Climatic factors controlling the distribution of certain frozen ground phenomena

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Geografiska Annaler, 43, 3/4, pp. 339-347, 1962-06-01

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Climatic factors controlling the distribution of certain frozen ground

phenomena

Williams, P. J.

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CLIMATIC FACTORS C O N T R O L L I N G

THE

D I S T R I B U T I O N O F CERTAIN

F R O Z E N G R O U N D P H E N O M E N A

BY P. J. W I L L I A M S

Division of Building Research, National Research Council, Ottawa, Canada

Introduction

Frost action processes can be related to the occurrence of patterned ground, solifluction, and allied natural phenomena to give some ex- planation of their geographical distribution. Knowledge of the conditions under which frost action causes these phenomena facilitates the use of fossil structures of similar origin in in- terpreting past environmental conditions (cf. Flint 1957, p. 205).

Frozen Ground Phenomena Requiring Deep Annual Frost Penetration

This group of frozen ground phenomena in- cludes the several forms of patterned ground and solifluction requiring a minimum seasonal frost penetration of perhaps a metre. Various involutions and contortions observed in strati- graphical sections, and dating from glacial or postglacial time, are believed to have originated from such features. Frost hummocks standing several decimetres high (Fig. 1) exist a t present in relatively warm areas and are possibly the form that has the widest distribution with respect to climate. Irregular hummocky features are abundant even in forest, although less conspicuous there. Gradationally related are similar features, where most of the soil movements occur below the ground surface, the surface expression being less marked and where the term "patterned ground" is inappropriate. It is generally agreed that these phenomena result from the expansions and differential movements of the soil, which are associated with seasonal formation of ice aggregations in the soil by water movement during freezing (frost heave) (cf. for example Penner 1958, Beskow 1935).

Depth of Annual Freeze-thaw

The thickness of the layer of annually freezing soil, therefore, substantially determines the degree and depth of development of this group of frozen ground phenomena.

T h e "frost index", obtained by multiplying mean air temperatures below 0°C by the time during which these temperatures occur, and expressed as -"C hours, is a measure of the intensity of the winter cold. It is often supposed that the latter determines the annual frost penetration (i-e. in non-permafrost areas) and hence the presence or absence of these phenomena. However, the frost index is, in fact, a poor indication of the depth of freeze-thaw in the soil, on other than a rather local basis. Ottawa, Canada, experiences winters of similar severity and frost index values to those of parts of Trollheimen, Norway. Frozen ground phenomena of the type described are virtually absent a t the former place, while the latter is an area of almost treeless tundra with locally striking frozen ground phenomena of several types. An even more striking comparison can be made with Iceland, where many areas of even milder winters have conspicuous solifluction a n d patterned ground features. Table I shows conditions a t several widely dispersed places, together with frost index values and mean annual air temperatures. It appears that the frozen ground phenomena at those places near the southern limit of their development are related to the nearness of the mean annual temperature to 0°C rather than to the frost index value. The relationship of mean annual air temperature to frost index is shown for Canada in Fig. 2.

With mean annual temperatures above On@, the depth of frost penetration decreases, being only perhaps 70 cm a t

+

4°C even with con-

G E O G R A F I S k A A N N A L E R

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X L l l l 1 1 0 B I l

.

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P. J. WILLIAMS

Fig. 1. Frost Hummocks, Rondane, Norway.

siderable intensity of winter cold. I t must be emphasized that the depth of frost penetration may vary by 2 factor of 2 or more within a small area because of variations in thermal properties of the soil or soil cover (cf. f o r example Keranen 1923). The values discussed throughout this paper must therefore be re- garded as characteristic values, subject to local modifications.

F o r mean annual air temperatures several degrees above 0°C the occurrence of a large frost index value must necessarily be arsociated with a correspondingly warm summer (com- prising a continental climate). In such cases, the stored summer heat below the frost line hinders its penetration.

Frost Penetration in Relation to World Climates

T h e frost line ceases to penetrate further into the ground when the loss of heat towards the

ground surface (9,) through the frozen layer, is equalled by that being supplied to the frost line from below (q,,). The loss of heat towards the surface depends on the temperature gradient, together with the conductivity in the frozen and unfrozen ground. T h e magnitude o f this gradient depends in part o n climatic conditions, such a s the air temperature, the nature of the surface cover (snow, vegetation, etc.), and the annual mean ground temperature.

Beskow (1935) proposed the following equation for maximum frost penetration:

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CLlMATlC FACTORS CONTROLLING FROZEN GROUND PHENOMENA

Table I. The development of frozen ground phenomena is seen to be substantially independent of intensity of winter cold (frost index value or the mean monthly temperature of coldest month), but the importance of mean annual

temoerature is indicated. Ottawa, Canada S.E. of Snae- fell, Iceland Location Godthaab, . Greenland Trollheimen, Norway Mean Monthly temperature coldest month (1,) "C Calgary, Canada Lat. No patterned ground or solifluction Solifluction and Hummock formation, spora- dic permafrost6 Hummock formation' Large-scale soli- fluctionZ and patterned ground Patterned ground and solifluction Mean Ann' A I ~ Temp "C

I

1

1 I

(

not reported

'

Extraoolated from observations for Sunndal and Berkak (Det Norske Met. Inst. 1951-55). Long.

So'ifluction and Patterned

Ground

williams (1957). Wilkins and Dujay 1954. Thomas 1953.

Smithsonian Misc. Coll. 1947, 194411927;

"

Extrapolated from observations for Teigarhorn. Thorarinsson, 1951.

'

Bocher 1954. Approx. Frost index value - "C (hr)

where I,, = the maximum frost penetration to = temperature a t frost line (i.e. freez-

ing point)

t, = temperature a t ground surface

1,

= conductivity of unfrozen ground

I.,

= conductivity of frozen ground a = temperature gradient' below frost

line

q,, = heat flow t o the underside of the freezing line

q, = h e a t flow towards the ground surface

This equation is not entirely satisfactory. T h e value to be attached t o t, is uncertain. Beskow assumed t, to equal the monthly mean air temperature of the coldest month. Similarly uncertainties may exist a s to the temperature change a n d distribution in the frozen layer a t the time of maximum frost penetration. H o w - ever, the equation provides a simple qualitative picture.'

Ker3nen (1920) gives a detailed discussion of values of qa and qfand the position of the frost line, o n a month by month basis, which is supported by his field measure- ments. .Type of Climate Somewhat continental Maritime Maritime Somewhat maritime Continental

T w o general cases c a n now be considered: 1. T w o places, assumed to have t h e same soil a n d cover conditions, have the s a m e extremes of winter cold (i.e. the same t,) but different mean annual a i r temperatures. A similar difference in mean annual ground temperatures t,,, is assumed.2 T h e place with the higher mean annual temperature has therefore, a warmer summer. A t this place, maximum frost penetration, according t o Beskow's equation, will be less, since a a n d hence q,, are greater (see Fig. 3a).

2. T w o places alike in soil and cover conditions have the same m e a n annual temperature but the annual extremes (and hence t,) differ. F o r the more extreme (continental) climate (to-t,) is greater, but so is q,,, and therefore the increase in frost penetration is smaller than might be expected (see Fig. 3b).

I n addition to the mean temperature of the coldest month it is apparent that t h e mean annual temperature is a very significant factor It should be pointed out that mean annual air temperatures commonly exceed the mean annual ground temperature by I-2°C and that the amount varies (cf. Chang 1953, Beskow 1935).

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Fig. 2. Relationship of Mean Annual Air Temperature and Winter Cold (25,00OoC Hr Frost Index Value) under Maritime and Continental Conditions in Canada.

in determining frost penetration since the value of q,, (and q,) also depend on this.3 Further illustration is provided by Table 11.

In summary, with increase in mean annual temperatures several degrees above O°C, the reserve of heat (represented by q,,), which limits frost penetration, increases rapidly. With such mean annual temperatures the frost penetration in the cold winters of continental climates in particular, is resisted by the relatively large amount .of heat stored in the hot summers.

It is therefore proposed that the group of frozen ground phenomena involving typical frost penetration of at least 70 cm only occurs where the mean annual temperature is lower than a b o ~ t

+

4°C (corresponding to a n air temperature about 1 to 2°C lower).

It is clear from the foregoing discussion that a single climatic variable, the mean annual temperature, will not sharply delineate the limits of development of such complex phenom-

It is the reserve of heat in the winter (represented by temperatures above 0°C) below the frost line that is significant. This is defined for given soil condit~ons when the mean annual temperature and the range of temperature (repre~nted by t y ) are glven.

ena. In particular, an extremely large amplitude (very continental climate) o r an extremely small amplitude (very maritime climate) may always require consideration also of winter temperatures. However, where a single criterion is desired, the mean annual temperature is more simple and rational than the frost index, or similar measure of the winter cold.

The accuracy of this limit of

+

4"C, and the extent to which other local factors control frost penetration, can only be established with further observations of frozen ground phenomena and ground thermal regime'

Snow cover is clearly, particularly important in determining frost penetration (cf. Gold 1958). Beskow (1935) has calculated hypo- thetical values for I,, under various snow covers and for various values of q,,. I t is commonly observed, however, that particular frozen ground phenomena have rather similar snow cover conditions, in widely spaced localities. Difference in snowfall in the wind-swept tun- dras is reflected to a considerable extent only

Lachenbruch, 1959, gives a detailed theoretical analysis of related problems.

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CLIMATIC FACTORS CONTROLLING FROZEN GROUND PHENOMENA

TEMPERATURE TEMPERATURE

FREEZING POINT= t o tml

t m t GROUND t4, t o =FREEZING POINT

t

Y t 8

A A SURFACE

PENETRATION

CLIMATE

W

THE EFFECT OF A LOWER M E A N . G R O U N D TEMPERATURE WITH UNCHANGED MEAN ANNUAL TEMPERATURE (tml. (trn,1 IS TO INCREASE FROST PENETRATION A COLDER. MEAN TEMPERATURE OF THE COLDEST CONSIDERABLY, E V E N WHEN THE M E A N TEMPERATURE MONTH GIVES LITTLE INCREASE IN FROST

OF ME COLDEST MONTH (tq,) IS U N C H W G E D . PENETRATION.

Fig. 3. Diagrams of Temperature Distribution at Time of Maximum Frost Penetration (all drawn to same scale). in enlarged drifts. Similarly, although soil type

and especially moisture content and vegetation cover greatly influence frost penetration, these are of course rather similar for any particular frozen ground phenomenon.

Large-Scale Solifluction Features

These appear generally as terrace-like or lobate features where downslope movement occurs

(Fig. 4). The depth of movement is often indicated by a conspicuous layer of organic matter in the soil profile resulting from over- run vegetation. I t may also be indicated by irregular layering of soil, or boulder orientation. Solifluction, in general, involves the movement of soil during the spring thaw, over a deep-lying still-frozen layer. Although permafrost is not necessary, it does require fairly deep frost penetration and furthermore that thawing Table 11. Climate and soil thermal condition

Place Sodankyla Moscow Oktyabrsky Gorodok Leningrad Ottawa

Mean annual ground temperature 2.g°C (see footnote 2, p. 5).

"ukianov and Golovko give a map of q , for the U.S.S.R. (based largely on extrapolation, although checked by field observations) showing values of 9 K-cal/m2/hr in the south t o less than 3 in the north.

Mean Temp' of coldest month (tY) "C -14 - 10 -13.7 - 8.6 - 1 1 Lat. 67" N 56" N 52" N 60" N 4 5 " N Reference Keranen 1920,1929 Lulcianov and Go- lovko 1957 Ditto Ditto Gold 1959 Long. 27" E 37" E 45" E 30" E 7 6 " W Average q~ K-cal/m2/hr period Mean Annual Air Temp. OC - 1.6l

+

4.7

+

4.4

+

5.2 + 5 . 0 1.0 5.2 7.0 4.5 4.8 Nov-Mar Oct-April Winter (freezing period) Ditto Nov-Feb

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P. J. WILLIAMS

Fig. 4. Large scale solifluction, Trollheimen, Norway (cf. Table I).

take place very largely from the ground surface downwards. In many areas of quite deep frost penetration thawing in the spring takes place in considerable degree from the bottom of the frozen layer upwards (cf. for example Beskow 1935, Keranen 1920). If no thawing from below is to occur, q,, must be less than or equal to qf. Once thawing from the surface has begun q f approaches nil, since both the upper a n d lower surfaces of the frozen layer a r e a t thawing point. F o r a deep-lying frozen layer to persist during the spring thaw requires, therefore, a particularly small q,,. T h e value of

q,, during the spring is rather closely related t o the mean annual ground temperature, decreas- ing a s this approaches 0°C. By spring the ground has lost most of the heat gained during the summer (Beskow 1935, p. 210-212, Lukianov a n d Golovko 1957, p. 58).

A mean annual air temperature of about

+

1 ° C is therefore suggested as t h e rough limit in both maritime and continental climates f o r the development of large-scale solifluction features. This is supported by the frequent o b - servation that large-scale solifluction is found a t somewhat higher altitudes t h a n frost hummocks a n d other features also associated with deep frost penetration.

Large-scale solifluction can occur in permafrost areas where there is sufficiently deep a n - nual thaw.

Significance of Agorestation

T h e tree line (roughly coinciding with the 1 1 ° C July isotherm) i n maritime climates lies south of the permafrost boundary. I n continental Canada a n d Russia, the tree line lies north o f t h e permafrost boundary. T h e ability of trees t o hinder movement of soils by solifluction is in doubt. However, large-scale solifluction giv-

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Fig. 5. Stony earth circles, near Opdal, Norway.

ing terrace forms and the characteristic well- defined. organic layers cannot occur where there is heavy tree growth.

In continental climates widespread solifluction of this type will therefore be restricted to permafrost localities, but only because the forest covers the coldest non-permafrost areas. Stony Earth Circles: A Regelation Feature These are non-sorted circles (in Washburn's

(1956) teminology), where circular or ovate patches up to 2 m. in diameter are free of vegetation and covered with small stones (Fig. 5). They occur in striking "fields" in lichen-heath tundra, where there is considerable exposure to wind. I n a detailed investigation described fully elsewhere (Williams 1959), they were shown to be formed primarily by repeated short-term freezing cycles of several days duration, and

penetrating only some centimetres into the ground.

They are typical of a group of regelation features. The annual frost penetration is not a limiting factor. The most important factor is the absence of snow cover commonly due to wind. This is because short-term fluctuations around freezing point are damped out by even a thin snow cover. T h e absence of a thick vegetation cover is essential for the same reason. I t is likely that a lichen mat is necessary to provide conditions for their best development.

If these conditions are met, however, varia- tion in the number of freeze-thaw cycles in the air temperature appears of minor significance, since stony earth cycles are widespread and occur under a variety of frost climates. The importance of lack of cover in regelation proc-

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P. J. WILLIAMS esses, rather than a particular frequency of freeze-thaw cycles, is also supported by studies of the latter (Fraser, 1959).

The Time Factor in the Distribution of the Fossil Forms

Most solifluction and patterned ground features form under a range of climatic conditions and thus a n y specific feature may be found over a large area. Factors such as soil type, water availability, vegetation, and exposure play a major role within such areas and determine the exact location of the features. I n the author's opinion, observed variations in density of OC- currence of specific fossil frozen ground phenomena a r e not so much indicative of particular climatic conditions but of length of time during which the features could be formed.'

Several investigations of the rate of movement of soil in solifluction have been published. a s well a s accounts of the rates of development of patterned ground features (for example Rud- berg 1958, Dahl 1956, Williams 1959). These investigations polnt to movements of material of only millimetres or centimetres per year, with only scattered and occasional movements a t intervals of a few years, of decimetres or more. Isolated rapid movements involving hundreds of cubic metres of material, to be considered as minor landslides although perhaps due to frost action, probably are far rarer. If the post-glacial land surface is considered, in the northern hemisphere gen'erally, there has been a period of less than 12,000 years during which patterned ground and solifluction phenomena might develop. Most solifluction movements will have moved materials n o more than 10 or 20 metres downslope in this period.

While climatic conditions may have changed quite substantially in this time, this is less true o f , purely local environmental factors. The "micro-geomorphology" will have remained much as it is now, so that the scattered distribution of well-defined solifluction a n d patterned ground features will have been similar to that seen in the present areas of frozen ground phenomena. I t is not surprising, therefore, that fossil features of this type are not particularly conspicuous in late Wiscosin-Wiirm strati- graphy, and are often absent on the surface in The density of reporter1 occurrences may also be a reflection of the number of geologists working in the area.

areas which experienced a fairly rapid a n d persistent temperature rise after deglaciation.

By contrast, the abundance of fossil features south of the southern margin of the maximum expansion of the Pleistocene ice sheet, shown clearly by Poser (1947), suggests the existence of cold conditions during a very long period. During this time, quite substantial changes in the essentially local geomorphology will have resulted from over-all erosion a n d other geo- morphological processes. These will have pro- duced, a t one time or another, local environmental conditions conducive to frozen ground phenomena over much of the sufficiently cold area, approximately delimited southward in the case of large-scale solifluction b y .the

+

1 ° C mean annual air temperature isotherm.

Conclusions

1. The development of patterned ground a n d solifluction requiring deep annual frost penetration depends o n the nearness of the mean a n - nual (ground) temperature t o 0 ° C rather than o n the intensity of winter cold. T h e mean a n - nual air temperature of

+

3°C is suggested a s marking a rough limit of their development.

2. Large-scale solifluction requiring a deep- lying still-frozen layer in the spring, is restricted t o areas with somewhat lower mean annual air temperatures;

+

1 ° C is suggested a s a n approximate limit. If forest is present this will prevent its development a t even lower mean annual air temperatures.

3. The distribution of stony earth circles, a form of patterned ground due t o regelation, is not closely related to annual frost penetration. T h e frequency of fluctuation of air temperatures around 0 ° C is generally n o t a limiting factor, but the absence of snow a n d of a thick vegetation cover a r e necessary.

4. Variations in density of occurrence of fossil forms of patterned ground and solifluction are to be related to the period of time during which conditions might have existed f o r the essentially slow movements o f material in- volved.

Acknowledgements

Constructive criticism by colleagues in the Snow a n d Ice and Northern Building Sections, Divi- sion of Building Research, National Research Council, is gratefully appreciated. This paper is published with the approval of the Director of the Division, M r . R . F . Legget.

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LACHENBRUCH, A. H., 1959. Periodic Heat Flow in a Stratified Medium with Application to Permafrost Problems. U.S. Geol. Surv. Bull. 1083-A.

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