Permafrost

Permafrost is unconsolidated material and bedrock that has remained below 32 degrees F (O degrees C) at least from one winter through the next. This is the minimum duration of permafrost; much has been in existence for tens of thousands of years. Permanently frozen ground prevails throughout most of Alaska (Figure 96), ranging from less than a foot in depth at the southern margin to 2,000 feet (609.6 m) at Prudhoe Bay. Local variations in thickness, areal extent, and permafrost temperature depend on differing thermal properties of earth materials and on local differences in climate, topography, vegetation, geology, hydrology, and rate of heat flow within the earth. In many places these local variations mask the regional southward decrease in areal extent and thickness and southward increase in permafrost temperatures. Areas around large water bodies and thermal springs are generally free of permafrost because of increased amounts of heat flow in the ground. The shape of the permafrost and subpermafrost tables reflects the presence of these features and large-scale surface topography (Figure 97).

The permafrost table is the upper boundary of permanently frozen ground. The area above it is called the suprapermafrost layer. The active layer is that part of the suprapermafrost zone that freezes in winter and thaws in summer. When winter freezing does not extend all the way down to the permafrost table, an unfrozen layer remains between the permafrost and the frozen active layer. Such unfrozen ground surrounded by frozen ground is known as talik (Figure 98). Groundwater trapped in taliks may be stored under great hydrostatic pressure. If disturbed, springs may burst to the ground surface and freeze, producing a thick and often widespread ice sheet or ice mound called aufeis. This process is known as icing. Since water deposits tend to reduce temperature fluctuations from season to season, thawing usually reaches deeper in drier materials. Permafrost thickness is aggrading as it thickens and degrading as it thins.

The extent and thickness of permanently frozen ground in the Yukon Region varies from thick, continuous permafrost in the northern part of the region to moderately thin, discontinuous permafrost in the southern part of the region (Figure 100). Climate is a primary factor in the formation of permafrost. However, especially in Alaska, this is complicated by complex relief characteristics. Seasonal temperature variations are reflected to a certain depth depending on climate and terrain. Below that depth permafrost is at its coldest, warming gradually thereafter with depth until it passes 32 degrees F (O degrees C), indicating the subpermafrost boundary (Figure 99). In the Yukon Region, the boundary between the continuous and discontinuous permafrost zones intersects the 17.6 degrees F (-8 degrees C) isotherm.

The age of permafrost varies from place to place. Areas that were ice-covered during the Pleistocene may have been free of permafrost until the glaciers melted (Hopkins et al. 1955). The ice may have insulated the ground from low air temperatures at the same time that permafrost was aggrading in adjoining ice-free areas. Possibly, permafrost in areas that were never ice-covered during the Pleistocene may be much older. Some ice wedges in northern and central Alaska have been dated from 14,000 to 32,000 years old. After the ice disappeared, permafrost existing in areas of post-glacial flooding, such as pro-glacial lakes and marine transgressions, probably dissipated and was not able to reform until several thousand years later.

Various terrain conditions have important effects on permafrost growth and thickness (Brown and Pewe 1973). Surface relief directly influences permafrost formation by controlling the amount of solar radiation received by the ground. Permafrost in the discontinuous zone may occur only on north-facing slopes, which receive less solar radiation; in the continuous zone, permafrost on north-facing slopes may be thicker with a thinner active layer. Relief also influences snowfall accumulation and vegetative cover, which in turn also affect permafrost thickness.

The type of ground surface is an important factor in determining permafrost conditions. The thermal conductivity of silt is about one-half that of coarse-grained sediments and several times less than that of bare rock with its high reflectivity value. The active layer is thinnest and the permafrost layer generally thickest in fine-grained soils such as silt and clay.

Moss and peat are of special importance in insulating the ground and protecting permafrost. Little alteration of the permafrost may occur through removal of trees or brush, but severe damage occurs when the moss or peat cover is disturbed. Vehicle traffic across tundra areas is a major cause of this type of disturbance. The resulting permafrost thaw and subsidence often leads to local flooding, drainage diversion, and soil erosion. Though low vegetation is of prime importance, trees also affect permafrost by shading the ground and intercepting some snowfall. Also, the density and height of trees influence the effects of ground level winds, lowering velocities where trees are dense. This affects the transfer of heat. Permafrost considerably affects the rooting of plants by keeping the root zone temperature well below the optimum for plant growth and reducing water absorption by plant roots. Also, the permafrost layer prevents the penetration of roots, resulting in a shallow, lateral root system. Permafrost prevents water percolation, so the active layer is boggy, poorly aerated, and improperly nourished. Solifluction, a direct result of permafrost, disturbs vegetation stability by continually shifting the root zone.

Thaw bulbs are present under large bodies of water (Figure 97). Their extent depends on the depth and temperature of the water body and the type of bottom sediments. Well-sorted sand and gravel are generally free of permafrost.

Snow cover influences permafrost distribution and thickness by controlling heat transfer to the ground from the atmosphere. Snowfall conditions and the length of time snow is on the ground are important. Early, heavy snowfall in winter effectively insulates the ground from the severe winter cold, preventing permafrost aggradation. Thick snow cover that lasts late into spring delays spring thaw and can bring about aggradation of permafrost. The thickest and most extensive permafrost and thinnest active layer exist in areas of thin snow cover. Some investigators believe that variations in snow cover have more effect on permafrost than variations in vegetative cover.

The influence of fire on permafrost depends on vegetation, moisture, and the rate of burning. If a fire moves rapidly through an area or if the surface peat and moss layer is moist, the ground vegetation may be little affected and the permafrost undisturbed. However, when surface vegetation is very dry, fire burns the insulative cover and exposes the permafrost layer. In the discontinuous zone, masses of thin permafrost below a fire of this type may disappear.

Several types of geomorphic features are produced by permafrost and frost action.

Thermokarst Topography—Thermokarst topography consists of mounds, sink holes, tunnels, caverns, short ravines, lake basins, and circular lowlands. Local melting of ground ice and the subsequent settling of the ground creates this uneven topography, so it is most common where massive ice formations such as ice wedges and thick, segregated ice exist. Melting can result from the disturbance or removal of vegetation or by a warming trend in climate. Even small disturbances, such as a vehicle driven across the tundra, can create thermokarst features.

Thaw Lakes—Thaw lakes, the most common thermokarst feature, usually start as slight ground surface depressions such as low-centered polygons. Water pools and begins to thaw the permafrost immediately beneath. As thaw continues along lake margins, the basin extends and often merges with other adjacent lakes. Some thaw lakes are connected by stream channels while others have neither an active inlet nor outlet stream. Since underlying permafrost prevents percolation into underground aquifers, water is trapped until the lake drains. Thaw extension continues until intervening higher ground is breached, creating an outlet channel. The lake drains, leaving behind a low, marshy basin surrounded by the higher topography of the original tundra surface. Repeated formation and drainage of thaw lakes often creates an overlapping series of drained basins with only isolated ridges or platforms of the original surface remaining. Thaw lakes are most numerous in the Yukon Flats, Kanuti Flats, Koyukuk Flats, Yukon delta, and north of the Tanana River near Minto.

Ice Wedge Polygons—Polygonal or patterned ground forms when the ground contracts during extreme low temperatures. Water and snow collect in the contraction cracks, eventually turning to ice and producing ice wedges that surround each polygon. The addition of ice to the wedges is accompanied by upthrusting of material adjacent to them, both by the expansion of ice during freezeup and of the ground during summer warm-up. This tends to form ridges of material on each side of the ice wedge.

If drainage is sufficient, ice wedges thaw and form troughs that the drainage tends to perpetuate. The ridge-forming material may slump back into troughs and develop high-centered polygons. In areas of poor drainage, such as marshy, drained lake basins, ice wedges are not eroded, and ridges continue to develop. They stand in significant relief above polygon centers, producing low-centered polygons. Normally, polygon crack systems form at random in areas of uniform temperature stress. However, on receding shorelines, in drained lakes, and in abandoned stream channels, crack systems often form both parallel and perpendicular to the receding shorelines because of horizontal stresses caused by temperature gradients.

Active ice wedges—those still growing by the addition of new ice—occur predominantly within the continuous permafrost zones. Inactive wedges characterize the discontinuous zone (Figure 101). Wedges are active in areas where winter ground temperatures at the top of the permafrost are five degrees F (-15 degrees C) or colder. Low-centered polygons are uncommon in the inactive wedge region because the ice is not growing. Inactive wedges do not have a seam or crack extending to the surface in the spring and usually exist only in silt, having thawed and disappeared in sand and gravel areas. Most regions of inactive, discontinuous permafrost no longer exhibit patterned ground as they have been covered by recent sediments.

The former presence of permafrost is indicated by ice-wedge casts, which form by slow collapse of sediments into the cavities formed by melting ice wedges. Some of the oldest casts known in North America, estimated at 1.5 million years old, are near Fairbanks.

Beaded Streams—A beaded stream consists of a series of elliptical pools, approximately three to 10 feet deep and a few yards in length and width, connected by short, usually straight water courses that generally follow ice wedges and connect at their intersections.

Pingos—Pingos, the Eskimo word for "small hills," are small, conical, ice-cored mounds found in northern and central Alaska and northwestern Canada. Open-system pingos are characteristic of the discontinuous permafrost zone in the Yukon Region (Figure 101). Open system pingos form when subpermafrost or intrapermafrost water penetrates the permafrost layer under hydrostatic pressure. A large water-ice lens forms below the tundra that heaves up into a mound from a combination of freezing pressure and hydrostatic pressure. They generally lie near the base of slopes, where water is apparently able to enter the subsurface system from the nonpermanently frozen areas upslope. Almost all open-system pingos are on south- or southeast-facing slopes of alluvium-filled valleys and are composed of a variety of surficial materials, primarily silty colluvium and valley-fill material. None has been found on glacial drift. They nearly all lie in areas that have been unglaciated for at least 25,000 years.

Nearly 300 pingos or pingo-like mounds have been located in central Alaska between the Alaska and Brooks Ranges in the forested discontinuous permafrost zone. In some cases they are as dense as 10 per 100 square miles (260 km***).

Pingos have definite, although limited, practical value. Crater ponds and springs, which flow even in very cold winters, are direct sources of water and should be good indicators of ground water at moderate depths. Some steep-sided pingos with ground ice at shallow depths could be tunneled and used as semi-permanent cold storage facilities, a technique employed in northwestern Alaska (Porsild 1938). Pingos indicate very poor conditions for buildings, roads, towers, or other permanent structures.

Frost Mounds—Frost or ice-cored mounds are smaller, but related in origin to pingos, and have maximum relief of about four feet and diameters ranging from 10 to 15 feet. They may exist anywhere on the tundra, but are most common in marshy, drained, lake basins. Although some, especially the largest, have their cores rooted in permafrost, most probably are restricted to the active layer.

Frost Creep and Solifluction—Frost creep and solifluction are probably the most common forms of mass wasting in permafrost environments. They occur on slopes as gentle as three degrees and often create lobes, sheets, and terrace-like features. Frost creep is a downslope movement of soil particles caused by frost heaving of the ground perpendicular to the slope, followed by vertical settling upon thawing. Solifluction is the slow downslope flow of soil due to permafrost preventing the absorption of surface water into the subsurface, causing the surface sediments to be water-saturated.

The elevation of well-developed solifluction features in central Alaska increases from west to east. Near Nulato they occur predominantly near 1,300 feet (396.2 m) elevation; near Fairbanks they are near 3,000 feet (914.4 m) elevation; and farther east near Eagle, they occur at nearly 4,000 feet (1,219.2 m) elevation.

Stone Nets, Garlands, and Stripes—Stone nets are soil structures with centers of clay, silt, and gravel and roughly circular or polygonal borders of coarse stones. When isolated they are known as stone rings. Their netlike arrangement may extend downward as much as two feet, and they have diameters from a few inches to more than 30 feet. Stone nets are found on flat or nearly flat ground. On slopes of moderate inclination (about five to 15 degrees) they are drawn out downslope by solifluction into tongue-like or elliptical shapes known as stone garlands. On steeper slopes (five to 30 degrees) garlands give way to stone stripes—parallel stony and earthy bands.

 

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