![Skeiðarársandur, Iceland](https://upload.wikimedia.org/wikipedia/commons/a/ad/Skei%C3%B0ar%C3%A1rsandur%2C_Iceland.jpg)
An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a
plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
formed of
glaciofluvial deposits
Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers.
They are transported, sorted and deposited by streams of water.
The deposits are formed beside, below or downstream ...
due to meltwater outwash at the
terminus
Terminus may refer to:
* Bus terminus, a bus station serving as an end destination
* Terminal train station or terminus, a railway station serving as an end destination
Geography
*Terminus, the unofficial original name of Atlanta, Georgia, United ...
of a
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#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
. As it flows, the glacier grinds the underlying rock surface and carries the debris along. The meltwater at the snout of the glacier deposits its load of sediment over the outwash plain, with larger boulders being deposited near the terminal
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 shee ...
, and smaller particles travelling further before being deposited. Sandurs are common in Iceland where geothermal activity accelerates the melting of ice flows and the deposition of sediment by meltwater.
Formation
Sandurs are found in glaciated areas, such as
Svalbard
Svalbard ( , ), also known as Spitsbergen, or Spitzbergen, is a Norwegian archipelago in the Arctic Ocean. North of mainland Europe, it is about midway between the northern coast of Norway and the North Pole. The islands of the group range ...
,
Kerguelen Islands
The Kerguelen Islands ( or ; in French commonly ' but officially ', ), also known as the Desolation Islands (' in French), are a group of islands in the sub-Antarctic constituting one of the two exposed parts of the Kerguelen Plateau, a large ...
, and
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
. Glaciers and icecaps contain large amounts of
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel when ...
and sediment, picked up as they
erode
Erode () is a city in the Indian state of Tamil Nadu. Erode is the seventh largest urban agglomeration in the state, after Chennai, Coimbatore, Madurai, Tiruchirapalli, Tiruppur and Salem. It is also the administrative headquarters of the E ...
the underlying rocks when they move slowly downhill, and at the snout of the glacier, meltwater can carry this sediment away from the glacier and deposit it on a broad plain. The material in the outwash plain is often size-sorted by the water runoff of the melting glacier with the finest materials, like
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel when ...
, being the most distantly re-deposited, whereas larger boulders are the closest to the original terminus of the glacier.
An outwash plain might contain surficial braided stream complexes that rework the original deposits. They may also contain
kettle lake
A kettle (also known as a kettle lake, kettle hole, or pothole) is a depression/hole in an outwash plain formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of dead ice left behind by retreating gla ...
s, locations where blocks of ice have melted, leaving a depression that fills with water. The flow pattern of glacial rivers across sandar is typically diffuse and unchannelized, but in situations where the glacial snout has retreated from the terminal
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 shee ...
, the flow is more channelized.
Sandurs are most common in
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
, where
geothermal activity beneath ice caps speeds up the deposition of sediment by meltwater. As well as regular geothermal activity,
volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
activity gives rise to large
glacial bursts several times a century, which carry down large volumes of sediment.
The
Gaspé Peninsula
The Gaspé Peninsula, also known as Gaspesia (; ), is a peninsula along the south shore of the Saint Lawrence River that extends from the Matapedia Valley in Quebec, Canada, into the Gulf of Saint Lawrence. It is separated from New Brunswick o ...
that makes up the essential part of southern
Quebec
Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...
(Lower St-Lawrence and Gaspé areas) also contains several example of paleo-sandar, dating from the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
ice melt.
The prototype sandur
One of the sandurs from which the general name is derived is
Skeiðarársandur
Skeiðarársandur () is an Icelandic glacial outwash plain, a vast expanse of sand generated by the transport of debris by the Skeiðará and other rivers, whose flow is generated by the Skeiðarárjökull glacier and fed by the volcanic system ...
, a broad sandy wasteland along Iceland's south-eastern coast, between the
Vatnajökull
Vatnajökull ( Icelandic pronunciation: , literally "Glacier of Lakes"; sometimes translated as Vatna Glacier in English) is the largest and most voluminous ice cap in Iceland, and the second largest in area in Europe after the Severny Island ice ...
icecap and the sea. Volcanic eruptions under the icecap have given rise to many large glacial bursts (''jökulhlaups'' in
Icelandic), most recently in 1996, when the
Ring Road
A ring road (also known as circular road, beltline, beltway, circumferential (high)way, loop, bypass or orbital) is a road or a series of connected roads encircling a town, city, or country. The most common purpose of a ring road is to assist i ...
was washed away (minor floods have also occurred since then). This road, which encircles Iceland and was completed in 1974, has since been repaired. The 1996
jökulhlaup
A jökulhlaup ( ) (literally "glacial run") is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages.
It originally referred to the well-known subglacial outburst flood ...
was caused by the eruption of the
Grímsvötn
Grímsvötn (; ''vötn'' = "waters", singular: ) is a volcano with a (partially subglacial) fissure system located in Vatnajökull National Park, Iceland. The volcano itself is completely subglacial and located under the northwestern side of the Va ...
volcano, with peak flow estimated to be compared to the normal summer peak flow of . Net deposition of sediment was estimated to be .
The main braided channels of Skeiðarársandur are the
Gígjukvísl and
Skeiðará
Skeiðará () is a relatively short glacier river (about 30 km long). It has its source on the glacier Skeiðarárjökull, one of the southern arms of the Vatnajökull in the south of Iceland.
In spite of its short length, this river has a b ...
rivers, which incurred net gains of respectively during the 1996 jökulhlaup. In the Gígjukvísl there was massive sediment deposition of up to , which occurred closest to the terminus of the glacier. The erosional patterns of Skeiðarársandur can be seen by looking at the centimetre-scale elevation differences measured with repeat-pass laser altimetry (
LIDAR
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
) flown in 1996 (pre-flood), 1997, and 2001. Of the overall deposition during the 1996 jökulhlaup, nearly half of the net gain had been eroded 4 years after the flood. These two rivers on the sandur display drastically different erosional patterns. The difference in sediment erosion can be attributed to the wide trench near the terminus where the Gígjukvísl flows, in contrast with the Skeiðará, which has braided flows directly onto the outwash plain. The Gígjukvísl river is where some of the highest level of sediment deposit occurred and also where the largest erosion happened afterward. This indicates that these massive jökulhlaup deposits may have a large geomorphic impact in the short term, but the net change on the surface relief could be minimal after a couple years to a decade.
The observed change of Skeiðarársandur from a diffuse to a channelized distributary system where it has the most observed sediment deposit has a significant impact on the development of the fluvial succession in the proximal zone. However, in order to have sustained active accretion across the entire sandur there needs to be a diffuse, multipoint distribution system. The system of accumulation on Skeiðarársandur, which is a product of glacier retreat, can be seen as multiple regions of differing
channel patterns
Channel patterns are found in rivers, streams, and other bodies of water that transport water from one place to another. Systems of branching river channels dissect most of the sub-aerial landscape, each in a valley proportioned to its size. Whethe ...
that distribute sediment across the plain in dynamic configurations.
Fossil sandar
Fossil sandar (i.e. no longer active) are found in areas which were formerly glaciated. An example would be the
Usk Valley of South
Wales
Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...
where towards the end of the
last ice age, the receding Usk valley glacier left behind a series of
recessional moraines
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 shee ...
and sandar deposits down-valley of them. Many of the sandar surfaces are still visible albeit degraded over succeeding millennia. Extensive sandar are also recorded in the eastern part of the
Cheshire Plain
The Cheshire Plain is a relatively flat expanse of lowland within the county of Cheshire in North West England but extending south into Shropshire. It extends from the Mersey Valley in the north to the Shropshire Hills in the south, bounded b ...
and beneath
Morecambe Bay
Morecambe Bay is a large estuary in northwest England, just to the south of the Lake District National Park. It is the largest expanse of intertidal mudflats and sand in the United Kingdom, covering a total area of . In 1974, the second larges ...
, both in northwest
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
. 'Valley sandur' deposits are recorded from various localities in that same region.
See also
*
*
References
* Church, Michael A. (1972), Baffin Island sandurs: a study of arctic fluvial processes.
* Garvin J.B. (2001), ''Topographic Dynamics of Kerguelen Island: A Preliminary SRTM Analysis'', American Geophysical Union, Fall Meeting 2001
* Gomez B., Russell A.J., Finnegan D.C., Smith L.C., Knudsen O. (2001), ''Sediment Distribution on Skeidararsandur, Southeast Iceland'', American Geophysical Union, Fall Meeting 2001
* Hardardottir J., Snorrason A., Zophoniasson S., Jonsson P., Sigurdsson O., Elefsen S.O. (2003), ''Glacial Outburst Floods (Jökulhlaups) in Iceland'', EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6–11 April 2003
* Magilligan F.J., Gomez B., Mertes L.A.K., Smith, L.C. Smith N.D., Finnegan D., Garvin J.B., ''Geomorphic effectiveness, sandur development, and the pattern of landscape response during jökulhlaups: Skeiðarársandur, southeastern Iceland'', Geomorphology 44 (2002) 95–113
* Hétu, B., ''La déglaciation de la région de Rimouski, Bas-Saint-Laurent (Québec): Indices d'une récurrence glaciaire dans la mer de Goldthwait entre 12400 et 12000 BP'', Géographie physique et Quaternaire, 1998, vol. 52, n.3, p. 325-347
* Smith L.C., Sheng Y., Magilligan F.J., Smith N.D., Gomez B., Mertes L., Krabill W.B., Garven J.B., ''Geomorphic impact and rapid subsequent recovery from the 1996 Skeiðarársandur jökulhlaup, Iceland, measured with multi-year airborne lidar''. Geomorphology vol. 75 Is. 1-2 (2006) 65-75
External links
NASA page about the SkeiðarársandurA study of sandur formation in western Greenland
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Glacial landforms
Glaciology