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Permafrost: Patterned Ground

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The repeated annual freezing and thawing of the active layer in permafrost soils can produce very interesting features, termed patterned ground. These features include polygons, circles, nets, steps, and stripes, where each is divided into nonsorted and sorted varieties. Additional features are frost mounds, including frost blisters, pingos, and palsas. Mass-wasting related to frost actions, such as active-layer failures, retrogressive thaw slump, solifluction, and rock glaciers can also produce patterned ground. The formations of some of these patterned ground features are still not fully understood.

 
  Polygons Return to Top  

Nonsorted polygons occur from contraction of the soil from desiccation and/or coldness. The contraction is forceful enough that the ground cracks in a polygon pattern in order to release tensile stress. The polygons that form desiccation, or dryness cracks are seen globally in features such as dried mud flats. The polygons that form from thermal contraction only happen in very cold environments, usually only in the continuous permafrost zone. The thermal contraction polygons usually develop ice wedges within the cracks. Both polygons can be classified as micro-scale and macro-scale. The micro-scale is less than seven feet in diameter and usually related to desiccation polygons. The macro-scale is commonly 50 to 100 feet in diameter and related to ice wedge polygons, the diameters have also been seen to exceed 330 feet. The polygons can form in fine-grained and coarse-grained soils, and commonly on flat horizons but can be seen on slopes. The polygons can either have cracks or furrows, cracks are where the borders have visible cracks. Furrows are depressions that have a concentration of vegetation, these are usually related to ice wedges. Ice wedge polygons can be classified as low-centered or high-centered polygons. For low-centered polygons, the edges, above the commonly forming ice wedge, are higher and the centers become depressions that often pond water during the summer. For high-centered polygons, the centers are higher than the edges, and it is sometimes linked to the ice wedges thermally eroding. The first photo (left) is an example of micro-scale polygons with furrows, where the clay centers frost heave severely enough to destroy any roots of plants trying to grow there. The second photo (right) is low-centered polygons on the Alaskan coast near Barrow.

Photo depicting a micro polygon pattern. Photo depicting a low center polygon pattern.

Ice wedge polygons will actively form in continuous permafrost regions, however it can grow in discontinuous permafrost under the right conditions. More commonly, the ice wedges seen in discontinuous permafrost are inactive and called fossil ice wedges. Within the Tunnel, the ice wedges are inactive and buried by silt deposition under syngenetic permafrost conditions. Amazingly, the winze of the Tunnel cuts through a great ice wedge intersection of three ice wedges. The three-dimensional aspect of ice wedges are hard to picture, so the diagram below (right) illustrates a cross-cut of an ice wedge polygonal ground. The photo (below, left) is a gravel road that has been severely damaged by the thawing of ice wedges.

Photo depicting a gravel road damaged by the thawing of ice wedges. Diagram illustrating a cross-cut of an ice wedge polygonal ground.

Photo showing an example of sorted polygons. Sorted polygons differ from nonsorted polygons because they contain a border of stones. The largest macro-scale polygon for sorted polygons is much smaller than the nonsorted variety at approximately 30 feet. In addition, the sorted polygons have never been seen on slopes. There is a relationship between the size of the stones within the border and the size of the polygon, where larger polygons have larger the stones. The border stones will also decrease with size with depth, no matter the size of the polygon. Some stone borders with decrease in width with depth until it ceases to exist, and some stone borders will widen with depth and grow into a continuous stone layer. The creations of these polygons are related to the frost cracking of the ground along with the seasonal frost heave and thaw settlement of the ground to cause sorting. The photo to the right is an example of sorted polygons.
 
 
  Circles Return to Top  

Photo depicts a nonsorted circle. Nonsorted circles are circular-like shaped features without a stone border, but with a vegetation border. They are also known as mud circles, earth hummocks, frost boil, etc. The features are commonly slightly dome shaped with small polygonal cracks through it. They usually form on flat terrain with one or groups of circles. The soils within the circles are usually fine-grained, such as silt and clay, and can sometimes contain few coarse-grained soils. However, nonsorted circles can form in coarse-grained soils, such as gravels, and are called stony earth circles. The circles are usually 1.5 to 10 feet in diameter. After the circle has been inactive for some time, vegetation will slowly begin to grow on it. The processes behind nonsorted circles vary and are not precisely known for each type of circle. The diagram does show a few of the processes for hummocks. Majority of the processes relate to seasonal frost heave with segregated ice and thaw settlement or pressurized super-saturated mud that gets pushed to surface in order to release pressure. The photo to the right shows a nonsorted circle, this example is often called a mud circle.
 

Photo depicts a sorted circle. Sorted circles are similar in description as nonsorted circles, except they have a stone border. The center of the circle is commonly fine-grained material and sometimes contains stones. These are formed through the seasonal frost heave and thaw settlement that causes sorting of the coarse-grained and fine-grained soils. The photo to the right is an example of a sorted circle.

 
  Nets Return to Top  

Photo depicts a group of hummocks. Both sorted and nonsorted nets are very similar to circles and polygons. Nets are simplistically the intermediate form between the shapes of polygons and groups of circles. The terminology and formations of nets are similar to the circle and polygon counterparts. Hummocks type terrain generally fall under nonsorted nets rather than nonsorted circles. The photo to the right is an example of a group of hummocks.

 
  Steps Return to Top  

Photo shows sorted steps, taken at Glacier National Park in Montana. Steps are step-like features that form on slopes. The borders are vegetation for nonsorted and stones for sorted. Steps form from parent features, such as circles, polygons and nets. Nonsorted steps form from mainly hummocks of nonsorted circles or nets. Sorted steps form from sorted circles or polygons. The formation processes of the steps will be similar to the formation processes of the parents feature. The photo to the right shows sorted steps in the lower half, taken in Glacier National Park in Montana.

 
  Stripes Return to Top  

Photo shows sorted stripes in its lower half, taken at Glacier National Park in Montana. Stripes are very similar to steps, except they form a series of vertical stripes down hillsides. Nonsorted stripes have vegetation borders and sorted stripes have stone borders. The parent features for nonsorted stripes are nonsorted circles and small nonsorted polygons. The parent features for sorted stripes are sorted polygons and sorted nets. The photo to the right shows sorted stripes in its lower half, taken in Glacier National Park in Montana.

 
  Active-Layer Failure Return to Top  

Photo shows an example of an active-layer failure. Active-layer failure is a broad term where the active layer over permafrost on a hillside fails and slides down the slope for any reason. One common type of this active-layer failure is detachment failure, where warmer than normal temperatures causes the depth of thaw to become deeper. Then the active layer fails due to the thawing of ice-rich sediments within the very top layer of permafrost. The features are commonly known as skin flows within the United States. After the active layer fails, it exposes the underlying permafrost which could create more thermal degradation and mass-wasting features. The photo to the right shows an example of an active-layer failure, where the light brown stripe is the freshly exposed permafrost about a half-mile long.

 
  Retrogressive Thaw Slump Return to Top  

Retrogressive thaw slump is formed from thaw slumping processes. Thaw slumping happens when ground ice melts causing the soils to become overly moist, which causes a slope to fail and move down the hill. The result becomes a debris fan of thawed sediments and melt-water known as retrogressive thaw slump.

 
  Solifluction Return to Top  

Photo shows an example of solifluction lobes near Chicken Creek, Alaska. Solifluction is a slow movement of the soils and vegetation down a slope. The shape of the feature can be lobes, a step (or terrace), or a sheet of the active layer. The formation is due to the visco-elastic creep of the frozen ground, and possibly the action of seasonal frost heaving and thaw settlement. The photo to the right is an example of solifluction lobes near Chicken Creek, Alaska.

 
  Rock Glaciers Return to Top  

Photo shows a rock glacier taken from above in the Chugach Mountains, Alaska. Rock glaciers are basically what they sound like, they are a mass of rocks and soils that show evidence of movement or past movement. Active rock glaciers commonly move a few feet every year, however they have been seen to move up to 150 feet a year on steep slopes. The reason they move or have moved is not fully understood. There are three theories that include permafrost creep, ice-cored, and rock slides. The permafrost creep is similar to solifluction where rock glacier is a matrix of rock and ice and slowly creeps down hillsides. The ice-cored theory is basically a glacier that is covered with debris of rocks and soils, where the movement is due to the movement of the glacier underneath. The rock slide is a land slide that has no continuous movement or significant ice involvement. The photo shows a rock glacier taken from above in the Chugach Mountains, Alaska.

 
  Frost Blisters Return to Top  

Frost blisters are a seasonal frost mound feature. They form because the freezing ground traps water in between the bottom of freezing front and an impermeable layer, such as permafrost. The trapped water becomes pressurized and pushes up on the top frozen layer to release the pressure. The water will inject itself under this mound and freeze in a dome shape. Frost blisters are usually small, and will thaw and disappear away in a year or two.

 
  Pingos Return to Top  

Photo shows a pingo with ice wedge polygons on its top. Pingos range in height from several feet to greater than 200 feet and in diameter from several feet to about 2000 ft. The core of pingos is pure ice that forms from a slow injection of water into the mound from pressurized or artesian water. There are two types of pingos that are open-system and closed-system. Open-system pingos have an open-system water source, such as ground water moving through unfrozen ground or artesian groundwater. Cold-system pingos form due to the water within moist soil, such as a drained thaw lake. The water becomes pressurized from the top of the active layer freezing, similar to the frost blister. Open-system pingos are related to discontinuous permafrost and cold-system pingos are commonly related to continuous permafrost. As a pingo erodes, the center will sometimes collapse and fill with water this creating a pingo remnant. Further erosion can sometimes result in thaw lakes. In the diagram below, the various lifecycles of hummocks, pingos, and palsas (described below) are illustrated. The photo to the right shows a pingo that has ice wedge polygons on its top.

Diagram showing lifecycles of hummocks, pingos, and palsas.
Reprinted with permission from the Arctic Institute of North America. Diagram from Lundqvist, Jan (1969) Earth and ice mounds: a terminological discussion. In The Periglacial Environment: Past and Present, ed. Troy L. Péwé, 203–215. Ottawa, Canada: Arctic Institute of North America.

 
  Palsas Return to Top  

Palsas differ from pingos and frost blisters because they include segregated ice processes within their formation. Palsas form in circular mounds, as well as ridges and winding ridges. The heights vary from 3 to 20 feet, the widths vary from 30 to 100 feet, and the lengths vary from 50 to 500 feet. One theory of segregated ice formation is due to the injection of pressurized water during the ice lens formation to create thicker ice lenses. The life cycle of a palsa is depicted in the above diagram.


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