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Results for permafrost:

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permafrost

CATEGORY: geology
DEFINITION: Permanently frozen subsoil with a temperature below 0 C (32 F) continuously for two or more years. The permafrost line is a line demarcating regions where the subsoil is permanently frozen. Permafrost is overlain by a surface layer that is subject to thawing during the warmer seasons of the year. This zone of seasonal freezing and thawing is termed the active layer. Permafrost is related to the tree line, because the frozen ground prevents tree roots from penetrating deeply and inhibits the subsurface drainage of meltwater. Permafrost is estimated to underlie 20 percent of the Earth's land surface and reaches depths of 1,500 meters (5,000 feet) in northern Siberia. It occurs in 85 percent of Alaska, more than half of Russia and Canada, and probably all of Antarctica. Permafrost has preserved the carcasses of extinct Ice Age mammals; one or two almost complete, frozen mammoths dating from at least 10,000 years ago have been reported from Siberia.

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cryoturbation

CATEGORY: geology
DEFINITION: The disturbing or mixing of soil by frost action and the freezing of the active layer of permafrost late in the melting season. The soil in regions close to an ice sheet contains a good deal of water, and when it refreezes after the seasonal thaw the pressure of growing ice crystals tends to rotate and rearrange the stones. The presence of such a structured soil indicates former cold climatic episodes.

fossil ice wedges

SYNONYM: foliated ground ice, wedge ice
CATEGORY: geology
DEFINITION: Soil features caused when the ground freezes and contracts, opening up fissures in the permafrost that fill with wedges of ice. The fossil wedges are proof of past cooling of climate and of the depth of permafrost. Foliated ground ice, or wedge ice, is the term for large masses of ice growing in thermal contraction cracks in permafrost.

gelifluction

CATEGORY: geology
DEFINITION: A geologic process occurring when the active layer of permafrost moves under the influence of gravity. The soft flowing layer is often folded and draped on hillsides and at the base of slopes as solifluction, or gelifluction, lobes. Gelifluction can cause destruction or redistribution of archaeological deposits.

ice wedge

SYNONYM: ice-wedge; foliated ground ice
CATEGORY: geology
DEFINITION: Large masses of ice growing in thermal contraction cracks in permafrost. In periglacial conditions, alternating freeze and thaw can lead to the formation of vertical, narrow, and deep wedges of ice in gravels. After melting, these tend to fill with sediment, forming a cast of the ice wedge seen as dark bands, easily confused with manmade features, in aerial photographs. Casts of fossil ice wedges are one of the few true indicators of former permafrost conditions. Fossil ice-wedges of this kind are seen in many sections of sand and gravel deposits in Europe. They have been used to reconstruct the extent of the periglacial zone which developed around the Devensian and Weichselian ice-sheets.

involution

CATEGORY: geology
DEFINITION: A structure that develops within the active layer of the Periglacial (permafrost) zone. Cryoturbation (seasonal freezing) causes movement within the layer and sorting of its component materials. Involutions help to define the area of ancient periglacial zones but their action can cause disturbance or mixing of archaeological deposits. Involutions may also be confused with archaeological features.

periglacial

CATEGORY: chronology
DEFINITION: A term describing cold-climate processes and landforms, an environment with severe frost in non-glacial conditions and have much ground ice, mass movements, and strong winds. It applies to the region surrounding a glacial area and regions immediately beyond the ice-front during a glaciation. In a periglacial zone, part of the ground is perennially frozen. This so-called permafrost layer is covered by a layer which thaws and freezes seasonally, the active layer. Such seasonal changes give rise to several processes, some of which sort the constituents of the active layer and are collectively known as cryoturbation. A variety of landforms, including involutions, ice wedges, and pingos, are formed in the active layer and permafrost. Hillslopes become mantled with frost-shattered rubble that move downslope during cycles of freezing and thawing. Rivers are usually seasonal in the periglacial zone, and erosion by frost action is dominant. Wind erosion and deposition is often an important factor, and caused the formation of the huge deposits of loess and cover-sands in Europe and Asia. The periglacial zone is of interest because it would have been the environment in which man lived for long periods of time during the Devensian/Weichselian cold stage. During the coldest periods of the Quaternary (the last 1,600,000 years), the periglacial zone was enlarged to approximately twice its present size.

pingo

CATEGORY: geology
DEFINITION: An ice-cored mound that develops within the active layer and permafrost of the periglacial zone. These conical mounds can be up to 40 meters high and 600 meters wide. Layers of ice may separate out by percolation of water, or form by the injection of water-charged sediment from below. When a pingo melts, its center collapses, leaving behind a circular 'rampart' of material (a circular ridge with a central basin, often filled with peat). Pingo ramparts have been used to reconstruct the extent of the periglacial zone which developed around the Devesian/Weichselian ice-sheets.