Periglacial: Meaning and Mechanism | Glaciers | Geography

In this article we will discuss about:- 1. Meaning of Periglacial 2. Features of Periglacial 3. Mechanisms 4. Genetic Classification.

Meaning of Periglacial:

The term periglacial literally means around the ice or peripheral to the margins of the glaciers but now this term is used for both ‘periglacial landscape’ and ‘periglacial climate’. Periglacial areas are those which are in permanently (perennially) frozen condition but without permanent ice cover on the ground surface. The periglacial climate is characterized by mean an­nual temperature ranging between 1°C and – 15°C and mean annual precipitation of 120 mm to 1400 mm (mostly in solid form).

In fact, periglacial areas are characterized by permanently frozen subsoil (perma­frost), seasonally thawed topsoil (active layer), fre­quent changes of temperature and an incomplete veg­etation cover. The term periglacial was first used by W. Lozinski in 1906, though some subglacial processes were already described by earlier scientists e.g., nivation process by F.E. Matthes in 1900, subglacial climate and process (dominanted by solifluction) by J.G. Anderson in 1906 etc. Later on D.D. Cairnes (equiplanation), H.M. Eakin (altiplanation), B. Hogbom (frost heave) etc. studied different periglacial proc­esses.

Periglacial areas are of two types viz.:

(a) The present day periglacial zones are found in the Arctic regions of Alaska, Canada, Greenland and Sibe­ria and also in Antarctica, and

(b) The fossil zones of Pleistocene and other past Ice Ages. Permafrost and active layer are the two most striking features of periglacial areas.

Features of Periglacial:

The most striking feature of periglacial areas is the permafrost or permanently fronzen ground without permanent ice cover. The term permafrost was first used by S.W. Muller while K. Bryan used pergelisol (pergelisol = per, meaning permanently + gelare, mean­ing to freeze + solum, meaning soil = permanently frozen soil). The depth of permafrost varies from place to place. The greatest depth of 600 m has been discovered near Nordvic (northern Siberia). The depth of perma­frost has been noted as 500 m in Taymyr Peninsula of Siberia, 314 m near Cape Simpson in Alaska and 450 m in northern Canada.

Permafrost is classified in 3 categories e.g.:

(i) Continuous permafrost,

(ii) Discontinuous perma­frost, and

(iii) Sporadic permafrost.

About 50 per cent and 47 per cent areas of Alaska and Canada and earstwhile USSR are covered by continuous and dis­continuous permafrost. Nearly 20 to 25 per cent of the geographical area of the globe is covered by perma­frost.

Active Layer:

The top layer of permafrost is called active layer which is characterized by diurnal freeze (during night) and thaw (during day time) cycle during the interven­ing periods of summer and winter seasons. It is com­pletely frozen during winter and is thawed during late summer. The depth of active layer ranges from a few centimetres to 3 metres. All the periglacial processes viz., congelifraction, congelifluction, frost heave, nivation etc., operate in the active layer and the main driving force of the periglacial processes is related to thermal conditions i.e., seasonal and diurnal change of temperature.

The active layer thaws during day time and freezes during night as temperature rises above 0°C but remains completely thawed during late sum­mer. This layer again freezes (during night) and thaws (during day) with the onset of winter season and becomes completely frozen during winter season.

This is why this layer is called active layer because this becomes active due to alternate freeze and thaw mecha­nism while the permafrost below the active layer is inactive because it is permanently frozen throughout the year. Kirk Bryan used the term molisol (moller meaning thereby to make soft and solum means soil (fig. 23.1.).

Mechanism of Periglacial:

Several periglacial processes have been described by many periglaciologists but they do not agree collec­tively on their exact mode of mechanism. F.E. Matthes (1900) described the process of nivation which becomes active in such climate which is characterized by alter­nate freeze and thaw due to changes in temperature but there is no complete glaciation.

J.G. Anderson (1906) described the process of solifluction which becomes operative in subglacial climate. Subsequently, the proc­esses of ‘planation by frost action’, equiplanation—a process of down-wastage responsible for the planation of land, altiplanation—a special process operating over the hill tops and hillslopes, frost heaving, cryoplanation, were described by various scientists. The main periglacial processes include congelifraction or frost weathering, congelifluction or solifluction or soil creep, frost heave, nivation, cryoturbation or cryoplanation, aeolian and fluvial processes.

1. Congelifraction:

Weathering processes under periglacial climates include ‘freeze-thaw action’, ‘contraction cracking’ and chemical weathering but freeze-thaw action is by far the most active mechanism of rockshattering. Congelifraction, simply known as frost weathering (Latin word congelare-to freeze and fracture-to break), includes freezing of moisture and water during night and subsequent thawing during daytime (in summer) making a complete ‘diurnal freeze thaw cycle’ which disintegrates the rocks because of continuous alternate expansion and contraction.

Freeze-thaw is confined only to the active layer because it experiences tempera­ture changes. A few periglaciologists have expressed their doubt about the effectiveness of congelesifraction. Apart from the voices raised against the effectiveness of freeze-thaw weathering it is only significant process for rock-weathering and commination of larger blocks under periglacial conditions.

2. Frost Heaving:

Frost heaving is connected with freeze thaw cycle but it is given separate entity as it helps in moving the coarse grains upward. Frost heaving is defined as bulging and subsequent subsidence of the ground sur­face by expansion in water to form ice.

Frost heave works in two ways viz.:

(i) Heaving by vertical thrust, and

(ii) Heaving by leteral thrust.

Ice segretation is one of the most important process of frost heave.

Frost susceptible grains (0.01 mm or less in size) in the ‘active layer’ are frozen during night in summer and, therefore, they expand their sizes by 9 to 10 per cent. The repetition of this process results into ice segregation which includes ice in several forms like ice lenses, ice layer parallel to the ground surface, pipkraker etc. Slowly and slowly segregated ice grows in size and heaves up coarser unfrozen materials coming from nearby areas.

The repetition of this mechanism brings larger particles on the ground surface and it appears as if ground vomits stones. This vertical thrust gives rise to the formation of patterned ground including stone circles, stone polygons, stone nets, stone garlands and stone stripes. I. Belockrylov explained and validated the functioning of the mechanism of frost-heave through an experiment (fig. 23.2). It is apparent from fig. 23.2 that a wooden log fixed in the active layer was pushed upward by the process of frost heaving and it ulti­mately fell on the ground.

Lateral thrust is caused when the upper part of thawed active layer starts freezing with the advent of winter season and proceeds downward. Meanwhile there is an intermediate layer in unfronzen state be­tween the frozen part of active layer above and perma­frost below unless the whole of active layer is frozen during late winter.

The intervening unfrozen but mo­bile zone comes under heavy pressure exerted by expanding and growing frozen part of active layer. This vertical pressure sets up lateral thrust in the constituent materials of intervening layer causing heav­ing of upper layer.

Segregated ice in the frozen active layer in late winter also heaves up the ground surface forming pingos and contorted surface. Frost heaving also helps the congelifluction process (transportation of water soaked soil downslope).

3. Congelifluction:

The process of debris-movement in periglacial regions has been variously defined and a number of terms have been suggested. First J.G. Anderson (1906) proposed the term solifluction (solum-soil, fluere-flow) for slow movement of debris, soaked with water, from higher to lower slopes. Solifluction term was replaced by congelifluction of J. Daylik (1951) to incorporate only soilflow in the periglacial climate having perma­frost lying below an active layer. K. Bryan (1946) used the term cryoturbation which included all types of mass-movement of regolith under periglacial environ­ment. Recently, gelifluction is used in place of congelifluction.

The mechanism, rate of movement and morpho­logical effects of solifluction (congelifluction) need critical analysis. Ground surface freezes during winter season resulting into extensive formation of ground ice which causes expansion in the active layer so that a state of minimum packing of grains is activised result­ing into loosening of soil texture.

Frozen ground sur­face starts thawing with the beginning of summer providing sufficient water in the active layer to reduce its shear strength. The melt-water acts as a lubricant with the result water soaked debris finds it easy to flow downslope under the force of gravity. The local condi­tions including grain-size, composition of materials, amount of melt-water, depth of freeze-thaw and the presence of vegetative cover control the rate of solifluction.

It may be pointed out that the rate of solifluction is exceedingly slow as it ranges between 3 cm and 30 cm per year. Solifluction is most active during transitional periods between winter and sum­mer seasons as these periods experience active freeze- thaw cycles. Solifluction stops during late summer because melt-water is evaporated and during winter because all moisture is frozen.

Solifluction is mainly a process of transporta­tion of regoliths but it also smoothens relief while passing over it and hence has some geomorphlogical significance. Solifluction helps in cryoplanation (planation by frost action) by smoothening the interfluves and by aggradation of valley floors. Solifluction also helps in the sculpturing of patterned ground. Differen­tial movement of debris over the slope forms stone- banked terraces and lobes, stone streams, earth wrin­kles etc. (erosional features). Talus cones, and plications may be regarded special features of solifluction.

4. Nivation:

Nivation is a wide term which includes a variety of sub-processes related to the snow patches either immobile or semi-permanent. The process of nivation includes the sub-processes of weathering under a snow- patch, melt-water erosion from beneath a snow patch and downhill erosive creep of water saturated snow. Since snow-patches are considered to be immobile and hence corrasive power cannot be assigned to them but freeze-thaw is most effective around the edges and at the bases of snow-patches.

5. Fluvial Process:

It is commonly agreed that the running water as an erosive agent under periglacial conditions is of less significance than other processes as flow of stream depends on the mercy of temperature changes. There is marked irregularity and fluctuation in the stream flow.

Winter freezing suspends all streams flows whereas summer thaw provides some water but the streams become sluggish because they are overloaded by large amount of detritus fed by solifluction and hence over­loaded streams cease to be an active erosive agent. Sometimes, sudden flux of water by summer thaw results in floods of catastrophic dimension charged with enormous erosive power.

According to J.L. Jenness (1952) periglacial streams are powerful erosive agents in Arctic Canada as they have carved out ravines and gullies of 60-90 m depth. On the other hand, L.C. Peltier and A. Rapp have opined that streams in the periglacial areas are only the agents of transportation of debris. J. Corbel has maintained that the streams in maritime arctic climate are capable of removing debris coming from the side walls of the valley and making vertical erosion whereas the streams become sluggish in continental arctic climate because of supply of enormous amount of debris which limit the flow of streams resulting into aggradation.

6. Eolian Process:

Wind action becomes effectively operative in the penultimate stage of periglacial cycle when the relief is subsequently reduced resulting into gentle slope of 5° or less and weathering materials are commi­nuted so finely that they can easily be transported by wind. It is obvious that wind sweaps away the fine grains from one place and deposits them in other-places making erosional (faceted, flutted and grooved sur­faces) as well as depositional (loess and sand deposts) features.

Genetic Classification of Periglacial Landforms:

Savindra Singh (1974) attempted to present genetic classification of periglacial landforms (fig. 23.3) on the basis of the works of various periglaciologists, although many exponents are not unanimous on the exact mode of their origin. It is difficult to name a particular landform on the basis of a single process, as generally many processes operate collectively at varying magnitudes in producing a single landform.

Fig. 23.3: Periglacial landforms

In such cases the macro-level landforms have been named after the most dominant process. Regional variations in the general characteristics of the periglacial landforms pose further difficulties in their genetic classification because periglacial areas are so varied in nature that their complete similarity becomes a remote possibility.

The variations in the periglacial landforms are largely due to the differences found in permafrost, depth of active layer, geological structure, vegetation cover, mechanism of periglacial processes etc. Further subdivisions of these landforms have been made on micro-levels on the basis of their size, shape, pattern, stage of formation, erosion, deposition etc.

In the classification the following abbreviations have been used to designate different processes responsible for the genesis of these landforms:

C = congelifraction

S = solifluction

FH = frost heave

N = nivation

1. Congelifractate landforms:

(a) Involutions^FH,C (on the basis of shape):

(i) Fold involutions

(ii) Pillar involutions

(iii) Amorphous involutions

(b) Hummocks^C (on the basis of form):

(i) Earth hummock

(ii) Turf hummock

(iii) Mima mound

(iv) Palsa

(c) Pingo^c (on the basis of the form of the top):

(i) Rounded-top pingo or closed pingo

(ii) Crater pingo or open pingo (on the basis of regional characteristics)

(i) Mackenzie-type pingo (closed pingo)

(ii) East Greenland-type pingo (open pingo) (on the basis of shape)

(i) Circular pingo

(ii) Elongated pingo

(iii) Fossil pingo

(d) Thermokarsts^c (on the basis of size):

A-macro-landforms (due to subsidence of ground surface)

(i) Thermokarst or thaw lakes

(ii) Cauldron subsidence or subsidence ba­sin

(iii) Dry valleys

(iv) Caves

B-micro-landforms (due to minor subsidence of surface)

(i) Funnel shaped sink and pit

(ii) Sink holes

(iii) Thaw sink

C-fossil thermokarsts

(e) Frost-riven cliffs:

(f) Frost polygons:

2. Patterned Ground^FH,s (on the basis of shape):

(1) Gentle slope-seated landforms:

(i) Stone cricles: sorted and unsorted.

(ii) Stone nets: sorted and unsorted.

(iii) Stone polygons: sorted and unsorted.

(2) Steep slope-seated landforms:

(iv) Stone garlands: sorted and unsorted.

(v) Stone stripes: sorted and unsorted.

3. Contorted surface^FH,C:

(1) Frost heave-riven contorted surface.

(2) Freeze-thaw-riven contorted surface.

4. Solifluctuate Iandforms^S (due to differences in the movement of solifluction):

(1) Solifluction Terraces:

(i) Stone-banked terraces.

(ii) Turf-banked terraces.

(2) Solifluction L obe:

(i) Stone-banked lobe.

(ii) Turf-banked lobe.

(3) Plication:

(4) Depositional Landforms:

(i) Talus or scree.

(ii) Stratified scree.

(5) Stone Streams:

5. Applanation landforms ^S,N,FH,C (polyprocess or high altitude landforms):

(1) Frost-riven landforms having moving veneer of debris:

(i) Altiplanation terraces.

(ii) Altiplanation cliffs.

(2) Landforms due to differential weathering:

(i) Tors or ‘stone cities’.

(ii) Frost-riven cliffs.

(iii) Boulder fields.

(3) Depositional landforms:

(i) Block fields.

(ii) Stone streams.

6. Nivation Landforms^N:

(1) Landforms due to snow-patches:

(i) Nivation hollows (on the basis of shape)

(a) Longitudinal hollows.

(b) Transverse hollows.

(c) Circular hollows.

(2) Depositional landforms:

(i) Nivation terraces.

(ii) Nivation platforms.

(iii) Nivation ridge.

(iv) Nivation fan.

7. Eolian landforms:

(1) Erosional Landforms:

(i) Grooved bedrock surfaces,

(ii) Periglacial loess, and 

(iii) Ventifacts.

(2) Depositional Landforms:

(i) Periglacial loess.

(ii) Periglacial sand dunes.

8. Periglacio-fluvial Landforms:

(i) Asymmetrical valleys,

(ii) Thaw-gullies, and

(iii) Thaw-ravines.

Involutions:

Are contorted forms of stratified deposits of unconsolidated materials just below the ground surface of the permafrost areas. They are formed due to squeezing and buckling of stratified but unconsolidated materials. Sometimes the interpenetration of deposits due to acute squeezing is so com­plex that the form of the deposits is so greatly distorted that their original forms cannot be identified.

On the basis of shape involutions are divided into:

(i) Fold involution,

(ii) Pillar involutions, and

(iii) Amorphous involutions.

Hummocks:

Small upstanding wrinkles on the surface of permafrost are called hummocks. These are formed due to squeezing of the ground surface because of lateral pressure exerted by freezing of active layer.

The squeezing of the ground surface results in the formation of small knots. According to Taber hum­mocks are formed due to frost heaving.

Hummocks are divided into two types on the basis of presence or absence of vegetation i.e.:

(i) Turf hummocks (with small vegetation), and

(ii) Earth hummocks (without vegetation).

Palsa:

A special category of hummock found in swampy areas and composed of peats having thin ice layers inside is called palsa which is about 10m in height and 10 to 20 m in diameter and is mostly found in the periglacial environment of arctic and subarctic areas. Palsa may be destroyed by rise in water table in the nearby swamps or by fracture in its upper surface. It is formed by frost heaving under the influence of ice segregation.

Pingos:

Pingo is an Eskimo word which means isolated dome-like low mounds or hills found in per­mafrost areas. This word was first used by A.E. Porsild in the year 1938. They are found in continuous as well as discontinuous permafrost areas. They are abun­dantly found in the arctic areas (65° N ) of Canada, Alaska, Greenland and Siberia. They range in height from a few metres to 60 metres (sometimes they are as high as 100 metres) and from a few metres to 300 metres in diameter. Small pingos have closed tops whereas big pingos have open tops.

They are classified in two types on the basis of shape:

(i) Closed pingos, and

(ii) Open pingos (fig. 23.4).

According to frost heaving hypothesis pingos are supposed to be formed in two ways:

(i) They are formed due to rise of ground surface caused by lateral thrust in the materials of active layer. Lateral thrust is caused due to downward increasing pressure exerted by freezing upper surface of actie layer,

(ii) The ice lenses formed in the active layer grow in size to form segregated ice due to ice-segregation. This segregated ice exerts pressure upward and the ground surface rises to form dome-shaped pingos.

According to J. R. Mackay (1962) pingos are not formed due to frost heaving. According to him pingo can be formed due to frost heaving only when the materials are frost susceptible i.e., theirdiameteris0.02 mm or less whereas the materials involved in the formation of most of pingos have diameter of the particles between 0.1 and 0.5 mm. According to him pingos are formed in the following manner. There is a frozen lake which is surrounded by permafrost. There is unfrozen land beneath the frozen lake (fig. 23.5-A).

There is still some water at the bottom of the lake but slowly and slowly the whole water is frozen (fig. 23.5-B) and thus unfrozen land is trapped between the lake ice and permafrost. Expanding permafrost exerts pressure on unfrozen land (fig. 23.5- B) and hence unfrozen portion causes the ground surface to rise upward (fig. 23.5-C). The water of the unfrozen land is converted into ice which collects and grows just below the bulge (fig. 23.5-D). This ice core causes further rise in the bulge. Thus, a pingo is formed.

Thermokarst:

Thermokarsts, though similar to the karst topography of carbonate rocks in appearance, vary significantly from the karstic landforms because the latter are formed due to chemical reactions of water and consequent dissolution of carbonate rocks (lime­stones and dolomites) whereas thermokarsts are formed due to thawing of frozen ground in permafrost areas because of changes in thermal conditions.

In fact, thermokarst refers to negative landforms (sinks and depressions) which are formed due to collapse of ground surface because of thawing of the ice of the active layer of permafrost due to rise in temperature. Thus, the karstic landforms are lithologically controlled whereas the thermokarstic landforms are thermally controlled.

Thermokarsts include surface pits, sinks (fun­nel sinks, sink holes), hollows, ravines, dry valleys, caves, thaw lakes, subsidence cauldron etc. These landforms are formed due to collapse of upper surface because of melting of ice in the active layer due to rise in temperature.

The change in the temperature of the rocks may be due to:

(i) Removal of vegetal cover, ploughing of the land and construction of tanks and lakes, and

(ii) Climatic change.

Patterned Ground:

Patterned ground refers to the development of such landforms in the periglacial areas which have the geometrical shapes (like circles, polygons, nets, stripes and garlands) and are so sys­tematically arranged according to the ground slope that they look like patterns of landforms as if arranged by man.

A.L. Washburn (1956) classified the patterns on the basis of their shapes and sorting of materials in 5 types e.g.:

(i) Stone circles (sorted and unsorted),

(ii) Stone polygons (sorted, and unsorted),

(iii) Stone nets (sorted and unsorted),

(iv) Stone stripes (sorted and unsorted), and

(v) Stone garlands (sorted and unsorted).

These features are further divided into two groups on the basis of slope gradient e.g.:

(i) Patterns developed on flat ground surface having slope gradient up to 6° (e.g., stone circles and stone polygons), and

(ii) Patterns developed on slopy surface having the slope gradient between 6° and 30° (e.g., stone nets, stone stripes and stone garlands) (fig. 23.6). Frost heaving and solifluction play major roles in the development of patterned ground.

Stone Glacier:

Stone or rock glacier consists of two layers. The upper layer carries rock pieces and rock blocks while the lower layer consists of silt, sand, mud etc. These materials are moved upward and are brought to the ground surface because of frost heaving. There are contrasting opinions about the movement of stone glaciers. According to A. Chaix (1923, 1943) uper part of rock glacier moves down the slope at the rate of 1.0 to 1.5 m per year whereas the lower part moves at the rate of 0.3 m to 1.0 m per year.

Block Fields:

Block field refers to the natural collection of large stone blocks at the flat surface of the hill tops in the periglacial areas. These block fields are also called blockmeer and falsenmeer. The stone blocks are formed due to frost weathering (congelifraction).

Stone Streams:

Accumulations of rock debris in the valley floors are called stone streams or boulder fields. The stone is well sorting of rock debris in the stone streams. The upper layer consists of large and coarse debris while the lower layer is dominated by fine materials. Water channel is developed between the valley walls and stone stream. Sorting of rock debris occurs through the process of frost Heaving. Stone streams move downslope due to the force of gravity, frost heaving and solifluction.

Applanation Terraces:

Flattened summits and bench-like features developed on higher altitudes of spurs and hill sides in periglacial areas are called applanation terraces. These terraces are separated by scarps which range in height from 2m to 12 m. Terraces are 10 to 90m long and upto 800m wide. The angle of the scarps varies from 15° to 22°. The base of the scarps may be characterized by frost riven cliffs. These ter­races are the result of the process of applanation which includes frost sapping, congelifraction and congelifluction (solifluction).

Tors:

Tors, one of the most controversial landforms, are piles of broken and exposed masses of hard rocks having a crown of rock blocks of different sizes on the top and clitters (trains of blocks) on the sides. The rock blocks, main components of tors, may be cuboidal, rounded, angular, elongated etc. in shape.

They may be seated at the top of the hills, on the flanks of the hills or on flat basal platforms ranging from 6 m to 30 m in height. They are found in different climates varying from cold to hot and dry to humid. Though tors have developed over almost all types of rocks but they are frequently found in the regions of granites.

Various theories of tor formation have been put forth (e.g., 1. Pediplanation Theory of L.C. King, 2. Deep Basal Weathering Theory of D.L. Linton, 3. Periglacial Theory of J. Palmer and R.A. Neilson, 4. Two-Stage Theory of J. Demek, 5. Glacial Theory of R. Dalh etc.) but there is no unanimity among the expo­nents because tors are not confined to a particluar rock type and climate. In periglacial areas tors are formed due to weathering of rocks along the joints through the process of congelifraction (frost weathering due to freeze and thaw cycle) and removal of weathered mate­rials through the process of congelifluction (solifluction).

Nivation Hollows:

Hollows produced by snow- patch erosion or nivation are called nivation hollows which are generally found along the hillsides in vari­ous forms. They extend from a few metres to 1.5 kilometres.

They are classified on the basis of shape into:

(i) Transverse hollows.

(ii) Longitudinal hollows.