Periglacial (composed of Greek peri, "around, around" and Latin glacies, "ice") describes a geomorphological process in physical geography and geology that affects the landscape-defining effect of frost, but also azonal processes that occur with snow, flowing water and wind are connected. The distinct geomorphological processes that occur in unglaciated areas are shaped by the thawing and freezing of ground ice, which can occur permanently, seasonally or daily. The frost effect must be so intense that it can be detected in the landscape. Areas with periglacial landscapes are predominantly in the continental tundra climate . Landscapes that were periglacial in the geological past are called paraglacial. The adjective 'periglacial' characterizes both the corresponding climatic conditions and the geomorphological processes occurring under these conditions . Also high mountains between the subarctic and the inner tropics between the snow and tree line have landscapes in which periglacial processes take place, these are often referred to as solifluction stage (= "periglacial stage"), because there is soil solifluction due to higher precipitation sums and greater relief energy trains. As in the tundra, there is a development of soil and vegetation with specialized adaptations of plants (alpine frost and rubble vegetation , snow valley communities).
The term periglacial
The term “periglacial” was coined by Lozinski in 1909 and was intended to designate geomorphological processes and the surface shapes that occurred in the immediate vicinity of glaciers. This close spatial connection to the direct vicinity of glaciers is no longer part of the definition, since the decisive factor of the periglacial is the permanent, seasonal or diurnal ground ice. Freezing and thawing of the soil through alternation of frost then cause the periglazal morphodynamics. Areas dominated by frost can occur far away from present-day or prehistoric glaciation, for example in central Siberia . The term periglacial, which had become misleading due to this change in meaning, was retained, as attempts at a new terminology (especially Washburn : "Geocryology") could not prevail.
In the 1960s, the term was redefined by Tricart and Cailleux and Péwé . Their definition shows after-effects to this day: These authors linked the term 'periglacial' to the occurrence of permafrost . This had the advantage that the boundaries of the periglacial areas could be determined relatively easily. In the German-speaking, general geomorphological literature, this definition has also been partially preserved, but it is unanimously rejected today among specialist scientists, which also corresponds to the international literature. The reason for this rejection is to be found in the fact that two of the most important geomorphological processes that all authors count as periglacial ( gelifluction and cryoturbation , see below) are clearly not restricted to areas with permafrost.
Thus, today, periglacial is mainly delimited according to the occurrence of at least these two processes. However, although this leads to a coherent definition with regard to the geomorphological procedures and processes, it makes it more difficult to draw an exact boundary because, in contrast to the two-year, random sampling of the permafost, complex measurements of the process would be required. Although the processes mentioned give rise to very specific surface shapes, it is often difficult to decide whether these were created recently or under prehistoric, formerly periglacial conditions.
The ambiguity of the term has led to the fact that various attempts have been made to separate partial aspects from the overall complex of the periglacial through new names. The term “paraglacial” was introduced for the immediate vicinity of glaciers, in which, in addition to the periglacial i. e. S. also the glacial formation and its long-range effects through meltwater play an important role. In the German technical language, the term "periglacial" can be found, with which the periglacial processes are summarized. However, all of these terms could hardly prevail.
For periglacial morphodynamics, temperature is only partially the decisive criterion. So that frost phenomena have an impact on the landscape, soil moisture, rock lithology, soil texture, and the distribution of rock the size of regoliths are decisive. Frost changes in air and ground temperature are therefore only representative physical quantities for freeze-thaw cycles in the ground ice, which were often taken as determining quantities due to their simpler measurement. The production, presence and melting of bottom ice are actual parameters that cannot be determined using a simple temperature criterion. Interactions with alternation of frost are only transferred to periglacial processes via certain soil properties.
Periglacial processes are characterized by a permanently or seasonally frozen subsoil. In summer, the topsoil is thawed ( thawing soil ) and is therefore susceptible to fluvial erosion processes , mass self-movements and, in the case of greater drought, also to deflation . These processes create characteristic sediments and geomorphological manifestations.
The processes can be subdivided into those that are associated with no or at most small-area displacement of the substrate, i.e. are essentially limited to a flat relief:
- Frost weathering ,
- Cryoturbation through frost stroke ,
- Deep frost shrinkage in the permafrost soil , which leads to volume losses at ice temperatures below approx. −20 ° C,
- Ice intrusion , which means that the pore volume of the affected sediment is no longer sufficient to absorb the ice volume increased by freezing, so that ice lenses or layers form, which exert pressure on the surrounding substrates through their growth (freezing thrust and compression); with strong (frozen lake sediments with primarily high thickness and large water contents) and especially sustainable ( artesian or thermal ) water supply, large forms ( pingos ) can also arise,
- Formation of segregation ice , in which the hygroscopic migration of the pore water towards the freezing front creates ice lenses or layers in the substrate, which can considerably intensify the effects of the ice intrusion,
- Thermokarst ,
and in processes with spatial displacement of material, i.e. on inclined slopes or in the foot of the slope, where the influences of a nearby slope have an effect, or in areas free of vegetation that offer the wind attack opportunities:
- Gelifluction ,
- Nivation ,
- Avalanche ,
- Periglacial irrigation denudation (see denudation ),
- Deflation .
Periglacial forms in the narrower sense are those that only occur in this form in periglacial areas and that are either closely linked to at least seasonal ground frost :
- Frost pattern floor ,
- Bag bottom ,
- Thufur (Isl.), A round hill up to 2 m in diameter and ½ m high, which usually has a core of segregation ice that has arched the substrate above ,
- Leveling niche that arises locally, where piles of snow promote nivation over a longer period of time,
- Snow pile moraine ( Protalus Rampart ), a special form of block pile that is deposited in winter at some distance from the wall from which the material fell, but which then rolls over a snow pile over the base of the wall,
or commonly associated with permafrost:
- Pingo ,
- Palsa ,
- Rock glacier ,
- Ice wedge , which can also be recognized as a surface shape in the form of polygonal ice wedge networks ,
- Frost pattern floors and pocket floors are considered to be permafrost phenomena when they reach sizes> 60 cm.
In a broader sense, forms are counted among the periglacials that can also arise under other conditions, but which occur more frequently in periglacial areas or are particularly promoted by the periglacial conditions:
- In the middle latitudes, valley races were largely climatically controlled and were modified by tectonic processes . They go back to a cyclical sequence of certain periglacial processes. The beginning of a cold period with still relatively warm and thus abundant sources of moisture ( oceans ) but already disturbed vegetation leads to strong, temporally concentrated meltwater runoff, which manifests itself in the rivers as lateral and deep erosion. Gelifluction becomes more important with decreasing discharge volumes, as a result of which the rivers 'drown' in sediment, which they can no longer fully transport. In the late glacial period , global warming leads to the melting of the permafrost stored in the cold period and thus to the rivers cutting in again. As a result of the multiple changes in the course of the Pleistocene , stepped valley transverse profiles were created in most of the valleys of the former periglacial areas, which reflect the sequence of deepening and accumulation of gravel. Glacier meltwater can support these processes, but is not necessary for the formation of valley races. The ground plans of the cold-age rivers were usually branched , which explains the broad formation of most of the valley floors.
- Slope dents are usually small, trough-like valleys that have been deepened into slopes by snowmelts .
- asymmetrical valleys have a steep and a flatter sloping valley slope. There are several possible explanations for it, most of which , if not all, can be explained by the theory of the underwashing of the steeper valley slope by a stream line pushed away by the action of wind .
- sand dunes
- Loess blankets
The sediments, too, can be divided into exclusively periglacial and those that preferentially, but not only, arise under periglacial conditions. However, since only the top layers are clearly periglacial and this is already at least controversial for the loess, this differentiation is omitted here:
- Periglacial cover layers ( periglacial layers ) are sediments that are mainly formed by gelifluction , but in which an admixture of aeolian sediment still plays a role.
- Block heaps
- Drift sand
- Flushing sediments
- Gravel body of the river terraces
These sediments can be overprinted by phenomena such as ice wedges or frost compressions , which supports their interpretation as periglacial sediments.
Areas in which periglacial processes are at work are called periglacial areas.
Due to the decrease in temperature with altitude, all high mountains have a periglacial altitude level (in the tropics:> 4000 m above sea level; in middle latitudes, e.g. the Alps :> 2000 m above sea level). Overall, around 25% of the earth's mainland area is covered by permafrost, so the proportion of periglacial areas is even greater.
In the cold ages of the Ice Age , the periglacial areas extended far towards the equator and included, for example, all of Central Europe . In this way, landscapes were also reshaped in Central Europe that were not covered by inland ice , and in which periglacial forms and deposits are still widespread today.
Periglacial climates are climates that enable periglacial processes.
It is not possible to define the periglacial space in its entirety by means of exact climatic measured variables, since ultimately the interaction of several climatic parameters (in addition to temperature, snow cover, water balance, etc.) with azonal influences (relief, substrate) determines the occurrence of periglacial processes and forms. In 1979, Karte made an attempt to classify the periglacial areas on the basis of zonal location, continentality and altitude in connection with an assignment of climatic limit values to the individual types.
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