PolygonsPolygons

OutlineOutline

DefinitionsDefinitions PeriglacialPeriglacial

Patterned GroundPatterned Ground PolygonsPolygons

Types and Formation TheoriesTypes and Formation Theories ExtraterrestrialExtraterrestrial

Periglacial Environments:Periglacial Environments:

A number of definitionsA number of definitions Definition from Physical Geography.net Definition from Physical Geography.net

“Landforms created by processes associated “Landforms created by processes associated with intense freeze-thaw action in high latitude with intense freeze-thaw action in high latitude areas or near an alpine or continental glacier”areas or near an alpine or continental glacier”

Features Include:Features Include: PingosPingos Ice wedgesIce wedges ThermokarstsThermokarsts Patterned groundPatterned ground

Polygons are a type of patterned groundPolygons are a type of patterned ground

PolygonsPolygons

Non-sorted: “Are polygonal-shaped Non-sorted: “Are polygonal-shaped features often delineated with a features often delineated with a furrow or crack but without a border furrow or crack but without a border of stones…vegetation is frequently of stones…vegetation is frequently concentrated in the furrow and helps concentrated in the furrow and helps to emphasize the pattern” (Price to emphasize the pattern” (Price 1972).1972).

Usually found on near Usually found on near horizontal surfaces but may be found horizontal surfaces but may be found on slopes.on slopes.

Occur only in groupsOccur only in groups Range in size from a few cm up to Range in size from a few cm up to

100m in diameter.100m in diameter. Similar to mud beds but on a larger Similar to mud beds but on a larger

scalescale Largest size polygons are the result Largest size polygons are the result

of ice wedges.of ice wedges.

Sorted: “are Polygonal features given Sorted: “are Polygonal features given definition by a border of stones definition by a border of stones surrounding a central area of finer surrounding a central area of finer material” (Price 1972).material” (Price 1972).

Form on fairly level groundForm on fairly level ground Small to large sizes from 10cm-10m Small to large sizes from 10cm-10m

in diameter.in diameter. Occur in groupsOccur in groups Stone size in the border increases Stone size in the border increases

with the size of the feature and with the size of the feature and decreases with depth.decreases with depth.

Stones on border are often on their Stones on border are often on their edge and oriented parallel to the edge and oriented parallel to the border, which may or may not be border, which may or may not be coincident with crack patterns.coincident with crack patterns.

Polygon FormationPolygon Formation

Usually the result of cracking due to dessication or thermal Usually the result of cracking due to dessication or thermal contraction.contraction.

Cracks are vertical and are a result of horizontal contraction of the Cracks are vertical and are a result of horizontal contraction of the surface layer.surface layer.

Sorting sometimes occurs by movement of coarse material Sorting sometimes occurs by movement of coarse material towards and into the cracks.towards and into the cracks.

In an experiment by Pissart (1974) in which the ground was In an experiment by Pissart (1974) in which the ground was artificially leveled and mixed found that it took small stones two artificially leveled and mixed found that it took small stones two years to accumulate on the surface and those that did moved years to accumulate on the surface and those that did moved toward and eventually fell into thermal contraction cracks. This toward and eventually fell into thermal contraction cracks. This material was caused when the ground warms and expands. Fine material was caused when the ground warms and expands. Fine grained material below the polygon is pushed upward and inward. grained material below the polygon is pushed upward and inward. Eventually stones reach the surface of the raised, fine grained Eventually stones reach the surface of the raised, fine grained material and gradually fall into the bordering cracks. The process material and gradually fall into the bordering cracks. The process then continues.then continues.

Types of PolygonsTypes of Polygons

There is a variety of polygon formationsThere is a variety of polygon formations Different types may include:Different types may include:

Frost-crack polygonsFrost-crack polygons Ice-wedge polygonsIce-wedge polygons Sorted and non-sorted circlesSorted and non-sorted circles Stone or soil stripesStone or soil stripes

Frost Crack Polygon:Frost Crack Polygon:

““Frost cracks are steep Frost cracks are steep fractures formed by thermal fractures formed by thermal contraction in rock or frozen contraction in rock or frozen ground with substantial ice ground with substantial ice content. They commonly content. They commonly intersect to create polygonal intersect to create polygonal patterns, which may lead to patterns, which may lead to the formation of wedges of ice the formation of wedges of ice and surficial material. The and surficial material. The frequency of cracking is frequency of cracking is linked to the intensity of linked to the intensity of winter cold..”winter cold..”

((http://www.gcrio.org/geo/frozen.htmlhttp://www.gcrio.org/geo/frozen.html)) Pic:(http://uregina.ca/~sauchyn/geog323/419.jpg)Pic:(http://uregina.ca/~sauchyn/geog323/419.jpg)

Ice Wedge Polygons:Ice Wedge Polygons:

Begin with cracking due to intense Begin with cracking due to intense cold and thermal contraction. cold and thermal contraction.

The cracks gradually fill with iceThe cracks gradually fill with ice The vertical wedges of ice shape the The vertical wedges of ice shape the

polygons which occur below the polygons which occur below the active seasonal freeze-thaw layer.active seasonal freeze-thaw layer.

The visible surface formation is a The visible surface formation is a result of the soil slumping above the result of the soil slumping above the ice wedges.ice wedges.

(Williams and Smith 1989)(Williams and Smith 1989) May be a few metres to over 100 May be a few metres to over 100

metres in diameter.metres in diameter. Can form low-centered or high Can form low-centered or high

centered polygons:centered polygons: Low centered if ice wedge is growingLow centered if ice wedge is growing High centered if thawing and erosion High centered if thawing and erosion

are more prevalent causing stream are more prevalent causing stream channels along ice-wedgeschannels along ice-wedges

Sorted and Non-sorted CirclesSorted and Non-sorted Circles

SortedSorted http://www.ucsc.edu/currents/02-03/art/patterns.03-01-http://www.ucsc.edu/currents/02-03/art/patterns.03-01-

20.240.jpg20.240.jpg

Non-sortedNon-sorted www.://uregina.ca/~sauchyn/geog323www.://uregina.ca/~sauchyn/geog323

Sorted and Non-Sorted CirclesSorted and Non-Sorted Circles

--Some classify them as a separate categorySome classify them as a separate category

SortedSorted Have a definite ring of stones or Have a definite ring of stones or

rocks in a circular formationrocks in a circular formation Defined as “circular accumulations Defined as “circular accumulations

of stones around a center of finer of stones around a center of finer material.material.

Vary in size from a few Vary in size from a few centimeters to over 3m on centimeters to over 3m on diameter and can extend to depths diameter and can extend to depths of about 1m. of about 1m.

Stones are largest at the surface Stones are largest at the surface and decrease with depth.and decrease with depth.

Occur singly or in groups on Occur singly or in groups on horizontal surface.horizontal surface.

If the surface is not horizontal, If the surface is not horizontal, elongation may occur creating elongation may occur creating stripes.stripes.

Non-sortedNon-sorted Lack definite ring of stones.Lack definite ring of stones. Classified as “bare circular Classified as “bare circular

areas margined by areas margined by vegetation” vegetation” (Price,1972).(Price,1972).

Usually 0.5-3m in diameter.Usually 0.5-3m in diameter. Can be found singly or in Can be found singly or in

groups on fairly level groundgroups on fairly level ground..

Stone StripesStone Stripes

StripesStripes

One definition of stripes is One definition of stripes is “linear patterns of soil or “linear patterns of soil or vegetation on slopes without vegetation on slopes without related lines of stones…related lines of stones…frequently consist of parallel frequently consist of parallel lines of vegetation and lines of vegetation and intervening strips of relatively intervening strips of relatively bare ground oriented down bare ground oriented down the steepest slope” ??? the steepest slope” ??? (Price, 1972)(Price, 1972)

Are believed to be created as Are believed to be created as stone circles but elongate due stone circles but elongate due to slope and creep.to slope and creep.

Range in size from a few cm Range in size from a few cm to 1-2m in width and can to 1-2m in width and can extend down slope several extend down slope several tens of metres but are often tens of metres but are often discontinuous.discontinuous.

Can be sorted or unsortedCan be sorted or unsorted Sorted Stripes often have Sorted Stripes often have

stones turned on edge stones turned on edge parallel to the stripe.parallel to the stripe.

Extraterrestrial:Extraterrestrial: MarsMars Have identified three types:Have identified three types:

Small ScaleSmall Scale Large Scale polygonal nets on level terrainLarge Scale polygonal nets on level terrain Large scale polygonal nets in cratersLarge scale polygonal nets in craters

Are thought to be caused from freeze Are thought to be caused from freeze and thaw frost crackingand thaw frost cracking

Mars impact Crater: Polygon FormationsMars impact Crater: Polygon Formations-Each section slightly larger than a football field-Each section slightly larger than a football field

(http://www.msss.com/mars_images/moc/7_19_99_fifthMars/07_npolys)(http://www.msss.com/mars_images/moc/7_19_99_fifthMars/07_npolys)

(A) Polygon troughs highlighted by frost as the south polar cap retreats during spring. The circular features are the (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 locations of buried craters that were originally formed by meteor impact. This image was acquired on 1 October 2001. 2001.

(B) Summertime view of polygons, highlighted by dark, windblown sand, on the floor of a crater. Obtained on 21 (B) Summertime view of polygons, highlighted by dark, windblown sand, on the floor of a crater. Obtained on 21 January 2002. January 2002.

(C ) Polygon troughs highlighted by the retreating south polar frost cap during southern summer. This picture was (C ) Polygon troughs highlighted by the retreating south polar frost cap during southern summer. This picture was taken on 13 January 2000. taken on 13 January 2000.

(A) (B) (C )(A) (B) (C )

Cycle of polygons, thermokarst lakes and pingosCycle of polygons, thermokarst lakes and pingos

http:/http:/arctic.fws.gov/permcycl.htmarctic.fws.gov/permcycl.htm

Thank You!Thank You!

www.isgs.uiuc.edu/quaternary/valleytrain.htmlwww.isgs.uiuc.edu/quaternary/valleytrain.html