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Sandur

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   A satellite image of the Skeiðarársandur in Iceland
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   A satellite image of the Skeiðarársandur in Iceland

   In geology, a sandur (plural sandar) is a plain formed by meltwater
   from glaciers, also known as glacial outwash or merely outwash. Sandar
   are usually wider than their length, and consist of soft sediments
   washed downstream from a terminal moraine and deposited over a
   relatively flat plain criss-crossed by braided streams of meltwater.

Formation

   Sandur are found in glaciated areas, such as Svalbard, Kerguelen and
   Iceland. Glaciers and icecaps contain large amounts of silt and
   sediment, picked up as they erode the underlying rocks as 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,
   being the most distantly re-deposited, whereas, large boulders are the
   closest to the original terminus of the glacier.

   An outwash plain might contain surface meandering streams that rework
   the original deposits. They may also contain kettle lakes, 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, the flow is more channelized.

   Sandur are most common in Iceland, where geothermal activity beneath
   ice caps accelerates the deposition of sediment by meltwater. As well
   as regular geothermal activity, volcanic activity gives rise to large
   glacial bursts several times a century, which carry down large volumes
   of sediment.

   The Appalachian peninsula that composes the essential part of southern
   Québec (Lower St-Lawrence and Gaspé areas) also contains several
   example of paleo-sandurs, dating from the Pleistocene ice melt.

The prototype sandur

   The original sandur from which the general name is derived is
   Skeiðarársandur, a broad sandy wasteland along Iceland's south-eastern
   coast, between the Vatnajökull icecap and the sea. Skeiðarársandur is
   the largest sandur in the world, covering an area of 1300 km², and
   volcanic eruptions under the icecap have given rise to many glacial
   bursts (jökulhlaups in Icelandic), most recently in 1996. The peak flow
   of the 1996 jökulhlaup was estimated to be 50,000 m³/s, compared to the
   normal summer peak flow of 200-400 m³/s, and the net deposition of
   sediment was estimated to be 12.8 million cubic metres. The level of
   the sandur was raised by up to 10 metres in places.

   Between glacial bursts, the Sandur is usually criss-crossed by glacial
   rivers in normal flow. The Ring Road which encircles Iceland was
   completed in 1974 with the stretch across the Sandur. It was washed
   away by the 1996 jökulhlaup, which was caused by the eruption of the
   Grímsvötn volcano, but has since been repaired.
   Retrieved from " http://en.wikipedia.org/wiki/Sandur"
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