Amateur Geologist Structured Geological Glossary: Ice and Glaciers

Structured Geological Glossary: Ice and Glaciers

ice shelf	"A sheet of very thick ice with a level or gently undulating surface. It is attached to the land on one side, but most of it is floating. On the seaward side it is bounded by a steep cliff (ice front) two to 50 m or more above sea level. Ice shelves have formed along polar coast (e.g., Antarctica and Greenland); they are very wide, with some extending several hundreds of kilometers toward the sea from the coastline. They increase in size from annual snow accumulation and seaward extension of land glacier. They decrease in size from warming, melting and calving."
ice cap	An ice cap is a dome-shaped cover of perennial snow and ice. Ice caps are actually small ice sheet, generally less than 50,000 square kilometres in size. Today, they are found in polar regions (e.g. Baffin Island). Like ice sheets, these ice caps are not constrained by the existing relief and exert a modifying effect on the climate. The cooling effect of an ice cap, however, is considerably less than an ice sheet and the effects are more localized.
ice sheet	A glacier of considerable thickness and more than 50,000 sq. km in area. It form a continuous cover of ice and snow over a land surface. An ice sheet is not confined by the underlying topography but spread outward in all directions. During the Pleistocene Epoch ice sheets covered large parts of North America and northern Europe but they are now confined to polar regions (e.g., Greenland and Antarctica). Also called continental glacier.
Cordilleran ice sheet	Ice cap that grew in western North America during the Pleistocene Epoch. It began growing first in Canada, eventually covering much of British Columbia, Alaska, the northern U.S., and parts of several western states.
glacier	"A mass of land ice that is formed by the cumulative recrystallization of firn. A glacier flows slowly from an accumulation area to an ablation area. Some well-known glaciers are: the Zermatt, Stechelberg, Grinelwald, and Les Diablerets in Switzerland; the Nigards, Gaupne, Fanarak, and Lom in Norway; the Wright Taylor and Wilson Piedmont glaciers in Antarctica; Grinnell glacier in Glacier National Park, Montana, U.S., and the Teton glacier in Teton National Park, Wyoming, U.S."
accumulation (glacial)	All processes including snowfall condensation avalanching snow transport by wind and freezing of liquid water that add snow or ice to a glacier floating ice or snow cover. The term also includes the amount of snow or other solid precipitation added to a glacier or snowfield by these processes.
iceberg	A floating ice mass up to 100 km long and 200-300 m thick that has broken off (calved) from ice shelves, glacier or coastal ice cliffs into the ocean.
glacier surge	A period of unusually rapid movement of one glacier, sometimes lasting more than a year.
ice stream	Large, fast glacier embedded in slow-moving ice. Flow velocities are a few hundred meters to kilometers per year.
valley glacier	A glacier that is smaller than a continental glacier or an icecap, and which flows mainly along well-defined valleys, many with tributaries.
glacial valley	A valley occupied or formerly occupied by a glacier, typically with a U-shaped profile.
alpine glaciers	A glacier occupying a valley, usually high in mountainous terrain.
zone of accumulation	The area above the snowline where snowfall exceeds snow melt and material is added to a glacier.
glacial maximum	The position or time of the greatest advance of a glacier (e.g., the greatest equatorward advance of Pleistocene glaciation).
zone of ablation (or wastage)	The area below the snowline where snow melt exceeds snowfall and material is lost from a glacier.
annual snowline	A term used by glaciologists (scientists who study glacier) for the boundary where the amount of snow loss from melting equals the amount of snow accumulation from snowfall (also called firn limit).
snowline	The lower limit of any year's permanent snowfall. Separates the Zone of Accumulation from the Zone of Ablation.
terminus	The downvalley end of a glacier. It is sometimes referred to as the glacier snout.
mass balance	"The application of the principle of the conservation of matter. For example, the mass of a glacier is not destroyed or created; the mass of a glacier and all its constitutive components remains the same despite alteration in their physical states. The mass balance of a glacier is calculated with the input/output relationships of ice, firn and snow, usually measured in water equivalent. Output includes all ablative processes of surface melting basal melting evaporation wind deflation calving and internal melting. Input includes direct precipitation avalanching and the growth of superimposed ice."
firn limit	A term used by glaciologists (scientists who study glacier) for the boundary where the amount of snow loss from melting and evaporation equals the amount of snow accumulation from snowfall (also called the annual snowline).
basal sliding	The movement or speed of movement of a glacier on its bed. Also called basal slip.
crevasse	A deep crack in a glacier caused by the stresses of the ice's movement.
neoglaciation	Neoglaciation refers to the advances made by mountain glacier since the great Pleistocene ice age. In the Cascades the advances have occurred since 6,600 years before present.
ablation (glacial)	All processes which include melting evaporation (sublimation) wind erosion and calving (breaking off of ice masses) that remove snow or ice from a glacier or snowfield. The term also refers to the amount of snow or ice removed by these processes.
equilibrium line	The level on a glacier where accumulation equals ablation and the net balance equals zero.
firn	Material that is transitional between snow and glacier ice. It is formed from snow after passing through one summer melt season and becomes glacier ice after its permeability to liquid water falls to zero.
ice core	Deep drill into the earth's permanent glacier revealing the history of the atmospheric gas and dust content, enabling deduction of former atmospheric temperatures.
granular snow	Snow that has been metamorphosed into small granules of ice.
Lake Missoula	Glacial lake whose flood when its ice damn broke created the Grand Coulee. Glaciers impounded Clark Fork River.
jökulhlaup	Icelandic term for glacial outburst flood. 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.
delta kame	A deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier.
pluvial lake	A lake formed in a land-locked basin during a period of increased rainfall associated with glacial advance elsewhere.
ice age	A glacial epoch or time of extensive glacial activity. Also, as Ice Age, which refers to the latest glacial epoch, the Pleistocene Epoch. Periods characterized by very low temperature worldwide and advancing glacier.
nunatak	Islands of mountain tops that pertrude above glacier.
tarn	Small lake left by the retreat of a glacier. May fill a basin formed by a moraine dam or eroded by the glacier into bedrock.
moraine	A hill-like pile of rock rubble located on or deposited by a glacier. An end moraine form at the terminus of a glacier. A terminal moraine is an end moraine at the farthest advance of the glacier. A lateral moraine forms along the sides of a glacier.
lateral moraine	A moraine formed along the side of a valley glacier and composed of rock craped off or fallen from the valley sides.
medial moraine	A long stripe of rock debris carried on or within a glacier resulting from the convergence of lateral moraine where two glaciers join.
terminal moraine	A sinuous ridge of unsorted glacial till deposited by a glacier at the line of its farthest advance.
end moraine	See terminal moraine.
rock flour	A glacial sediment of extremely fine (silt-and clay-size) ground rock formed by abrasion of rocks at the base of the glacier.
glacial rebound	The isostatic adjustment of previously glaciated areas after glacial retreat (e.g., the uplift of Scandinavia after the most recent glaciation.
ground moraine	A glacial deposit of till with no marked relief, interpreted as having been transported at the base of the ice.
till	An unconsolidated sediment containing all sizes of fragments from clay to boulder deposited by glacial action, usually unbedded.
glaclolacustrine	Used to describe sediment deposited in a lake in contact with or receiving most of its water and sediments from a glacier, and also landforms resulting from such deposition.
striations (glacial)	Grooves eroded into bedrock by rock debris frozen into the base of a glacier.
glacial abrasion	A common mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is mowing across.
glacial striations	Scratches left on bedrock and boulder by overriding ice, and showing the direction of motion.
Roche Mountonnée	A small symetrical shaped hill formed by glacial erosion. The upper sides are rounded and smoothed whilw the lower sides are rough due to quarrying by the glacier.
permafrost	"Perenially frozen ground that occurs whereever the ground temperatures remain continuously below 0ø C for two or more years. Discontinuous or patchy permafrost occurs north of the 0ø C mean annual air temperature isotherm; continuous permafrost up to several hundred meters in thickness is widespread in Siberia and northern Alaska and Canada."
freeze-thaw cycle	In colder temperate regions, water trapped in fracture and between grain of rocks repeatedly freezes, then thaws during the winter months. In some areas this occurs on a daily basis as water freezes at night, then melts in warmer daytime temperatures. Only in the coldest regions does water remain frozen throughout the winter.
solifluction	The flow of wet material that takes place at the surface when the ground in a permafrost area is patially thawed.
eskar (spell)	A glacial deposit in the form of a continuous, winding ridge, formed from the deposits of a stream flowing beneath the ice.
drumlin	A smooth, streamlined hill composed of till.
frost wedging	A process that mechanically breaks apart rock caused by expansion of water as it freezes in cracks and crevices.
kettle	A small hollow or depression formed in glacial deposits when outwash was deposited around a residual block of ice that later melted.
outwash	A glaciofluvial sediment that is deposited by meltwater streams emanating from a glacier.
outwash plain	An extensive, relatively flat area of sand, silt, and gravel formed below a glacier by meltwater streams and rivers.
sheetwash	Overland flow of water in thin sheets
drift	A collective term for all te rock, sand, and clay that is transported and deposited by a glacier either as till or as outwash.
U-shaped valley	A deep valley with steep upper walls that grade into a flat floor, usually eroded by a glacier.
hanging valley	A former glacial tributary valley that enters a larger glacial valley above its base, high up on the valley wall.
fjord	A former glacial valley with steep walls an a U-shaped profile now occupied by the sea.
fiord	See fjoid.
cirque	The head of a glacial valley, usually with the form of one half of an inverted cone. The upper edges have the steepest slope, approaching vertical, and the base may be flat or hollowed out and occupied by a small lake or pond.
arête	A sharp, narrow ridge separating two glacial valley.