Pleistocene Epoch

Hydrothermal alteration:

• 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:

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

Ice sheets:

Jökulhlaup:

• Icelandic term for Glacial outburst floods. Jökulhlaup'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:

• Refers to a wave of ice moving downglacier propagated by its increased thickness. The wave of ice may move at two to six times the velocity of surrounding thinner ice.

Lahar:

• A lahar is a mudflow or debris flow originating on a volcano. Jökulhlaups (see above) often become lahars when they incorporate the rock debris that lies within their path.

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:

• 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:

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

Moberg:

• Moberg is the Icelandic term for palagonite rock (Kjartansson, 1959). (Hammond, 1987)

Moberg hills:

• A moberg hill is a rounded landform composed chiefly of palagonitized hyaloclastite. Hyaloclastite is very fine-grained to coarsely fragmented rock, consisting of a high percentage of glass relative to crystalline rock fragments, generally of basaltic composition, and commonly poorly sorted. It is produced where lava flows or intrudes into water, ice, or water-rich sediment. (Hammond, 1987)

Moraine:

• Rock debris deposited by a glacier.

Neoglaciation:

• 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:

• 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.

Perfectly plastic solid:

• A solid that does not deform until it reaches a critical value of stress, after which it will yield infinitely. Some glaciologists say that ice is a perfectly plastic substance. (That is, brittle and capable of cracking like a solid, yet deformable and capable of flowing at other stresses.)

Pleistocene:

• 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:

• A 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:

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

Striations:

• 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:

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

Tarn:

• A small mountain lake or pool, especially one that occupies an ice-gouged basin on the floor of a cirque.

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:

• The downvalley end of a glacier. It is sometimes referred to as the glacier snout.

Till:

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

Trimlines:

• 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.

Tuya:

• A volcano that erupted under a glacier. (British Columbia Ministry of Environment)
• A tuya is a volcano that erupts initially beneath a glacier, melts through the ice, and develops an upper, subaerial part, which commonly consists of a flat-topped form capped by a lava flow. (Hammond, 1987)

• British Columbia Ministry of Environment, Lands & Parks Website, 2000
• Crandell, 1980, Recent Eruptive History of Mount Hood, Oregon, and Potential Hazards from Future Eruptions: USGS Professional Paper 1492
• Dolphin Reference Bood, Dictionary of Geologic Terms, 1962, Dolphin Book Publishers, N.Y., 545 p.
• Driedger, 1986, A Visitor's Guide to Mount Rainier Glaciers: Pacific Northwest National Parks and Forests Association
• Gardner, et.al., 1995, Potential Volcanic Hazards from Future Activity of Mount Baker, Washington: USGS Open-File Report 95-498
• Hammond, P.E., 1987, Lone Butte and Crazy Hills: Subglacial volcanic complexes, Cascade Range, Washington: IN: Hill, M.L., (ed.), 1987, Geological Society of America Centennial Field Guide Volume 1, Cordilleran Section of the Geological Society of America.
• Walder and Driedger, 1993, Glacier-generated debris flows at Mount Rainier: USGS Open-File Report 93-124

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