The patterned ground below Mugi Hill on Mount Kenya lies in an area of seasonal frost.[1]
A pingo and polygonal ground near Tuktoyaktuk, Northwest Territories, Canada

Patterned ground is the distinct and often symmetrical natural pattern of geometric shapes formed by the deformation of ground material in periglacial regions. It is typically found in remote regions of the Arctic, Antarctica, and the Outback in Australia, but is also found anywhere that freezing and thawing of soil alternate; patterned ground has also been observed in the hyper-arid Atacama Desert and on Mars.[2][3] The geometric shapes and patterns associated with patterned ground are often mistaken as artistic human creations. The mechanism of the formation of patterned ground had long puzzled scientists but the introduction of computer-generated geological models in the past 20 years has allowed scientists to relate it to frost heaving, the expansion that occurs when wet, fine-grained, and porous soils freeze.

Types

Patterned ground can be found in a variety of forms. Typically, the type of patterned ground in a given area is related to the prevalence of larger stones in local soils and the frequency of freeze-thaw cycles.[4][5][6][7][8][9]

Patterned ground in the form of soil polygons located in the hyper-arid Atacama Desert.[10]

Polygons

Polygonal soil patterns, typical of the Arctic Tundra

Polygons can form either in permafrost areas (as ice wedges) or in areas that are affected by seasonal frost. The rocks that make up these raised stone rings typically decrease in size with depth.[6]

In the northern reaches of the Canadian Boreal forests, when bogs reach a eutrophic climax and create a sedge mat, tamarack larch and black spruce are often the early colonists within such a polygonal climax sedge mat.[11]

Circles

Partially melted and collapsed lithalsas (heaved mounds found in permafrost) have left circle-like structures on the Svalbard Archipelago.

Circles range in size from a few centimeters to several meters in diameter. Circles can consist of both sorted and unsorted material, and generally occur with fine sediments in the center surrounded by a circle of larger stones. Unsorted circles are similar, but rather than being surrounded by a circle of larger stones, they are bounded by a circular margin of vegetation.[12][6]

Steps

Steps can be developed from circles and polygons. This form of patterned ground is generally a terrace-like feature that has a border of either larger stones or vegetation on the downslope side, and can consist of either sorted or unsorted material.[4][6]

Stripes

Periglacial stone stripes in Antarctica[13]

Stripes are lines of stones, vegetation, and/or soil that typically form from transitioning steps on slopes at angles between 2° and 7°. Stripes can consist of either sorted or unsorted material. Sorted stripes are lines of larger stones separated by areas of smaller stones, fine sediment, or vegetation. Unsorted stripes typically consist of lines of vegetation or soil that are separated by bare ground.[14][15][6]

It has been conjectured that periglacial stripes on Salisbury Plain in England, that happened by chance to align with the solar sunrise at mid summer and sun set at mid winter , gave rise to awe and veneration by prehistoric people that eventually culminated in the building of the Stonehenge.[16]

Formation

Patterned ground in the polar region of Mars.

In periglacial areas and areas affected by seasonal frost, repeated freezing and thawing of groundwater forces larger stones toward the surface, as smaller stones flow and settle underneath larger stones. At the surface, areas that are rich in larger stones contain much less water than highly porous areas of finer grained sediments. These water-saturated areas of finer sediments have a much greater ability to expand and contract as freezing and thawing occur, leading to lateral forces which ultimately pile larger stones into clusters and stripes. Through time, repeated freeze-thaw cycles smooth out irregularities and odd-shaped piles to form the common polygons, circles, and stripes of patterned ground.[17]

Patterned ground occurs in alpine areas with freeze thaw cycles. For example, on Mount Kenya seasonal frost layer is a few centimetres (inches) below the surface in places.[1] Patterned ground is present at 3,400 metres (11,155 ft) to the west of Mugi Hill.[18] These mounds grow because of the repeated freezing and thawing of the ground drawing in more water. There are blockfields present around 4,000 metres (13,123 ft) where the ground has cracked to form hexagons. Solifluction occurs when the night temperatures freeze the soil before it thaws again in the morning. This daily expansion and contraction of the soil prevents the establishment of vegetation.[19]

Frost also sorts the sediments in the ground. Once the mantle has been weathered, finer particles tend to migrate away from the freezing front, and larger particles migrate through the action of gravity. Patterned ground forms mostly within the active layer of permafrost.[17][20]

See also

References

  1. 1 2 Grab, Stefan W.; Gatebe, Charles K.; Kinyua, Antony M. (2004). "Ground Thermal Profiles from Mount Kenya, East Africa". Geografiska Annaler: Series A, Physical Geography. 86 (2): 131–141. doi:10.1111/j.0435-3676.2004.00219.x. ISSN 0435-3676. JSTOR 3566103. S2CID 129324724.
  2. ↑ Sager, Christof; Airo, Alessandro; Arens, Felix L.; Schulze-Makuch, Dirk (2021-01-15). "New type of sand wedge polygons in the salt cemented soils of the hyper-arid Atacama Desert". Geomorphology. 373: 107481. Bibcode:2021Geomo.37307481S. doi:10.1016/j.geomorph.2020.107481. ISSN 0169-555X.
  3. ↑ "Southern Hemisphere Polygonal Patterned Ground". Mars Global Surveyor: Mars Orbiter Camera. Malin Space Science Systems. Archived from the original on 27 October 2016. Retrieved 8 November 2013.
  4. 1 2 "Patterned Ground". Archived from the original on 29 March 2017. Retrieved 21 September 2016.
  5. ↑ Ballantyne, C.K. (1986). "Non-sorted patterned ground on mountains in the Northern Highlands of Scotland". Biuletyn Peryglacjalny. 30: 15–34.
  6. 1 2 3 4 5 Allaby, Michael (2013). A Dictionary of Geology and Earth Sciences. Oxford University Press. p. 429. ISBN 978-0-19-107895-8.
  7. ↑ Ólafur, Ingólfsson (2006). "Glacial Geology Photos". Retrieved March 4, 2007.
  8. ↑ Kessler M.A.; Werner B.T. (January 2003). "Self-organization of sorted patterned ground". Science. 299 (5605): 380–3. Bibcode:2003Sci...299..380K. doi:10.1126/science.1077309. PMID 12532013. S2CID 27238820.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ↑ Marchant, D.R.; Lewis, A.R.; Phillips, W.M.; Moore, E.J.; Souchez, R.A.; Denton, G.H.; Sugden, D.E.; Potter Jr., N.; Landis, G.P. (2002). "Formation of Patterned Ground and Sublimation Till over Miocene Glacier Ice in Beacon Valley, Southern Victoria Land, Antarctica". Geological Society of America Bulletin. 114 (6): 718–730. Bibcode:2002GSAB..114..718M. doi:10.1130/0016-7606(2002)114<0718:fopgas>2.0.co;2.
  10. ↑ Sager, Christof; Airo, Alessandro; Arens, Felix L.; Schulze-Makuch, Dirk (2021-01-15). "New type of sand wedge polygons in the salt cemented soils of the hyper-arid Atacama Desert". Geomorphology. 373: 107481. Bibcode:2021Geomo.37307481S. doi:10.1016/j.geomorph.2020.107481. ISSN 0169-555X.
  11. ↑ C. Michael Hogan. 2008. Black Spruce: Picea mariana, GlobalTwitcher.com, ed. N. Stromberg Archived 2011-10-05 at the Wayback Machine
  12. ↑ Hallet, Bernard (2013). "Stone circles: form and soil kinematics". Phil. Trans. R. Soc. Lond. A. 371 (2004): 20120357. Bibcode:2013RSPTA.37120357H. doi:10.1098/rsta.2012.0357. PMID 24191111.
  13. ↑ Davies, Bethan. "stone stripes". AntarcticGlaciers.org. Retrieved 2022-03-24.
  14. ↑ King, R. B. (1971). "Boulder polygons and stripes in the Cairngorm Mountains, Scotland". Journal of Glaciology. 10 (60): 375–386. Bibcode:1971JGlac..10..375K. doi:10.1017/s0022143000022073.
  15. ↑ Ballantyne, Colin K. (2001). "The sorted stone stripes of Tingo Hill". Scottish Geographical Journal. 117 (4): 313–324. doi:10.1080/00369220118737131. S2CID 128558678.
  16. ↑ Yirka, Bob; Phys.org. "New dig suggests Stonehenge was built to align with summer and winter solstice". phys.org. Retrieved 2022-03-24.
  17. 1 2 Easterbrook, Don J. (1999). Surface processes and landforms (2nd ed.). Prentice Hall. pp. 418–422. ISBN 978-0-13-860958-0.
  18. ↑ Baker, B. H. (1967). Geology of the Mount Kenya area; degree sheet 44 N.W. quarter (with coloured map). Nairobi: Geological Survey of Kenya.
  19. ↑ Allan, Iain (1981). The Mountain Club of Kenya Guide to Mount Kenya and Kilimanjaro. Nairobi: Mountain Club of Kenya. ISBN 978-9966985606.
  20. ↑ Perkins, S. (17 May 2003). "Patterns from Nowhere; Natural Forces Bring Order to Untouched Ground". Science News. 163 (20): 314–316. doi:10.2307/4014632. JSTOR 4014632.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.