Periglacial

Periglacial (composed of gr peri, "around, around " and Latin glacies " ice " ) referred to in physical geography unvergletscherte areas that are dominated by the effects of frost. Moreover, with, periglacial ' refers to the geomorphic processes occurring there, and the climate which prevails in such areas.

The term periglacial

The term " periglacial " was coined in 1909 by Lozinski and should geomorphological processes and thereby resulting surface shapes in the vicinity of glaciers, respectively. This close spatial binding to the direct vicinity of glaciers is no longer part of the definition, since the decisive factor of the Periglazials is the frost. Dominated by the frost areas may be far removed from today's or prehistoric glaciation occur, for example in central Siberia. The term periglacial through this change of meaning has become misleading was retained, since attempts a new terminology (especially Washburn: " Geocryology " ) were not successful.

In the 1960s, the concept of Tricart and pewe was redefined. Your definition shows up today after-effects: These authors tied the term periglacial ' to the presence of permafrost. This had the advantage that the limits of Periglazialgebiete could be determined relatively easily. In German, general - geomorphological literature, this definition has also partially preserved, among professional scientists but it is today unanimously rejected, which also corresponds to the international literature stand. The reason for this rejection is to be found in the fact that two of the most important, by all authors to the periglacial geomorphic processes counted ( Gelifluktion and Kryoturbation, see below) are clearly not limited to areas of permafrost.

Thus today periglacial is accrued at least these two processes for the occurrence majority. However, this does lead to a coherent in relation to the geomorphological processes and procedures definition, but complicates an exact demarcation, as opposed to two-year, random observation of Permafosts now consuming measurements of process action would be required. Although caused by the processes mentioned very specific surface forms, but is often difficult to decide whether caused or tart. Among prehistoric, formerly periglacial conditions

The ambiguity of the term has meant that on several attempts have been made to separate aspects of new terms from the total complex of the Periglazials. Thus, the term " Paraglazial " for the direct area of glaciers has been introduced in the narrow sense yes next to the periglacial the glacial formation and their long-range effects by meltwater play a significant role. In the German technical language we find the term " periglacial " with which the periglacial processes are summarized. All these terms were to prevail, however, hardly.

Periglacial processes

Periglacial processes are characterized by a permanent or seasonal frozen subsoil. In summer, the upper soil is thawed ( Auftauboden ) and thus susceptible to fluvial erosion processes for mass- movements and with greater drought for deflation. These processes create characteristic sediments and geomorphological manifestations.

The processes can be divided into those that are associated with no or at most small -scale relocation of substrate, ie essentially are restricted to shallow relief:

• Frost weathering,
• Kryoturbation by frost heave,
• Deep frost shrinkage in the permafrost, leading to volume loss in ice temperatures below about -20 ° C,
• Eisintrusion, which means that the pore volume of sediment affected no longer sufficient to accommodate the enlarged by freezing ice volume, so that ice lenses or layers form, which by their growth pressure on the surrounding substrates exercise (frost thrust and deformation- ); with strong ( frozen lake sediments with primary high cardinality and large water contents ) and in particular sustainable ( artesian or thermal ) water supply, even large forms ( pingos ) can thereby arise
• Formation of Segregationseis, when caused by hygroscopic migration of pore water to the freezing front towards ice lenses or layers in the substrate, which may considerably enhance the effects of Eisintrusion,
• Thermokarst,

And in processes with spatial displacement of material, ie on inclined slopes or in Hangfußbereich, where the impact of the effects of a nearby slope, or vegetation-free areas that offer the wind attack options:

• Gelifluktion,
• Nivation,
• Avalanche,
• Periglacial Spüldenudation (see denudation )
• Deflation.

Periglacial forms

Periglacial forms in the strict sense are those that occur in this form in periglacial and which are closely tied either at least on seasonal ground frost:

• Frost pattern floor,
• Bottom of the bag,
• Thufur ( Isl ), an up to 2 m by measuring and ½ m high, rounded hills, a core of Segregationseis usually has that has bulged the substrate overlying
• Nivationsnische that occurs locally where snow piles promote Nivation a long time,
• Schneehaldenmoräne ( Protalus Rampart ), a special form of boulders which is deposited at some distance from the wall in winter, from which the material is overthrown, but which still continues to roll over a snow pile on the base of the wall beyond,

Or normally associated with permafrost:

• Pingo,
• Palsa,
• Rock glaciers,
• Ice wedge, which can be seen in the form of polygonal Eiskeilnetzen as surface shape
• Frost pattern floors and pocket bottoms are then viewed as phenomena of permafrost when they reach sizes > 60 cm.

In a broad sense forms are counted among the periglacial, which also can occur in other conditions, but frequently occur in periglacial or specifically targeted by the periglacial conditions:

• Valley terraces have arisen largely climatically controlled in the mid-latitudes and have been modified by tectonic processes at best. They go back to a cyclic sequence of certain periglacial processes. The beginning of a cold period but already disturbed vegetation So leads still relatively warm and thus abundant sources of moisture ( oceans ) to strong, temporally concentrated meltwater runoff, which manifest themselves in the rivers by lateral and vertical erosion. With decreasing runoff the Gelifluktion gaining in importance, so that the quasi rivers in sediment, drown ', which they can not fully carry on. In the late glacial climate warming leads to the melting of the permafrost cold temporally stored and thus back to the cutting of the rivers. Due to several changes during the Pleistocene stepped Talquerprofile have emerged in most valleys of the former Periglazialgebiete that mirror the sequence of depression and gravel accumulation. Glacier meltwater these processes can help, but are not necessary for the formation of valley terraces. The floor plans of the temporal cold rivers were usually branched, which explains the broad education of most valley floors.
• Hang dents are mostly small, trough-like little valley, which were dug out of snow meltwater in relationships.
• Asymmetric valleys have a steep and a shallow sloping side of the valley. There are several possible explanations for them, of which the theory of a washout of the steeper Talhangs by a abgedrängten due to the effect of wind power line the most, but can not explain all cases.
• Dunes
• Lößdecken
• Ventifacts

Periglacial sediments

The sediments settled in periglacial exclusively and those members who preferred, but arise not only under periglacial conditions. However, since only the outer layers are created uniquely periglazial and this is already the loess at least debatable, this differentiation is omitted here:

• Periglacial layers ( periglacial layers ) are sediments, which primarily arise from Gelifluktion, but where still partially an admixture of aeolian sediment plays a role.
• Loess
• Blockfields
• Quicksand
• Spülsedimente
• Gravel body of the river terraces

These sediments can be overprinted by phenomena such as ice wedges or frost compressions, making their interpretation is supported as periglacial sediments.

Periglazialgebiete

As Periglazialgebiete refers to areas in which periglacial processes act.

Periglazialgebiete can be found today in the Polar and Subpolargebieten Earth ( Arctic, North America, northern Asia, northern Scandinavia and unvergletscherte areas of Antarctica ).

Due to the decrease in temperature with height all the high mountains have a periglacial uplands ( in the tropics: > 4000 m ü.d.Meer, at mid-latitudes, such as the Alps :> 2000 m ü.d.Meer ). Overall, around 25 % of the fixed land area of permafrost are covered, the proportion of Periglazialgebiete is therefore greater.

In the cold periods of the Ice Age, the Periglazialgebiete stretched far from equatorward and included, for example, the whole of Central Europe. In this way, landscapes in Central Europe were formed, which were not covered by ice, and that are therefore periglacial forms and deposits are still widely used today.

Periglazialklima

Periglazialklimate are climates that allow periglacial processes.

A definition of Periglazialraums in its entirety by exact climatic indicators is not possible, because ultimately the interaction of several climatic parameters ( in addition to temperature and snow cover, water balance and many others) with azonal factors (relief, substrate) decides on the realization of periglacial processes and forms. An attempt of classification of the Periglazialgebiete on the basis of zonal location, continentality and altitude in connection with an assignment climatic limits to the individual types undertook Map 1979.