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An ice stream is a region of fast-moving ice within an
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
. It is a type of
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
, a body of ice that moves under its own weight. They can move upwards of a year, and can be up to in width, and hundreds of kilometers in length. They tend to be about deep at the thickest, and constitute the majority of the ice that leaves the sheet. In Antarctica, the ice streams account for approximately 90% of the sheet's mass loss per year, and approximately 50% of the mass loss in Greenland. The shear forces cause deformation and recrystallization that drive the movement, this movement then causes topographic lows and valleys to form after all of the material in the ice sheet has been discharged. Sediment also plays an important role in flow velocity, the softer and more easily deformed the sediment present, the easier it is for flow velocity to be higher. Most ice streams contain a layer of water at the bottom, which lubricates flow and acts to increase speed.


Mechanics

Ice streams are typically found in areas of low
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...
, surrounded by slower moving, higher topography ice sheets. The low topography arises as a result of various factors, the most prominent being that
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
accumulates at topographic lows. As water accumulates, its presence increases basal sliding and therefore
velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
, which causes an increase in sheet discharge. Another factor causing ice streams to be found in low regions is that thicker ice results in faster velocity. As the thicker an ice stream is, the greater the driving stress at the bed, and thus the greater the velocity. In addition to driving stress, ice streams have better insulation as the thickness of ice increases, due to it retaining higher temperatures better, it can increase the rate of deformation, as well as basal sliding. As a substance's volume increases, it requires more energy per unit volume to raise its temperature, one of the reasons why it is so difficult for oceans to freeze or evaporate, water is also a poor conductor of heat, so increased thickness will not only increase the amount of heat that can be retained, but also make more energy required for heat to be lost. In addition to thickness, water, and stresses,
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
and
bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...
play a key role in the rate at which ice streams drain. If the underlying sediment is too
porous Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
, allowing for too much water to seep into it, and therefore become saturated, it will be incapable of supporting the shear stress the ice stream places on the bed. The best type of sediment for increased speed of drainage is soft, deformable sediment, that allows the ice stream to flow over the combination of sediment and
till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
, while supporting against
shear stress Shear stress, often denoted by ( Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. '' Normal stress'', on ...
. If the underlying surface is bedrock, and not made of sediments, the speed will decrease. The bedrock acts to slow down the ice stream as it incises and deforms it. Flow velocity of the ice stream is not entirely constant, but in short time scales of days to weeks, it can be treated as such, over long scales, however, it is variable, depending on how the conditions of thickness, temperature, water accumulation, stresses, and base material have changed.


Antarctica

The
Antarctic Ice Sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost and ...
is drained to the sea by several ice streams. The largest in
East Antarctica East Antarctica, also called Greater Antarctica, constitutes the majority (two-thirds) of the Antarctic continent, lying on the Indian Ocean side of the continent, separated from West Antarctica by the Transantarctic Mountains. It lies almos ...
is
Lambert Glacier Lambert Glacier is a major glacier in East Antarctica. At about 50 miles (80 km) wide, over 250 miles (400 km) long, and about 2,500 m deep, it is the world's largest glacier. It drains 8% of the Antarctic ice sheet to the east and s ...
. In
West Antarctica West Antarctica, or Lesser Antarctica, one of the two major regions of Antarctica, is the part of that continent that lies within the Western Hemisphere, and includes the Antarctic Peninsula. It is separated from East Antarctica by the Transan ...
the large Pine Island and Thwaites Glaciers are currently the most out of balance, with a total net mass loss of per year measured in 2006.
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
has many ice streams that carry billions of tons of ice to the sea a year. The Pine Island and Thwaites streams have the highest amount of net discharge in west Antarctica while Lambert Glacier leads the way in
East Antarctica East Antarctica, also called Greater Antarctica, constitutes the majority (two-thirds) of the Antarctic continent, lying on the Indian Ocean side of the continent, separated from West Antarctica by the Transantarctic Mountains. It lies almos ...
. The rate at which the Antarctic ice sheet is losing mass is accelerating and the past and ongoing acceleration of ice streams and outlet
glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
is considered to be a significant, if not the dominant cause of this recent imbalance. Ice streams hold serious implications for
sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cr ...
as 90% of Antarctica's ice mass is lost through them. While
East Antarctica East Antarctica, also called Greater Antarctica, constitutes the majority (two-thirds) of the Antarctic continent, lying on the Indian Ocean side of the continent, separated from West Antarctica by the Transantarctic Mountains. It lies almos ...
is generally stable, ice loss in
West Antarctica West Antarctica, or Lesser Antarctica, one of the two major regions of Antarctica, is the part of that continent that lies within the Western Hemisphere, and includes the Antarctic Peninsula. It is separated from East Antarctica by the Transan ...
has increased by 59% in the past 10 years and by 140% in the
Antarctic peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martín in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctic ...
. Ice streams control much of the
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
mass budget as they dictate the amount of
discharge Discharge may refer to Expel or let go * Discharge, the act of firing a gun * Discharge, or termination of employment, the end of an employee's duration with an employer * Military discharge, the release of a member of the armed forces from ser ...
that comes off an ice sheet. Geomorphic features such as bathymetric troughs indicate where paleo-ice streams in Antarctica extended during the
Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eu ...
(LGM). Analysis of landforms diagnostic of paleo-ice streams, revealed considerable asynchronicity in individual ice stream retreat histories. This notion is important when considering how the underlying
geomorphology Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...
of ice streams control at what rate and how they retreat. Furthermore, this reinforces the importance of internal factors such as bed characteristic,
slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
, and
drainage basin A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
size in determining ice stream dynamics.


Greenland

Ice streams that drain the
Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equi ...
into the sea include
Helheim Glacier Helheim Glacier is a glacier in the Sermersooq municipality, Eastern Greenland. This glacier is named after Helheim, the world of the dead in Norse Mythology. Geography The Helheim Glacier is located on the eastern side of the Greenland ice she ...
,
Jakobshavn Isbræ Jakobshavn Glacier ( da, Jakobshavn Isbræ), also known as Ilulissat Glacier ( kl, Sermeq Kujalleq), is a large outlet glacier in West Greenland. It is located near the Greenlandic town of Ilulissat (colonial name in da, Jakobshavn) and ends ...
and
Kangerdlugssuaq Glacier Kangerlussuaq Glacier ( kl, Kangerlussuaq, meaning 'large fjord'; old spelling ''Kangerdlugssuaq'') is the largest glacier on the east coast of the Greenland ice sheet. It flows into the head of the Kangerlussuaq Fjord, the second largest fjord i ...
. With significantly more surface melt, only 50% of ice mass is lost through ice streams in Greenland, but they still are one of the primary modes of ice loss. the Northeast Greenland Ice Stream, at long, drains roughly 12% of the entire ice sheet through three outlet glaciers. The northeast Greenland ice stream behaves similarly to the Ross ice streams of West Antarctica with fast flow and a weak bed with low driving stresses. The basal
shear stress Shear stress, often denoted by ( Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. '' Normal stress'', on ...
balances the driving stress for several hundred kilometers in the center of the ice stream. Further upstream, the initiation of the ice stream (established by looking at velocity data) is caused by a weak bed.


Lesser Streams

Ice streams can also occur in ice fields that are significantly smaller than the Antarctic and Greenland ice sheets. In the
Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...
n region of southern
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sou ...
there are three main icefields - the North Patagonian Icefield, South Patagonian Icefield, and Cordillera Darwin Icefield that all exhibit ice streams. Ice streams are also important for ice sheet dynamics of Iceland's ice fields. In Iceland, areas with reticulated ridges, ribbed
moraine A moraine is any accumulation of unconsolidated debris ( regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sh ...
s, and trunk-flow zones have demonstrated no control over the direction and magnitude of ice streams.


Geomorphology

Ice streams have various impacts on the surrounding event. The most obvious one is the development of large
topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary scie ...
lows and
valleys A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams ...
after an ice stream has been completely drained from the ice sheet itself. The topographic lows are formed by glacial
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
as the stream carves through the underlain material, eroding it and pushing sediment into the water beneath the ice stream and through the drainage system. These low topographic areas can be up to a few kilometers in depth, and up to hundreds of kilometers in length. The resulting low regions act as a new drainage system for the ice sheet, as it allows movement of material through topographic low to increase, since the stream has left the sheet. Another problem arises from the discharge of the sheet through ice streams, which can be one of many factors causing small stage sheet collapse. In addition to this collapse, ice streams also act to increase the global sea level. As ice streams drain into the surrounding ocean, not only does this increase the sea level due to displacement of the ice runoff, but also by increasing the volumetric content of the oceans themselves, but this is almost negligible. As ice streams diminish in size, the pressure they exert on surrounding features like glaciers reduces, allowing the glacier that feeds into the sea to speed up and discharge more quickly, rising sea level. This rise in sea level affects both topography and
bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water ...
in the regions directly affected by the ice stream in question. As a result of this rise in sea level, albeit slow and almost minute in short scales but large over longer scales, the landscape will be altered. Rising sea levels will weather the surrounding sheet and cause erosion and deformation of the sheet itself, thus altering the landscape and morphology.


References


Further reading


''Fits and Starts – What regulates the flow of huge ice streams?''
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Stream A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long large streams ...
Glaciers Articles containing video clips