User:Iridia/Polygonal patterned ground

Polygonal patterned ground is a geometric landform that develops in periglacial geographic regions, which experience intense freezing and thawing cycles. It is seen on ground at high altitudes, in the Arctic, in the McMurdo Dry Valleys of Antarctica, and on the northern plains of the planet Mars. The accepted model for its formation requires several years of thermal cycles, due to seasonal change, in the active layer of ice-cemented soil that exists above a body of permafrost. The chill at the onset of winter causes the frozen ground to contract, creating a network of stress fractures. These cracks become braced open by long vertical wedges of ice and gravel, and can grow up to ten metres deep as the wedges progressively recrack and widen over successive seasons. The wedge-filled cracks spread and intersect to form a network of regular geometric shapes: these polygons generally begin as rectangles and acquire more sides as they subdivide with age. The expansion of the wedges deforms the layers of soil on either side, in some cases gently forcing them upwards to create raised rims around bowl-shaped polygons. Rocks on the surface are rearranged and sorted along the cracks by this cryoturbation.

On Earth, the size of individual polygons is normally between three and fifteen metres across, though ones as small as thirty centimetres diameter have been observed in the Arctic. Martian polygons can be considerably larger: while those seen at the landing site of the Phoenix spacecraft were of terrestrial scale, polygon fields exist with polygons more than fifty metres across. Polygon fields cover available flat or gently sloping ground; common sites of formation on Earth are former lakebeds and river deltas. Polygonal patterned ground near rivers may have prominent lines of alignment parallel with the riverbanks.

Formation

Aerial view of patterned ground in Alaska.

The amount of water available in the environment controls the development of the polygons. In relatively wet areas such as the Arctic, ice wedge polygons develop, with wedges that are nearly pure ice. In extremely arid areas such as the Dry Valleys, sand wedge polygons form. These have wedges that are more than 80% fines, pebbles and rocks, cemented by a small amount of ice.

Ice wedge polygons

In the fine sediments of the active layer, the expansion of water as it freezes pushes apart the grains of soil: when conditions are right, the force of expansion exceeds the tensile strength of the soil and a small vertical crack forms. Propagation is defined by the strength of the ground.

Water runs into the crack and freezes, wedging the crack open. The wedge cracks in the centre in the next thermal cycle, splits further apart, refills, and season by season the wedge grows larger and deeper.

Martian polygons

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  Comparison between polygons photographed by Phoenix on Mars...
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  ... with patterned ground on Devon Island in the Canadian Arctic, on Earth.
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  Orbital images: Patterned ground is rare to be found this far south (45 degrees north latitude). Picture taken by Mars Global Surveyor.
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  ... and as photographed (in false color) from Mars Reconnaissance Orbiter.

External links

Cartoon illustrating ice wedge formation [1]