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Montana University Glacial Geology Glossary
Category: Earth and Environment > Glacial Geology
Date & country: 13/09/2007, USA
Words: 90


Ablation
All processes by which snow and ice are lost from a glacier, floating ice, or snow cover; or the amount which is melted. These processes include melting, evaporation, (sublimation), wind erosion, and calving. Synonym wastage.

Abrasion
The mechanical wearing or grinding away of rock surfaces by the friction and impact of rock particles transported by wind, ice, waves, running water, or gravity.

Accumulation
All processes that add snow or ice to a glacier or to floating ice or snow cover snow fall, avalanching, wind transport, refreezing

Albedo
The percent of the incoming radiation that is reflected by a natural surface such as the ground, ice, snow, water. Atmospheric albedo includes clouds and particulates in the atmosphere. Synonym reflectivity

Alpine glacier
Any glacier in a mountain range which is dominantly confined by the surrounding topography. It usually originates in a cirque and may flow down into a valley previously carved by a stream. Synonym mountain glacier.

Braided Stream
A stream that becomes a maze of interconnected channels with excess sediment. See Outwash Plain and Sandur.

Calving
Breaking off and floating away as icebergs of either a tidewater glacier or an ice shelf. Calving is a very efficient form of ablation, thus helps stabilize the extent of ice sheets (like Antarctica) which might otherwise expand continuously from a positive mass budget.

Cavity
A hole or opening, as at the bed of a glacier. When the rate of deformation into a space behind an obstacle is less the rate of movement past the obstacle, a cavity will form.

Chattermark
A small, curved scar made by vibratory chipping of a bedrock surface by rock fragments carried in the base of a glacier. Each mark is roughly transverse to the direction of flow, and either convex ('lunate') or concave ('crescentic') toward the direction from which the ice moved.

Cleavage
1.The breaking of a mineral along its crystallographic planes, thus reflecting crystal structure. 2.The property or tendency of a rock to split along parallel, closely spaced planar surfaces.

Continentality
The distance between a site and open ocean water. Continental interiors (obviously!) and areas surrounded by nearly permanent, continuous sea ice are highly continental.

Crevasse
A crack in a glacier caused by rapid extension. Crevasses over 10 m deep would be healed by internal flow, but much deeper crevasses can be maintained by continued tension.

deposits.Maritime Glacier
A glacier in close proximity to open ocean water, thus dominated by high accumulation and ablation.

Drift
An archaic term for heterogeneous sediment (presumed to be deposited by drifting icebergs, perhaps in Noah's flood!). Includes and retained in stratified drift, but not in till.

Eccentricity
The degree to which the Earth's orbit around the sun varies from a perfect circle - it ranges between about 1% and 5% across a 100,000 year cycle.

Energy
Ability to do work. Most evident in glacial systems as radiant energy from the sun and as latent energy required to melt ice to water.

Esker
A sinuously curving, narrow deposit of coarse gravel that forms along a meltwater stream channel, developing in a tunnel within or beneath the glacier. The ice-contact margins of the esker are often slumped and mixed with till.

Feedback Loop
Reinforcement which either accelerates (positive feedback) or retards (negative feedback) a process. An example of positive feedback would be the accumulation of glacial snow and ice increasing the albedo of the surrounding region, thus cooling the air, thus accelerating glacier growth. An example of negative feedback would be mountain glacier retreat, eliminating low-elevation (ablation) area, thus reducing the rate of retreat.

Firn
A transition form between snow and glacial ice resulting from a summer's consolidation, metamorphosis, and melt/refreeze. Densities commonly between 400 and 830 kg·m-3.

Forebulge
An area farther from the ice than the peripheral depression which is bowed slightly upward by the lateral displacement of deformable material in the asthenosphere, beneath the rigid lithosphere. During deglaciation the forebulge will migrate in towards the glaciated region as the asthenosphere reacts to the removal of load.

Frost Action
The mechanical weathering process caused by repeated freezing and thawing of water in pores, cracks, and other openings, usually at the surface.

Glacial Drift
The general term for all glacial deposits, both unsorted and sorted (see Stratified Drift).

Glacial Ice
Compacted and intergrown mass of crystalline ice with a density of 830-910 kg·m-3.

Glacial milk
Term used to describe a sediment laden glacial stream. The stream described is usually laden with silt particles that are a result of glacial abrasion.

Glaciation
A long period of time (10,000+ years) characterized by climatic conditions associated with maximum glacial extent. Compare to 'interglaciation' and 'stade'. Also used to refer to covering of an area by ice see 'glacierize'.

Glacierize (glacierization)
A somewhat awkward term to explicitly denote covering an area with ice, as opposed to 'glaciate' (glaciation), which can include time, space, and climate.

Greenhouse Effect
Warming of global climate by retention of outgoing (long wavelength) radiation - inferred to be happening at present because of increasing atmospheric carbon dioxide content (CO2) driven by combustion of fossil fuels.

Grounding line
The point at which a tidewater glacier, or ice stream feeding an ice shelf, floats free of its bed. Inland from this point it acts on its bed, to seaward it floats, thus may accelerate, thin, and calve (if not restrained).

Hydroisostasy
Just as the loading and unloading of the crust by ice will cause isostatic depression and uplift, loading and unloading of ocean basins deforms the crust. Because the local magnitude of sea level change (<200 m) is much less than that by ice (>5000 m), hydroisostasy is a very subtle deformation.

Ice Cap
A dome-shaped cover of perennial ice and snow, covering the summit area of a mountain mass so that no peaks emerge through it, or covering a flat landmass such as an arctic island; spreading outwards in all directions due to its own weight; and having an area of less than 50,000 square kilometers.

Ice fall
A region on a glacier where rapid extension (as down steep slopes) causes brittle failure and intense crevassing.

Ice Field
An extensive area of interconnected glaciers in a mountain region, or of pack ice at sea.

Ice Sheet
A glacier of considerable thickness and more than 50,000 square kilometers in area, forming a continuous cover of snow and ice over a land surface, spreading outward in all directions and not confined by the underlying topography. Ice sheets are now confined to polar regions (as on Greenland and Antarctica), but during the Pleistocene Epoch they covered large parts of North America and northern Europe.

Ice Shelf
A continuous plate of floating ice which often extends seaward from a glacier or ice sheet on the shore.

Ice stream
A zone of high velocity within an ice cap or ice sheet. By analogy with flood waters, which have high velocities within their channels but low velocities in the flood plain, ice streams are often (but not necessarily) associated with a subglacial trough.

Insolation
INcoming SOLAr radiaTION. Short wavelength radiation - a major component of a glacier's energy balance.

Interglaciation
A long period of time (10,000+ years) characterized by climatic conditions associated with minimum glacial extent. Compare to 'glaciation' and 'interstade'.

Interstade
A short period of time (less than 10,000 years) characterized by climatic conditions associated with minimum glacial extent. Compare to 'interglaciation' and 'stade'.

Isostasy
(Greek iso-, 'same') Isostasy is the concept that the elevation of the Earth's surface (over tens of millions of years) seeks a balance between the weight of lithospheric rocks and the buoyancy of asthenospheric 'fluid' (nearly-molten rock). Gentle regional movement of the lithosphere occurs in response to short-term (thousands to millions of years) loading and unloading, as by ice, erosion and sediment deposition.

Joint
A fracture of rock without displacement (displacement defines faulting). Jointing of bedrock by pressure release, thermal stress, frost action, and chemical weathering between glaciations allows rapid, effective erosion during glaciations.

Jokulhlaup
(Icelandic) Outburst flooding from a glacial ice dam breakage or intense melt, as by volcanic activity.

Kame
A deposit, composed largely of material sorted by moving water, formed in direct contact with glacier ice. See esker, moulin kame, kame delta, and kame terrace.

Kame Delta
A deposit, often triangular, formed where a glacial stream entered into a proglacial lake. The ice-contact margin of the kame delta is often slumped and mixed with till.

Kame Terrace
A deposit, often sloping down-valley more steeply than the valley floor, formed where a glacial stream ran along the glacier margin. The ice-contact margin of the kame terrace is often slumped and mixed with till.

Kettle
Forms when an isolated block of ice persists in a ground moraine, and outwash plain, or valley floor after a glacier retreats; as the block melts, it leaves behind a steep-sided hole that is filled with water.

Lake Missoula
A glacial lake in northwest Montana (USA) during Pleistocene times which was formed by an ice dam of the Cordilleran ice sheet; this dam broke periodically, flooding a portion of current-day northern Idaho and Washington (see Jokulhlaup).

Laminar Flow
A type of flow in which the surface, bed, and internal flow vectors are all parallel to one another, thus there is no mixing. Contrasted to turbulent flow.

Latitude
Angular distance of a point on the earth's surface north or south or the equator, measured along a meridian, the equator being latitude 0°, the north pole latitude 90°N, and the south pole latitude 90°S.

Loess
Unconsolidated, wind deposited sediment composed largely of silt-sized quartz particles (0.015-0.05 mm diameter) and showing little or no stratification. It occurs widely in the central USA, northern Europe, Russia, China, and Argentina. In all but China, it is evidently derived largely from reworked glacial outwash

Mass Budget
On an annual basis, the difference between mass gained through accumulation and mass lost by ablation.

Mass Wasting
A general term for the downslope movement of soil and rock material under the direct influence of gravity. The debris removed is not carried within, on, or under another medium. However, mass wasting debris added to a glacier or glacier margin may be incorporated into till.

Moulin
(French, 'mill') A shaft by which supraglacial meltwater enters a glacier to become englacial or subglacial.

Moulin Kame
A deposit formed where a glacial stream falls into the glacier or to its bed, then loses gradient and velocity and drops its load in a pile. The ice-contact margins of the kame are often slumped and mixed with till.

Mountain Glacier
See Alpine glacier.

Nunatak
(Inuktitut) An area that is unglaciated, but surrounded by ice.

Orographic Uplift
Uplift of air masses encountering mountain ranges. As with convective and frontal uplift, causes cooling, thus precipitation. Unlike convective and frontal uplift, it is fixed in space, thus causes areas of high local precipitation, thus glacier growth.

Outwash
Meltwater-deposited sediment, dominantly sand and gravel, showing increasing rounding and sorting into layers with increasing distance from the ice margin. Often silt-rich, which can be reworked by wind to form loess.

Outwash Plain
A plain of glaciofluvial deposits of stratified drift from meltwater-fed, braided, and overloaded streams beyond a glacier`s morainal deposits.

Para
a prefix meaning 'subsidiary' or 'accessory' (e.g., paralegal).

Paraglacial
Refers to glacier-related processes and phenomena such as postglacial alluviation, loess deposition, and pluvial lake evolution.

Peripheral Depression
An area around an ice sheet which, although unglaciated, is pulled downward by the weight of the adjacent ice mass. [Think of a trampoline - the weight of the gymnast pushes the trampoline down, but pulls areas away from the direct weight down as well.]

Pleistocene
The epoch that extended from about 1.8 million years ago to 10,000 years ago on the geologic time scale; when the most recent glaciations occurred.

Plucking
A process of glacial erosion by which blocks of rock are loosened, detached, and borne away from bedrock by the freezing of water in fissures.

Pluvial lakes
(Latin pluves - rain) Lakes formed in closed basins as a result of climates which also encouraged glaciation globally colder and locally wetter.

Polar Climate
A type of climate of latitudes greater than 66°C characterized by temperature of 10°C and below. The two types of polar climates in Koppen's classification are tundra climate and perpetual frost climate (temperature always <0°C).

Polish
An attribute of surface texture of a rock or particle, characterized by high luster and strong reflected light, resulting from abrasion by very fine particles.

Pressure melting point
Increasing pressure with depth forces ice towards its more dense, liquid phase. Thus, the melting point of ice decreases at about 0.7oC per vertical kilometer of ice. If basal ice is at the PMP ('warm-based'), heat cannot escape, thus the presence of meltwater is assured. Also, ice may locally melt in high pressure regions and freeze in low-pressure regions, leading to regelation.

Pro
a prefix meaning 'in front of'.

Proglacial
Refers to the area immediately adjacent to a glacier, often affected by outwash and by ice- or moraine-dammed lakes.

Regelation
Refreezing of meltwater to ice at the bed of a glacier, often associated with the transition from high pressure (forcing melting) to low pressure (allowing refreezing) around a basal obstacle.

Sandur
(Icelandic) Plain, often wider than its length, where sediment, mostly gravel, is deposited from glacial meltwater. C.f. outwash plain, valley train. Icelandic sandurs may largely be deposited by jokulhlaup discharges.

Sea Ice
Ice which covers an ocean or sea; includes mostly continuous pack ice, broken only by narrow open water 'leads' or wider 'polynas', and discrete ice floes.

Snow
Distinct crystals (of many forms) of ice. Commonly accumulates with a density of 50 - 200 kg·m-3, although wind-abraded and -packed snow may have a higher initial density.

Stade
A short period of time (less than 10,000 years) characterized by climatic conditions associated with maximum glacial extent. Compare to 'glaciation' and 'interstade'.

Stratified Drift
Sediments deposited by glacial meltwater that are sorted and layered; a major subdivision of glacial drift that includes river, lake, and marine deposits

Stratosphere
The part of the atmosphere which lies above the troposphere (from 10-15 km to about 50 km up). Significant because fine particles like volcanic ejecta which are injected into the stratosphere tend to remain there for years, thus cooling global climate by raising the atmospheric albedo.

Stream Capacity
The maximum amount of sediment a stream can carry with a given discharge.

Striations
Multiple scratches or minute lines, generally parallel but occasionally cross-cutting, inscribed on a rock surface by a geologic agent. Common indicators of (at least the latest) direction of glacier flow.

Sublimation
The direct change of a material from a solid state to a gas state without turning to liquid in between. Can also occur in reverse, as crystallization from a vapor.

Temperate climate
A climate typical of the mid-latitudes, with neither exceptionally high (tropical) nor low (polar) temperatures and precipitation. May be either wet (maritime) or cold (continental).

Tidewater glacier
A glacier which terminates in ocean water. Flexing by tidal fluctuation may accelerate calving of icebergs, thus stabilize the glacier at or just beyond the grounding line.

Till
(or 'glacial till') Deposits of a glacier - usually described as massive (not layered), poorly-sorted, and composed of multiple types of angular to sub-rounded rocks, but varying greatly with source material.

Tillite
a sedimentary rock composed of till. Generally considered evidence of pre-Pleistocene glaciations, but identification (compared to landslide debris, for example) can be difficult.

Tilt
The angle of the Earth's rotational axis from the plane of its orbit around the sun. Axial tilt varies from about 21.5 to 24.5°, with higher values favoring seasonality, thus ice sheet growth.

Tropical
Low latitude (less than 20° latitude) areas characterized by high temperatures and high precipitation. At high elevations, however, tropical mountains may be both cold and relatively dry.

Troposphere
The lowest part of the atmosphere, from the ground or ocean surface to about 10-15 km up. Noteworthy for containing the vast majority of atmospheric moisture, and the location of nearly all weather phenomena. Above is the stratosphere.

Trough (or "U-shaped valley")
The steep-walled (though rarely vertical), broad-floored shape considered diagnostic of former mountain glaciation. Often contrasted to the 'V' shape typical of mass wasting slopes feeding river systems.

Turbulent Flow
Flow (as is common in air and water) in which the flow lines are confused and the fluid is heterogeneously mixed. Compare to laminar flow.

Valley Glacier
A subtype of alpine glacier or mountain glacier which is longer than it is wide, and flows along the floor of a mountain valley.

Valley Train
A narrow mass of outwash confined in a valley.