Pulsating pingos, Tuktoyaktuk Peninsula, N. W.T.

J. Ross MACKAY Department of Geography, University of British Columbia, Vancouver, B.C., Canada V6T 1 W5

Received 22 April 1976 Revision accepted for publication 24 September 1976

Field studies have been carried out on two pingos on Tuktoyaktuk Peninsula, N.W.T. One pingo was studied from 1969-1976; the other was studied from 1974-1976. Precise levelling of bench marks in permafrost shows that the tops of these pingos alternately rise and subside in response to the rate of accumulation and loss of water beneath them. The water lenses may exceed 50 cm in depth. The high pore water pressure that causes pingo uplift is produced by pore water expulsion adjacent to the pingo, where the thickness of permafrost is 2 to 3 times the pingo height. The pore water pressure beneath the permafrost surrounding the pingo may approach 100% of the lithostatic pressure. When uplift from the water lens exceeds the strength of the pingo, peripheral failure occurs, water escapes as a spring, and the pingo subsides. Pulsating pingos seem characterized by long radial tension cracks which extend far onto the drained lake floor.

The pulsation of pingos has also been experimentally achieved by drilling holes through two pingos to release spring flow from subpingo pore water. The field evidence, from precise before-and-after surveys, indicates that the two pingos and their adjacent drained lake floors are virtually 'afloat' on subpermafrost water.

On a effectuC des etudes sur le terrain de deux pingos dans la pkninsule de Tuktoyaktuk, T.N.-O., l'une de 1969 B 1976, l'autre de 1974 a 1976. Le nivellement de precision de repaires d'kltvation ancres dans le pergelisol montre que leurs sommets se soulkent et s'affaissent alternativement en rkponse aux Faux d'alimentation et de perte d'eau sous les pingos. Des lentilles d'eau Deuvent atteindre une e~aisseur superieure ?i 50 cm. Les pressions interstitielles elevkes qui causeit le soulevement du pingo se prod;isent par expulsion de l'eau interstitielle au voisinage du pingo, 1B ou l'kpaisseur de pergilisol excede de deux a trois fois la hauteur du pingo. La pression interstitielle sous le pergelisol autour du pingo peut atteindre 100% de la pression lithostatique. Lorsque la poussee causee par la lentille d'eau dCpasse la resistance du pingo, une rupture pkripherique se produit, l'eau s'kchappe le long des fissures et le pingo s'affaisse. Ces pingos oscillants semblent se caractkriser par de longues fractures radiales de tension qui s'ktendent jusqu'au lit d'un lac drain&.

La pulsation des pingos a aussi ete sirnulee expkrimentalement en forant des puits B travers deux pingos pour 1ibBrer l'exces d'eau interstitielle sous le pingo. Les observations sur le terrain, B partir de lev& de prkcision avant et aprts les forages, indiquent que les deux pingos et les fonds lacustres draines adjacents flottent littkralement sur l'eau en dessous du pergtlisol.

[Traduit par le journal] Can. J. Earth Sci., 14,209-222 (1977)

Introduction Pingos are ice-cored hills which can only grow

and be preserved in a permafrost environment. There are about 1500 pingos along the western Arctic coast with the main concentration on Tuktoyaktuk Peninsula and Richards Island, N.W.T. Most of the pingos have grown as a result of permafrost aggradation in the bottom of a drained lake, the specific pingo site being that of 'a shallow residual pond. Since 1969, the growth patterns of 15 pingos have been measured by precise surveying of bench marks installed in permafrost (Mackay 1973). It is the purpose of this paper to discuss the growth patterns of two unusual pingos which pulsate with alternating periods of uplift and subsidence. In the absence

of a recognized term, the two pingos will be referred to as pulsating pingos.

Field Studies The two pulsating pingos are at sites 1 and 2

(Fig. 1, inset map) on Tuktoyaktuk Peninsula. The lake bottom sediments, at both sites, are sands. The mean annual ground temperature at site 1 is about - 9 "C and at site 2 about - 8 "C (Mackay 1974).

Site 1 The pingo at site 1 is near the center of a

drained lake (Fig. 1). Lake drainage occurred prior to 1935 (air photo A5024-27L) but probably not long before 1915 (estimate from age of willows on the lake bottom). Drainage was

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210 CAN. J. EARTH SCI. VOL. 14, 1977

FIG. 1. Site 1 : pingos in the bottom of a drained lake. Inset location map showing sites 1 and 2.

probably caused by ice-wedge melting at the outlet. The greatest lake depth, before drainage, exceeded the minimum winter ice thickness of about 2 m. The mean annual lake bottom tem- perature, judging from other measured mean annual lake temperatures in the area, would have been about 4 "C. Therefore, a large talik (un- frozen zone) would have underlain the lake prior to drainage. Two pingos (Mackay 1973, pingos 15 and 16) have grown up on the lake bottom, but only the larger of the two (Fig. 1) is growing, and it is this pulsating pingo that will be dis- cussed below.

Levelling of Bench Marks In 1969, four bench marks (BM 25 to 28) were

frozen into drill holes in the pingo (Figs. 1,2). In 1972, eight additional bench marks (BM 91 to 98) of an antiheave type (Mackay 1973) were added to give a total of 12 bench marks (Figs. 1, 2). Considerable field experience with more than 100 bench marks has shown that they remain un- affected by freeze-thaw in the active layer but will respond to the movement of permafrost into which the bench marks are solidly frozen. How- ever, since any part of a recently drained lake bottom can heave, a control bench mark (BM 91) was located on the landward side of the drained lake where permafrost is deep (>300 m) and permafrost aggradation would not occur. The bench marks have been precisely levelled each summer (Wild NA 2; optical micrometer reading to 0.1 mm; invar rod and supporting struts). Every survey has been closed at least twice. The measurements have been made in July or August,

0 Metres SO Contour Interval l m I

FIG. 2. Site 1 : contour map of pingo. Datum is at the pingo periphery.

so short period fluctuations would go undetected. The 1969-1976 changes in altitude (AH cm) of BM 28 (top), BM 27 (middle), and BM 26 (bottom slope) are plotted in Fig. 3, using BM 25 at the periphery of the pingo as the reference datum. Precise levelling (Mackay 1973) has shown that bench marks on the periphery are usually very stable, because the pingos do not grow at the periphery. The 1972-1976 changes for BM 94 to BM 98 are plotted in Fig. 4, using BM 91 as datum. Consequently, as BM 25 remained stable during the 1972-1976 period, when referenced to BM 91, the combined data in Figs. 3 and 4 show that the pingo underwent differential uplift in 1969-1 970; subsidence in 1970-1971 ; uplift in 1971-1972; and general subsidence in 1972-1975, but with a slight tendency towards recovery of the lower slope (BM 95,96,97) in 1974-1975 and some recovery of the top in 1975-1976 (Fig. 4).

Tension Cracks Most pingos higher than several metres de-

velop radial tension cracks at the top, because the frozen overburden ruptures above the growing ice core. These radial tension cracks produce the

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MACKAY 21 1

30 " Icing Mound In July 1974, an icing mound (Fig. 5) which

had grown during the 1973-1974 winter was . , discovered on the edge of the pingo (Fig. 6) where

a rusty, iron stained patch of vegetation had been observed since 1969 (Mackay 1975). The icing mound was 2.3 m high, 35 m long, and the over- - 10 burden was ruptured by tension cracks. Ice was exposed on one side where the overburden was 45 f 5 cm thick, this being the depth of the

0 active layer at the end of summer. A spring bubbled up at the bottom of a small pool beneath the overhang (Fig. 7). The flow was

-10 difficult to estimate, but was perhaps 0.1 Is-'. lg70 lg7' 1972 1973 1974 1975 l g 7 ~ Attempts to probe down through the spring

FIG. 3. Site 1 : changes in altitude of BM 28 (bench orifice were U ~ S U C C ~ S S ~ U ~ . Although there was no u

mark 28) at the top, BM 27 at the middle, and BM 26 at spring flow, one year later in July 1975, artesian the bottom slope referenced to BM 25, on the periphery, flow, with an initial head of 2 to 3 cm above for 1969-1975. See Fig. 2 for location of the bench marks. ground level, was encountered in augered holes

Bottom Bench Mark

' .*--- ---.----- ---*

FIG. 4. Site 1 : changes in altitude of five bench marks on the pingo (for location see Fig. 2) for the 1972-1975 period. The datum was referenced to BM 91 (see Fig. 1).

typical star-shaped summits of the larger and steeper pingos. The tension cracks decrease in size from the pingo top to the pingo periphery where they usually terminate. However, the radial tension cracks of the pulsating pingos at sites 1 and 2 are unique because they extend far out onto the lake bed floor. In the summer of 1969, no fresh crack activity was observable either on the pingo or adjacent lake floor. When the pingo was next examined in early April 1970, several tension cracks on the pingo and lake floor had recently opened and the cracks were probed to a maximum depth of 4.5 m. Since 1970, the tension cracks have been largely inactive, except for the large crack between BM 28 and BM 98 (Fig. 2).

from a water lens beneath the ice core (Fig. 8). The abandoned drill holes quickly refroze, be- cause of the abundance of auger ice cuttings in them. Recharge to the water lens was evident, because artesian pressures would build up within the short interval, usually a matter of hours, between the augering of successive holes. It is quite possible that some water also seeped un- noticed into the wet lake flat downslope from the icing mound.

The surface of the icing mound subsided 10 to 20 cm between July 1974 and July 1975 (Fig. 8) and the tension cracks narrowed. Subsidence could not have been caused by surface melting of the ice core, which was in permafrost below the active layer, but it might have resulted from water loss from beneath the ice core. The icing mound remained stable and unchanged from July 1975 to July 1976 and will probably become a 'perma- nent' appendage to the pingo.

Site 2 .

Sometime prior to 1890 (age from willows) a large lake, about 6.5 km long, 1 km wide, and at least 2 m deep, became drained and four pingos have since grown on the lake bottom (Mackay 1973). The smallest pingo is inactive. The three other pingos are large and 12 to 13 m high. In 1972-1973, a total of 21 bench marks were installed across the drained lake floor and in two of the large pingos (Mackay 1973, pingos 13 and 14). The tops of the two pingos are growing at the rate of 4 to 6 cm yr-'. The present discussion deals primarily with the third large pingo, here

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212 CAN. J. EARTH SCI. VOL. 14, 1977

FIG. 5. Site 1 : the icing mound which grew during the winter of 1974-1975. Site 1.

FIG. 6. Site 1 : pingo and icing mound, marked by an arrow.

referred to as 'bubble pingo', because of its domed shape (Fig. 9), and with pingo 14 (Mackay 1973).

Icing Mound On 27 July 1974 an icing mound complex and

spring were discovered on the south side of bubble pingo (Fig. 9). The spring orifice was on the side of the pingo, 2 m above the former lake

floor. The spring had flowed intermittently for at least 15 years, judging from the old dead willows and ground birch which lay in the seepage path of the spring water. The spring flow was mea- sured at 3 to 4 ls-' on 27 July 1974; it decreased to about 0.8 Is-' on 2 August 1974 and ceased a week later. The total discharge during the observation period was about 1500 m3. On 15 March 1975, when the site was next checked, the

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MACKAY 213

FIG. 7. Site 1 : icing mound with ice exposed at the bottom of the active layer. The arrow marks the location of a spring. The trowel is about 20 cm long. July 1974.

3 1

METRES

FIG. 8. Site 1 : cross section of the icing mound in July 1975. Note subsidence of mound from 1974-1975 and free water, under artesian pressure, still below the ice core.

spring was again flowing at the icing mound (Fig. 10) and other springs issued along an active tension crack (Fig. 9, crack B) beside the icing mound. The icing mound was obviously growing rapidly, because the cover of windpacked snow was ruptured by dilation cracks 10 to 25 cm wide. The site was again checked at the end of July 1975 when a spring was flowing from the same orifice as in 1974 but with a 33 day period

(Fig. 11). In 1974-1975 the icing mound con- tinued to grow, with a maximum height increase of 14 cm; in 1975-1976, the increase was 47 cm.

A second icing mound grew up during the winter of 1975-1976 near the site from which a spring flowed in March 1975 (Fig. 10). The August 1975 thickness of the active layer at the icing mound site was 30 to 50 cm, whereas the overburden above the ice core when it froze was

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. . . . . . . . . . . . . . . . . . . . . . . . , . , . , . , , , , , . , . , . , , . , . . , , , , , , , , , , ,

. . , . , . , . , , , . , , , , ,

, , , , , . , , . , , . , , . . . . . . . . . . . . . . . . .

.... , , , , , , #

..

..............

0 1W 200

Mefrrr

---- Tension Cracks, Inactive - Tension Cracks, Active (A,B) A Main Icing Mound

Area of Ice (Water) Intrusion 69,72 Bench Marks

214 CAN. J . EARTH SCI. VOL. 14, 1977

In the summers of 1974 and 1975, only two tension cracks (Fig. 9, A and B) showed signs of current activity. In both 1974 and 1975 tension crack A possessed a series of discontinuous cracks averaging 5 cm in width. Tension crack B was very active in the winter of 1974-1975. There was faulting along a near vertical fault plane with an upthrow of up to 1 m; the uplift was traced horizontally for 13 m; and there was winter spring flow along the base of the fault scarp (Fig. 10). The second icing mound grew in the winter of 1975-1976 in close association with the fault scarp.

Permafrost Aggradation and Artesian Flow Drilling across the bottom of the drained lake

showed that permafrost in 1976 was at a depth of about 35 + 5 m. Artesian flow was encoun- tered in 10 holes which had been drilled through permafrost into the unfrozen ground below (Mackay 1975). In addition, one drill hole on the side of pingo 14 adjacent to bubble pingo and 6 m above the lake flat also yielded artesian flow. The holes were 10 cm in diameter and the artesian fountains often rose 1 m above ground level, the flow sometimes taking 3 days to stop. As a result, large volumes of entrained sand (from 10 to 25 m3) were deposited around the tops of most of FIG. 9. Site 2: bubble pingo. the drill holes. When artesian flow was first en- countered, the drilling crew estimated the dis- only 20 to 30 cm. Therefore, intrusion of spring charge at 0.04 to 0.06 m3 s- 1. A rough computa-

water into the active layer probably occurred in tion, based upon a fountain, gives 0.035 m3 October-November 1975 before the active layer s-l at the of flow. If the average had frozen through. The mound was about 4 m flow is assumed to be O.O1 m3 s- I on the first day, high and 15 by 20 m across. The exceptional 0.005 m3 s-2 on the second day, and 0.0025 m3 height, which exceeded any possible winter s - ~ on the third day, the total discharge for growth, was due to the presence of a large cavity hole would be about 1500 m3. Although the which reached a height of 1.65 m and was about recharge area for a drill hole is unknown, it is l2 by l5 The cavity dome contained interesting to note that three successive holes numerous horizontal 'water' contours which were

drilled at 170 intervals on the same day all water levels left behind as bulk wafer escaped yielded artesian flow which continued for several from a large water pool before complete freezing occurred. The icing mound collapsed in August 1976. Bench Marks

Tension Cracks Two bench marks (Fig. 9, BM 69 and BM 72) Bubble pingo is a rare example of a pingo with were installed 50 m apart on the top of bubble

an intersectingdension crack system, one radial pingo in July 1974 and an additional four bench and the other concentric (Fig. 9). The radial marks were added in 1975. BM 69 was located cracks on the pingo itself are very large (up to 1.5 m from an active tension crack (Fig. 9, 5 m in width and 3 m in depth) but on the sedgy tension crack A) and BM 72 was 4 m from an lake floor they resemble straight canals, about inactive crack. The two bench marks were tied in 50 cm wide. The concentric cracks, which are by precise levelling to a stable bench mark on very uncommon elsewhere, parallel the pingo high land above the former lake shore. When periphery and are smaller than the radial cracks. relevelled in July 1975, the top of bubble pingo at

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MACKAY

FIG. 10. Site 2: spring flow from tension crack B (Fig. 9) of bubble pingo in March 1975. Note frozen pools of water in the snow.

-. 0 \

'? -2.0 g AH (differential movement) of EM 69 with rerpect to EM 72 41

jULY 1975 AUG. 1975

1

Y K C 1

0

FIG. 11. Site 2: bubble pingo spring flow and differential movement of BM 69 compared to BM 72 in 1975.

BM 72 had grown 5 cm yr-', an amount com- parable to that of the two other pingos under survey. However, the top at BM 69 had grown 45 cm yr-', an amount twice that measured for any pingo under survey. It seems clear that the exceedingly high 45 cm yr-' growth of BM 69, the proximity of BM 69 to the two active tension cracks A and B (Fig. 9), the icing mound com- plex, and spring were all interrelated. On 2 August 1975, when it became evident that the

- /\,/ Spring Flow 14 I'. - 1 \ / ! 1 1 I \ r i I 1

I \ '. ,' 1 -.,..--,-*- I '-,

-, -_. .

spring was flowing with a 34 day period (Fig. 11) the height difference between BM 69 and BM 72 was levelled 7 times in a 36 h period (Fig. 11). The 33 day cycle was too brief to permit repeated levelling from either BM 69 or BM 72 to a stable bench mark beyond the old lake shoreline. How- ever, since the top of bubble pingo at BM 69 grew 45 cm in 1974-1975, in contrast to the 5 cm of BM 72, the latter bench mark was used as the zero datum for the 36 h survey. As Fig. 11 shows,

1

2s 26 27 28 29 30 3 1 1 1 4 0

2

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216 CAN. J. EARTH SCI. VOL. 14, 1977

BM 69 dropped 5 mm during the rising stage of spring flow (31 July to 2 August) and then began rising during the recession stage on 2 and 3 August. Unfortunately, a longer period of measurement was impossible, but the relation between the spring hydrograph and the differ- ential uplift of the top of bubble pingo at BM 69 seems to be reasonably well demonstrated.

The 19 cm growth of BM 69 in 1975-1976 was small in comparison to the 45 cm growth in 1974-1975, but it was still exceptionally rapid considering the age and height of the pingo. Bench mark 72 grew only 2 cm in 1975-1976. The pingo was still pulsating with spring flow from the same site as in August 1975. The pulsation for the period 7 to 14 July 1976, using BM 72 as datum, is shown in Fig. 12.

Discussion The field evidence clearly indicates that pingo

pulsations are caused by alternating periods of accumulation and loss of subpermafrost ground water from beneath the pingo ice core. Although numerous spiings near pingos have been ob- served in the U.S.S.R. (e.g. Shumskii 1964), and springs have been observed in association with open system pingos in central Alaska (Holmes et al. 1968), only two small pingo springs have previously been reported from the western Arctic coast (Mackay and Stager 1966), insofar as the writer is aware. Enquiries of local in- habitants have so far produced no knowledge of spring flow although it would be most surprising if springs were not much more common than the two examples indicate.

western Arctic coast, permafrost commences to grow downwards on the exposed lake bottom. The depth (z) frozen, in time (t), can be approxi- mated by Stefan's solution (Carslaw and Jaeger 1959):'

The annual frozen increment (Az) at the bottom of permafrost, in finite form with At in years is:

From [l] and [2]

The value of b, for saturated sandy lake bottom sediments of Tuktoyaktuk Peninsula is about 300 cm yr-'I2 and for ice and icy soil about 160 cm yr-'I2 (Mackay 1973). 1

If all of the pore water froze in place, the ground surface heave would be

i I

Equation 4 can be used to estimate the extent to which pore water freezes in place. For example, if the porosity (17) is assumed to be 25%, if h, is 300 cm yr-'I2, and if all the pore water froze in place, the ground surface at site 1 overlying 25 m of permafrost would heave about 0.4 cm yr-'. However, since the bench marks on the lake bottom remained nearly stable for the 1972-1976 period, pore water was then being expelled.

The expulsion of pore water in advance of a freezing plane has long been documented in field and laboratory studies (e.g. Balduzzi 1959; Mackay 1973; McRoberts and Morgenstern 1975; Takashi and Masuda 1971 ; Takashi et al. 1974; Tsytovich 1975, p. 59). Pore water tends to be expelled in advance of the freezing plane when the effective stress (a - u) exceeds a soil con- stant (c) which is specific to the given soil:

Permafrost Growth When a lake becomes drained along the

FIG. 12. Site 2: bubble pingo differential movements of BM 69,70,78, and 71 compared to BM 72 in 1976.

Even though water may be expelled, some heave may still occur (Takashi and Masuda 1971 ; Takashi et al. 1974). The soil constant (c) for a sand is very small; Williams (1967) suggests a

'The variables in this equation and those that follow are defined in Table 1.

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MACKAY 217

TABLE 1. Definitions of symbols used

Symbols Definitions

A b bi bis

6s C

d E

; h

H k 1 L P R S

S t U

U

v x Y Y' z

a E

Y Y i Y I S

Ys

Y w A 7 8 0

Area Constant for Stefan's solution Constant for ice; assumed 160 cm year-'I2 Constant for pingo ice core and overburden; assumed 170 cm year- 'I2

Constant for sand; assumed 300 cm year-I'2 Soil constant; kg cm-2 Diameter of pipe (drill hole) Elliptic integral E(a, p / ( J m ) ) Acceleration due to gravity Friction coefficient Height of artesian fountain above ground level Heave of ground surface; height of pingo Thermal conductivity Liter Latent heat of fusion Dummy variable Radius Second Curve length Time; dummy variable Pore water pressure Velocity Volume Radial distance from center of pingo Variable Derivative of y Depth of freezing plane below ground sur- face Dummy variable Stretching of pingo overburden Bulk density Bulk density of ice; assumed 0.9 g Bulk density of high ice content pingo over- burden above ice core; assumed 1.4 g cm-3 Bulk density of low ice content frozen sand; assumed 2.2 g cm- Density of water Increment of some quantity Porosity Mean annual ground temperature OC Stress; lithostatic pressure-(thickness x bulk density) above freezing plane

value of less than 0.075 kg cm-'. If there is an open system with free drainage, the expelled pore water escapes, but in a closed system, pore water pressures can approach that of the load (Balduzzi 1959).

If there is no lake bottom heave (AH - 0) then the volume of expelled pore water (from [4]) beneath an aggrading permafrost of area A would be

In a closed system beneath aggrading perma- frost, positive pore water pressures would develop.

Artesian Pressures Site I Seismic and resistivity surveys suggest that

permafrost on the lake bottom is about 25 m thick and drilling shows permafrost to be 23 m thick at the base of the pingo. Drilling shows the overburden above the ice core to be about 3 m. The ice core thickness in the center is estimated at 12 m and the depth of the freezing plane below lake bottom level at 4 m. Assuming appropriate densities for the lake bottom sands (y, - 2.2 g ~ m - ~ ) , icy overburden (yis -- 1.4 g ~ m - ~ ) , and ice core (y, -- 0.9 g ~ m - ~ ) , the uplift pressure must exceed 1.5 kg ~ m - ~ , which is the weight above the freezing plane and beneath the top of the pingo. The piezometric surface is then a t least 11 m above the lake bottom level and the pore water pressure beneath the 25 m of perma- frost at least 3.6 kg cm-', or 65% of the litho- static pressure. In actual fact, the pore water pressure is probably much higher, because uplift also had to overcome the bending resistance of the pingo ice core and frozen overburden.

Site 2 Mention has already been made of artesian

flow from 10 holes through permafrost and 1 hole on the side of pingo 14 adjacent to bubble pingo. The hole on the pingo was 6 m above the lake floor and the bottom of the ice core was 16 m below the lake floor. Using the depths of the holes to the bottom of permafrost, the 1 m foun- tains, and the Bernoulli equation for flow through pipes (Urquhart 1959, p. 4-65), a rough estimate can be made of the subpermafrost pore water pressure and the height of the piezometric surface above the lake bottom from

In order to provide an estimate of the pressure, y will be taken as that of water (i.e. the entrained sand is omitted); a friction coefficient off = 0.06 will be used, although it might be higher for a rough hole in frozen sands; the limitations to flow by the permeability of the subpermafrost sands and the resistance of the overburden to bending will be ignored. Data from the lake bottom holes, the pingo hole, and the spring at bubble pingo give a piezometric surface at least

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218 CAN. J . EARTH SCI. VOL. 14, 1977

TABLE 2. Site 1 : changes in the pingo for 1969-1976

Volume change Date (m3> Uplift/subsidence Interpretation for center

1969 (June) (Datum) (Datum) Water lens > 40 cm thick 1969-1970 + 340 Uplift Water intrusion > spring flow; uplift of about 25 cm

so water lens > 60 cm thick 1970-1971 - 120 Subsidence Water intrusion < spring flow; water lens > 50 cm

thick Uplift Water intrusion > spring flow; water lens > 40 cm

thick Subsidence Water intrusion < spring flow ; water lens > 30 cm

thick Subsidence Water intrusion << spring flow; water lens > 20 cm

thick; growth of icing mound Subsidence Water intrusion < spring flow; thickness of water lens

unknown but probably > 10 cm Uplift Slight spring flow; water lens unknown

1969-1976 - 105 (approximate) Net loss of water in the 6 year period

20 m above the lake flats. The minimum pore water pressure beneath the 35 m of permafrost would then be about 70% of the lithostatic pres- sure and not greatly different from the minimum of 65% estimated for site 1.

Interpretation Pulsation at Site 1

The area of the lower permafrost surface be- neath the drained lake is about 200 000 mZ and the basal area of the pingo about 16 000 mZ. The estimated 1969-1976 volume changes, based upon a planimetering of Fig. 2 with upliftlsubsidence from Figs. 3 and 4 are given in Table 2. The extent of lake bottom heave surrounding the pingo is unknown, but BM 92 and BM 93 (Fig. 1) on the lake bottom did not heave 1 mm in the 1972-1976 period. If the entire lake bottom did not heave, the expelled pore water ([6] with 7 = 0.25, b, = 300 cm yr-', z = 25m, A = 200 000 mZ) would be 800 m3 yr- l , a volume amply sufficient to account for the 340 m3 volume increase in 1969-1970 (Table 2) and a surplus for spring flow in subsequent years.

Even though a water lens underlies a growing pingo, water should still continue to freeze at the bottom of the ice core. From [3] (b, = 160 cm y r - l ~ z . , z = 16 m), about 8 cm yr-' of ice could freeze at the bottom of the ice core. Alternatively, a rough estimate of Az can be obtained from the rate of heat flow:

Az ke -- -- ke N --

A t - yiLz Lz

For the pingo center (k - 0.005 cal cm-' s-' "C-l; 0 = -9 "C; z = 16 m; At = 1 year) [8] gives 12 cm of ice. For computational purposes, 10 cm yr-' (the average of 8 and 12 cm yr-') will be used for data in Table 2. Let us assume that in 1976 the water lens had just frozen solid, to give a 1969-1976 net height increase of 7 cm. However, in the 7 year period about 70 cm of ice would have frozen at 10 cm yr-l; this implies, there- fore, a 1970 free water lens of at least 57 cm (50 cm plus 7 cm), for otherwise the pingo would have grown appreciably in 1975-1976. Table 2 also shows that a net volume change of about - 105 m3 occurred for 1969-1976 which could be explained only if a thick water lens were present in 1969 and a thicker lens in 1970.

The mass transport of heat by expelled pore water to the subpingo ice lens is not considered a restrictive factor in the prevention of freezing and thus in the maintenance of the water lens. I t seems futile to speculate on numerous details of subpermafrost ground water flow, but let us assume that water at 1 "C flows to the water lens. The 1969-1970 addition of 340 m3 would only transport enough heat to thaw 5 m3 of ice, a relatively small amount compared to the volume of ice which could have been added by basal freezing at 10 cm yr-' beneath the central part of the ice core. Thus, although the mass trans- port of heat by groundwater might reduce the freezing of ice and warm the spring conduit, the gross effect can probably be disregarded.

In summary, the cumulative evidence suggests that in June 1969 a water lens at least 40 cm thick

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METRES

FIG. 13. Site 2: schematic cross section of bubble pingo with a 10 x vertical exaggeration. The freez- ing plane is shown as horizontal and 4 m below the ground surface, as it would be if the core were of pure ice and the residual pond in which the pingo grew was 1 rn deep. The freezing plane may be irre- gular, or concave down, and at a much greater depth than 4 m.

lay beneath the pingo center. In 1969-1970, about 340 m3 of water was intruded and the top rose about 25 cm; allowing for 10 cm yr-I of freezing of ice at the bottom of the ice core, the water lens in 1970 was probably at least 60 cm thick. Since 1970, there has been both water replenishment and loss by spring flow. The spring flow in 1970- 1974 and 1975-1976 was not observed. The icing mound commenced growth when freezing of the active layer in October or November 1973 blocked the spring outlet. Uplift and subsidence have represented quasi-equilibrium among three factors : (1) water intrusion and pingo uplift; (2) ice growth at the freezing plane; and (3) water loss from spring or other flow. The icing mound is a 'permanent' appendage to the pingo, because the ice core lies within permafrost.

Pulsation of Bubble Pingo, Site 2 The 45 cm uplift of the top of bubble pingo a t

BM 69 in 1974-1975 and the 19 cm in 1975-1976 exceeds by so much any computed basal freezing of ice (e.g. as at site 1) and the 5 cm yr-' growth of the two other pingos of similar height, that a water lens at least 40 cm thick probably underlay the pingo at BM 69 in July 1975. A schematic cross section of bubble pingo, with a 10 x verti- cal exaggeration, is shown in Fig. 13. The bottom of the ice core is drawn as horizontal, but some evidence suggests it is concave down. In July- August 1974 about 1500 m3 of water was dis- charged from the spring. Let us assume that a circular 1500 m3 water lens in 1974 was of three possible cross sections: (1) tabular and ofuniform

height; (2) conical; (3) spherical segment (plano- convex). Table 3, which shows the maximum heights for water lens radii of 50 to 200 m, sug- gests that a conical or spherical segment lens, of about 50 m radius, is of the right order to give a 40 cm uplift (1974-1975) with 1500 m3 of water. Such a water lens, if centered at BM 69 (Fig. 9), would encompass the icing mound and active tension cracks A and B. The pulsations of bubble pingo suggest a valve effect on the pingo peri- phery (Fig. 13) where uplift and subsidence open and close the entrance to the spring conduit.

Overburden Stretching When a pingo grows, the frozen lake bottom is

ruptured as it is stretched by the growing ice core. The total widths of the radial tension cracks, along any vertical cross section, correspond closely with the increase in length of the pingo profile. However, when a pingo pulsates, the surface length must also expand and contract. At site I, the distance between BM 28 and BM 98, on opposite sides of the central tension crack, was accurately taped at 1 1.726 m in 1973,11.705 m in 1974, 11.691 m in 1975, and 11.697 m in 1976. The maximum decrease was 3.5 cm in the 1973- 1975 period. In addition both BM 28 and BM 98 subsided in 1973-1975 (Figs. 3 and 4).

A vertical cross section through BM 28 and BM 98 approximates a cosine curve with the origin beneath the top, and pingo basal diameter 2R :

[91 y = H cos (7rx/2R)

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220 CAN. J: EARTH SCI. VOL. 14, 1977

TABLE 3. Maximum heights of a water lens of three different shapes for varying radii

Radius of water Height of tabular Height of conical Height of spherical lens (m) layer of water (cm) water lens (cm) segment water lens (cm)

50 19.1 57 38 75 8.5 25 17

100 4.8 14.3 9.6 150 2.1 6.4 4.3 200 1 .2 3.6 2.4

H, the pingo height, is given by the altitude of BM 28, and the pingo radius (R) is taken as 40 m. Solving for the curve length (S ) , using the equa- tion for the length of a curve and elliptic integrals, gives

sin t cos t - p2 JI + p2 sin2 t I

where E (a, p/,/m) is an elliptic integral. The stretching is

The total stretching in 1973 (using [9], [lo], at7d [I I]) was 3.63 m and in 1975,3.53 m. As the large tension crack at the pingo top (Fig. 2) is 3 to 4 m wide, nearly all of the failure seems to have taken place at the pingo top. If it is assumed that the entire pingo surface stretched and contracted uniformly, then a prorating of the stretching from 1973-1975 would give a decrease of 1.3 cm instead of the measured3.5 cm. Even if the pingo cross section shape were assumed to be parabolic or semi-elliptical, a similar stretching is obtained. Consequently, the overburden contraction was greatest at the top, where the subsidence was the greatest. The resultant movements of BM 28 and BM 98 were then centripetally downwards and inwards, as would be expected if water escaped from a free water lens beneath the center. At site 2, the separation between BM 69 and BM 70, on opposite sides of tension crack A (Fig. 9), increased from 10.485 m in 1975 to 10.630 in 1976, a distance of 14.5 cm. As there was no comparable rupture at the bottom of the tension crack, the implication is that the 14.5 cm increase

resulted from the infilling of numerous small cracks associated with the pingo pulsations.

Failure Patterns Pollard and Johnson (1973) have shown in

their studies of laccoliths, which have similarities with pingos, that the maximum bending strain and differential stress is a t the periphery of the laccolith where pzripheral diking may occur. The dikes strike parallel to the periphery of the intrusion and extend upwards and outwards from the periphery. Therefore, it is interesting to note that concentric tension cracks parallel the periphery of bubble pingo (Fig. 9) and that failure 1 has been noted along concentric faults in other pingos (e.g. Mackay and Stager 1966; Miilles 1959, Plate 11; Rampton and Mackay 1971; Washburn 1973, Fig. 4.62). In addition, the springs at sites 1 and 2 are on the periphery, and the icing mound core at site 1 (Fig. 8) strikes parallel to the base of the pingo and dips gently towards it. Thus, the peripheral failure patterns of the pulsating pingos seem similar to those of laccoliths, because both are domed by uplift: laccoliths by magma, the pulsating pingos by water.

Experimental Evidence Field experiments were carried out at sites 1

and 2, in the summer of 1976, in order to assess the effects of subpermafrost ground water loss on the heights of pingos and the surrounding drained lake bottoms. At site 1, a hole was hydraulically drilled (Judge et al. 1976) through 23 m of perma- frost at the base of the pingo (Fig. 1) to produce a spring which flowed for 4 days. A comparison of bench mark heights on 25 June 1976, prior to the drilling, with three resurveys on 27, 28, and 29 June 1976 after the drilling, shows that the ad- jacent lake bottom dropped about 1 mm and the sides of the pingo slightly more. The heights of the bench marks increased as spring flow ceased.

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MACKAY 22 1

At site 2, a hole was drilled half way up pingo 14 (Mackay 1973), a large pingo whose base is only 100 m from that of bubble pingo. Ground level was 6 m above the drained lake floor and permafrost was penetrated at a depth of 22 m below the surface. A geyser of clear water initially jetted 2.6 m into the air from a 7.5 cm diameter pipe (Fig. 14). Before-and-after surveys showed that the drained lake bottom had sub- sided, along the line of 16 bench marks, about 2.5 mm for a distance of 400 m from the drill hole, and the pingo top subsided 5.27 cm (Fig. 15). A resurvey on 11 August 1976 showed that par- tial recovery had occurred, both of the pingo and drained lake floor.

The experimental evidence from sites 1 and 2 shows, rather clearly, that the release of sub- permafrost pore water under artesian pressure can cause subsidence both of pingos and of the surrounding drained lake floor.

FIG. 15. The upper graph shows a profile of pingo 14, which is adjacent to bubble pingo, with the locations of the bench marks and drill hole. The lower graph shows the growth of the pingo from 27 July 1975 to 4 July 1976 and the subsidence resulting from the loss of subpermafrost water from the drill hole. Note that the subsidence caused the pingo to lose 1 year's growth. In addition, the sub- sidence pattern was similar to the growth pattern, i.t. least at the periphery and greatest at the top. When resurveyed a month later on 10 August 1976, the top had recovered 1 cm, an amount twice that of any growth for a comparable period from 1971-1976. The drained lake floor also showed partial recovery, thus demonsrating that 35 & 5 m of permafrost was being uplifted by sub- permafrost pore water, i.e. the uplift pressure approxi- mated the lithostatic pressure.

Conclusion

FIG. 14. The geyser on pingo 14 photographed several hours after drilling. The height was 2.6 m from a 7.5 cm diameter hole. The flow continued for 3 days and stopped from inward freezing of the hole, rather than from a cessation of pressure.

The pulsations of two growing pingos, on Tuktoyaktuk Peninsula, N.W.T., are caused by the build-up of a free water lens beneath the pingos, and the periodic escape of water by peripheral diking. The free water lenses have underlain one pingo (site 1) probably for 7 years (1969-1976) and have underlain another (site 2) probably intermittently for at least 15 years. The Ienses are believed to be plano<onvex or conical in cross section, rather than tabular. The maxi- mum water lens thickness beneath the pingo tops probably exceeded 50 cm. The water source is expelled pore water from beneath aggrading permafrost in saturated, sandy, drained lake bottoms in a closed or semiclosed system. The subpermafrost pore water pressures may ap- proach 100% of the lithostatic (overburden) pressure. The piezometric surface, at both sites, has been far above the tops of the respective pingos. The pore water uplift pressure beneath each pingo has been sufficient to both lift and overcome the bending strength of that part of the pingo above the water lens.

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222 CAN. J. EARTH

Judging by the two pulsating pingos under study, and the other growing pingos which do not pulsate, pingos with large free water lenses seem distinguished by: radial tension cracks which extend far out onto the lake floors; pulsations of the top; and peripheral failure resulting in springs, icing mounds, and intrusive ice beneath the active layer.

Field experiments on two pingos show that artificially induced spring flow can lower the heights of both pingos and the adjoining drained lake bottom flats. The evidence suggests that some pingos and their adjacent lake bottoms are virtually 'floating' on water.

Pingo pulsations, with the episodic intrusion of free water, may help to explain the alternate occurrence of segregated and injection ice (Pissart and French 1966) which has been ob- served in some pingos.

Acknowledgements The field work was supported by the Geological

Survey of Canada; equipment was provided by the National Research Council of Canada and the Department of Indian affairs and Northern Development. Logistic support and much help came from the Polar Continental Shelf Project (Department of Energy, Mines and Resources) and the Inuvik Research Laboratory. The writer would like to thank R. W. Boyd, J. D. Dwyer, R. W. Toews, and P. A. Wright for help in field surveys; C. Burrows, W. H. Mathews, R. Ryan, M. C . Quick, A. L. Washburn, and W. W. Shilts for helpful comments and discussion; and J. J. Solecki for assistance in translating Russian papers.

BALDUZZI, F. 1959. Experimental investigation of soil freezing (in German). Mitt. Versuchsanst. Wasserbau Erdbau, ETH, Zurich, No. 44. (Translated by D. A. Sinclair, National Research Council of Canada, Techni- cal Translation 912, 1960,43 p.)

CARSLAW, H. S. and JAEGER, J. C. 1959. Conduction of heat in solids. 2nd ed. Clarendon Press, Oxford, 510 p.

HOLMES, G. W., HOPKINS, D. M., and FOSTER, H. L. 1968. Pingos in central Alaska. Washington United States Geological Survey, Bulletin 1241-H, 40 p.

SCI. VOL. 14, 1977

JUDGE, A. S., HUNTER, J. A., GOOD, R. L., COWAN, J., and ROBB, G. 1976. Thermistor cable installation in per- mafrost materials with a water-jet drilling method. Geological Survey of Canada, Paper 76-IA, pp. 479-480.

MACKAY, J. R. 1973. The growth of pingos, western Arctic coast, Canada. Canadian Journal of Earth Sciences, 10, pp. 979-1004.

1974. Seismic shot holes and ground temperatures, Mackenzie Delta area, Northwest Territories. Geologi- cal Survey of Canada, Paper 74-IA, pp. 389-390. - 1975. Freezing processes at the bottom of perma-

frost, Tuktoyaktuk Peninsula area, District of Macken- zie (107 C). Geological Survey of Canada, Paper 75-IA, p. 471-474.

MACKAY, J. R. and STAGER, J . K. 1966. The structure of some pingos in the Mackenzie Delta area, N. W.T. Geo- graphical Bulletin (Canada), 8, pp. 36Ck368.

MCROBERTS, E. C. and MORGENSTERN, N. R. 1975. Pore water expulsion during freezing. Canadian Geotechnical Journal, 12, pp. 130-141.

M ~ ~ L L E R , F. 1959. Observations on pingos (in German). Meddelelser om Gr~nland, 153, 127 p. (Translated by D. A. Sinclair, National Research Council of Canada, Technical Translation 1073,1963,117 p.)

PISSART, A. and FRENCH, H. M. 1976. Pingo investiga- tions, north-central Banks Island, Canadian Arctic. Canadian Journal of Earth Sciences, 13, pp. 937-946.

POLLARD, D. D. and JOHNSON, A. M. 1973. Mechanics of growth of some laccolithic intrusions in the Henry Mountains, Utah, 11. Tectonophysics, 18, pp. 311-354.

RAMPTON, V. N. and MACKAY, J. R. 1971. Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards Island, and nearby areas, District of Macken- zie. Geological Survey of Canada, Paper 71-21,16p.

SHUMSKII, P. A. 1964. Principles of structural glaciology (in Russian). Translated by D. Kraus, Dover, N.Y., 497 p.

TAKASHI, T. and MASUDA, M. 1971. An experimental study of the effects of loads on frost heaving and soil migration. Journal of the Japanese Society of Snow and Ice, 33, pp. 109-119.

TAKASHI, T., MASUDA, M., and YAMAMOTO, H. 1974. Experimental study on the influence of freezing speed upon frost heave ratio of soil under constant effective stress. Journal of the Japanese Society of Snow and Ice, 36, pp. 49-68.

TSYTOVICH, N. A. 1975. The mechanics of frozen ground. (Translated from the Russian book of 1973). McGraw- Hill, N.Y., 426 p.

UROUHART. L. C. (Ed.) 1959. Civil Engineering Hand- book. 4thed. ~c~raw- ill, N.Y.

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WASHBURN, A. L. 1973. Periglacial processes and envi- ronments. Edward Arnold, London, 320 p.

WILLIAMS, P. J. 1967. Properties and behaviour of freezing soils. Norwegian Geotechnical Institute, Oslo, NR. 72, 119 p.

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