Polygons

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Polygons
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Outline

• Definitions
• Periglacial
• Patterned Ground
• Polygons
• Types and Formation Theories
• Extraterrestrial

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Periglacial Environments

• A number of definitions
• Definition from Physical Geography.net Landforms
  created by processes associated with intense
  freeze-thaw action in high latitude areas or near
  an alpine or continental glacierFeatures
  Include
• Pingos
• Ice wedges
• Thermokarsts
• Patterned ground
• Polygons are a type of patterned ground

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Polygons

• Non-sorted Are polygonal-shaped features often
  delineated with a furrow or crack but without a
  border of stonesvegetation is frequently
  concentrated in the furrow and helps to emphasize
  the pattern (Price 1972).
• Usually found on near horizontal surfaces but
  may be found on slopes.
• Occur only in groups
• Range in size from a few cm up to 100m in
  diameter.
• Similar to mud beds but on a larger scale
• Largest size polygons are the result of ice
  wedges.

• Sorted are Polygonal features given definition
  by a border of stones surrounding a central area
  of finer material (Price 1972).
• Form on fairly level ground
• Small to large sizes from 10cm-10m in diameter.
• Occur in groups
• Stone size in the border increases with the size
  of the feature and decreases with depth.
• Stones on border are often on their edge and
  oriented parallel to the border, which may or may
  not be coincident with crack patterns.

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Polygon Formation

• Usually the result of cracking due to dessication
  or thermal contraction.
• Cracks are vertical and are a result of
  horizontal contraction of the surface layer.
• Sorting sometimes occurs by movement of coarse
  material towards and into the cracks.
• In an experiment by Pissart (1974) in which the
  ground was artificially leveled and mixed found
  that it took small stones two years to accumulate
  on the surface and those that did moved toward
  and eventually fell into thermal contraction
  cracks. This material was caused when the ground
  warms and expands. Fine grained material below
  the polygon is pushed upward and inward.
  Eventually stones reach the surface of the
  raised, fine grained material and gradually fall
  into the bordering cracks. The process then
  continues.

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Types of Polygons

• There is a variety of polygon formations
• Different types may include
• Frost-crack polygons
• Ice-wedge polygons
• Sorted and non-sorted circles
• Stone or soil stripes

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Frost Crack Polygon

• Frost cracks are steep fractures formed by
  thermal contraction in rock or frozen ground with
  substantial ice content. They commonly intersect
  to create polygonal patterns, which may lead to
  the formation of wedges of ice and surficial
  material. The frequency of cracking is linked to
  the intensity of winter cold..
• (http//www.gcrio.org/geo/frozen.html)
• Pic(http//uregina.ca/sauchyn/geog323/419.jpg)

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Ice Wedge Polygons

• Begin with cracking due to intense cold and
  thermal contraction.
• The cracks gradually fill with ice
• The vertical wedges of ice shape the polygons
  which occur below the active seasonal freeze-thaw
  layer.
• The visible surface formation is a result of the
  soil slumping above the ice wedges.
• (Williams and Smith 1989)
• May be a few metres to over 100 metres in
  diameter.
• Can form low-centered or high centered polygons
• Low centered if ice wedge is growing
• High centered if thawing and erosion are more
  prevalent causing stream channels along ice-wedges

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Sorted and Non-sorted Circles

• Sorted
• http//www.ucsc.edu/currents/02-03/art/patterns.03
  -01-20.240.jpg

• Non-sorted
• www.//uregina.ca/sauchyn/geog323

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Sorted and Non-Sorted Circles -Some classify
them as a separate category

• Sorted
• Have a definite ring of stones or rocks in a
  circular formation
• Defined as circular accumulations of stones
  around a center of finer material.
• Vary in size from a few centimeters to over 3m on
  diameter and can extend to depths of about 1m.
• Stones are largest at the surface and decrease
  with depth.
• Occur singly or in groups on horizontal surface.
• If the surface is not horizontal, elongation may
  occur creating stripes.

• Non-sorted
• Lack definite ring of stones.
• Classified as bare circular areas margined by
  vegetation (Price,1972).
• Usually 0.5-3m in diameter.
• Can be found singly or in groups on fairly level
  ground.

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Stone Stripes
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Stripes

• One definition of stripes is linear patterns of
  soil or vegetation on slopes without related
  lines of stonesfrequently consist of parallel
  lines of vegetation and intervening strips of
  relatively bare ground oriented down the steepest
  slope ??? (Price, 1972)
• Are believed to be created as stone circles but
  elongate due to slope and creep.

• Range in size from a few cm to 1-2m in width and
  can extend down slope several tens of metres but
  are often discontinuous.
• Can be sorted or unsorted
• Sorted Stripes often have stones turned on edge
  parallel to the stripe.

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Extraterrestrial

• Mars
• Have identified three types
• Small Scale
• Large Scale polygonal nets on level terrain
• Large scale polygonal nets in craters
• Are thought to be caused from freeze and thaw
  frost cracking

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Mars impact Crater Polygon Formations-Each
section slightly larger than a football
field(http//www.msss.com/mars_images/moc/7_19_99
_fifthMars/07_npolys)
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• (A) Polygon troughs highlighted by frost as the
  south polar cap retreats during spring. The
  circular features are the locations of buried
  craters that were originally formed by meteor
  impact. This image was acquired on 1 October
  2001.
• (B) Summertime view of polygons, highlighted by
  dark, windblown sand, on the floor of a crater.
  Obtained on 21 January 2002.
• (C ) Polygon troughs highlighted by the
  retreating south polar frost cap during southern
  summer. This picture was taken on 13 January
  2000.
• (A)
  (B)
  (C )
• 
  

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Cycle of polygons, thermokarst lakes and
pingoshttp/arctic.fws.gov/permcycl.htm
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Thank You!www.isgs.uiuc.edu/quaternary/valleytra
in.html