A Conceptual Model of Hiorthfjellet Rock Glacier, Svalbard

A conceptual model of Hiorthfjellet rock glacier, Svalbard

R.S. Ødegård Gjøvik University College, Gjøvik, Norway K. Isaksen Norwegian Meteorological Institute, Oslo, Norway T. Eiken and J.L. Sollid Department of Physical Geography, University of Oslo, Oslo, Norway

ABSTRACT: Field studies of the Hiorthfjellet rock glacier (78°15N, 15°47E) near Longyearbyen, Svalbard, started with surface velocity measurements in 1994. This paper presents a conceptual model for the development of this rock glacier. There is particular focus on the complexity of the front processes. It is suggested that the surface slope is mainly controlled by the accumulation of debris at the base of the active creeping part close to the front, as the rock glacier advances. This process will depend on the flow regime, and could force the front to move upward in a more mature state, assuming a strong longitudinal stress coupling. The suggested conceptual model is discussed related to the overall morphology and dynamics of the rock glacier. The proposed model is consistent with available field measurements. Considerable additional field data will be needed to validate the model.

1 INTRODUCTION of dirty ice. This is one of the few known descriptions of the inner structure of Svalbard rock glaciers. His The Svalbard archipelago (74°N to 81°N, Figure 1) interpretation was that rockslide material accumulates has continuous permafrost with permafrost thickness on snow in early spring/summer and gradually trans- varying from less than 150 m near sea level to more forms into ice during summer. than 450 m in mountain areas (Liestøl 1977). MAAT Sollid & Sørbel (1992) suggest Holocene or even at Svalbard Airport, Longyarbyen, is 6.7°C for the pre Holocene age of some rock glaciers. André (1994) period 1961–90 (28 m a.s.l.). states that rock glacier formation started 3500 BP During the first part of the 20th century several based on dating of surface material with lichenome- interpretations existed on the origin of these features try. The surface at the top front of Hiorthfjellet rock in Svalbard. Liestøl (1962) was the first to link glacier was roughly dated to 4000 BP based on the Svalbard rock glaciers to slope processes. Talus terraces length/surface velocity ratio (LSVR) (Isaksen et al., in Svalbard were described as having a “typical flow 2000). Due to rock glacier dynamics and long-term structure some what like rock glaciers”. Liestøl inves- initial development the rock glacier probably started tigated a rock glacier in Longyearbyen in 1954 where to develop at the onset of Holocene. a slip had occurred. He found irregular layers of The Hiorthfjellet rock glacier is 400 m long with sharp-edged stones and gravel alternated with layers an estimated mean thickness of approximately 35 m (Figure 2). The first interpretations of field data from the geodetic survey, DC resistivity measurements and GPR sounding are presented in Isaksen et al. (2000). Normalized horizontal surface velocities were measured to 9.0–10.3 cm/year (Isaksen et al., 2000). This paper contains further interpretation of field data and a conceptual model is suggested for the dynamics at the front of Hiorthfjellet rock glacier. This is a contribution to understanding the long debated morphology of these features in Svalbard and to the discussion of the dynamics of rock glaciers in general.

2 FIELD DATA AND INTERPRETATION

The surface slope is decreasing from approx. 20° at the top to 10°–12° near the front. On the top front Figure 1. Key map showing the location of the field site. of the rock glacier 20–40 m up-glacier from the steep

839 frontal slope the surface gradient is again increasing glacier from what is interpreted as the accumulation to a maximum of 14°–15°. The average longitudinal area. There is no significant trend in the gradient of surface gradient is 16.6° calculated from the lower the reflection layers relative to the surface gradient, accumulation area to the top front. however large anomalies are detected, believed to orig- There are two zones on the frontal slope where the inate from the actual accumulation process (Figure 3). slope gradients are 40° or steeper. The upper zone is This implies that the layers, interpreted as rockslide extending down to 16–22 m below the top front of the deposits, reinforce the upper supersaturated part. If rock glacier. The second zone is at the foot of the the layering of the Hiorthfjellet rock glacier are simi- frontal slope. lar to the observations by Liestøl in 1954 on the other The GPR profile presented in Figure 3 was obtained side of the fjord (Liestøl, 1962), the layers of rockslides using a Pulse EKKO 100 (Sensors and Software Inc., are saturated with ice alternating with layers of ice. Mississauga, Canada) with an antenna frequency of The radar data give few indications of the thickness 50 Mhz. The profile follows the centre flow line. The of the rock glacier. Rough estimates of the thickness georadar measurements give no indication of any can be made assuming a basal shear stress of 100 kPa cumulative shear in the upper 20 m in the area down and an ice content of 75% giving an average thickness of the supersaturated layer of 35 m. This calculation is based on a strong longitudinal stress coupling with an average surface gradient of 16.6°.

3 A CONCEPTUAL MODEL FOR THE FRONT PROCESSES

Heaberli (1985) addressed processes at the front of a rock glacier, stating that “coarse debris, which falls off the top of active rockglacier fronts, is subsequently overridden, and may then form a relatively stiff basal layer”. The hypothesis was later confirmed by defor- Figure 2. The Hiorthfjellet rock glacier is in middle of the mation measurements in boreholes in the Eastern Alps photo with the steep front free of snow (June 1998). (e.g. Heaberli et al., 1998, Hoelzle et al., 1998). The

Figure 3. Radar profile and interpretation. In the lower plot the white dots are depths down to 9 m, grey dots 10–15 m depth and black dots 16–20 m depth.

840 borehole measurements at rock glacier Murtel– to the plane of shear, which causes volume expansion Corvatsch show that the basal debris layer is about 20 m (dilatancy) (Arenson and Springman, 2000). There is no thick. This mechanism has been conceptually illustrated reason to expect that pore water pressure influences the by Haeberli et al. (1998). In a model of pure plastic flow mechanical properties of the basal debris (permafrost). this mechanism will be less efficient, because the front The mechanical properties of coarse grained sedi- of the glacier is simply pushed forward, and there will ments with ice content at saturation level or below are be no gradual increase in surface gradient at the front. known from geotechnical investigations. Pore ice will The details on how a permafrost ice-rock mixture deform even at very low stresses. The long term strength deforms is far from understood (Haeberli, 2000). of these sediments is mainly controlled by inter-grain However, all deformation measurements in boreholes friction like non-permafrost sediments. Ice will add (Haeberli et al., 1998, Hoelzle et al., 1998) and geo- strength to cold permafrost consisting of coarse grained morphological indications strongly suggest that a pure sediments, but for simplicity the added strength is not plastic model is not applicable. This assumption is considered in the conceptual model. Sayles (1973) sug- made for the Hiorthfjellet rock glacier. There are no gested that for noncohesive materials in the unfrozen direct field measurements of strain at depth. state, the long term strength in the frozen state would The steep front of a rock glacier will cause a stress be roughly equal to that measured in triaxial tests on field that can be modelled based on well known meth- the freely drained unfrozen material. In the following ods devoloped for finite slopes. The stress distribution it is assumed that the internal shearing resistance of beneath a finite slope is more complex than below an the basal debris is constant. This is certainly not valid infinite slope. There are no convincing field data from when the normal stress increases as the material is the Hiorthfjellet rock glacier to make definite state- overridden by the rock glacier. The assumption should ments about how the stress field relates to the defor- be refined in future quantitative models, but the sim- mation at the front. A relative increase of the surface plification is probably not critical considering the gradient on top of the rock glacier close to the front is precision in the calculations. consistent with an increasing submerging (vertical) In a conceptual model of the rock glacier front velocity. Similarly, the bulge at the base of the steep processes the following assumptions are made: frontal slope can be explained by shear strain close to The flow regime at the front is mainly controlled by the base of the basal debris layer, but other explana- • the applied stress from the creeping rock glacier. tions like active layer block creep and a different angle The strain caused by the steep front is neglected for of repose could be suggested as well. • the supersaturated part of the rock glacier, but is In the suggested model it is assumed that the stress considered for the basal debris layer. field from the steep front can be neglected for the upper The flow of the rock glacier is caused by shear strain supersaturated part of the rock glacier. On the other • in supersaturated sediments. hand, compression and damped creep close to the Debris that melts out from the upper front area front in the basal debris layer could be significant for • accumulates further down the front slope- not in the overall morphology and dynamics of Hiorthfjellet front of the rock glacier. rock glacier. Zero pore water pressure in basal debris (pores Sediments in front of the rock glacier consist of lichen • filled with ice or partly filled with ice in coarse covered blocks interpreted as early or mid Holocene debris with permafrost) dirty snow avalanche deposits from the western flank The sediments accumulating at the front are non- of the Hiorthfjellet rockwall. Even close to the base of • cohesive with constant angle of shearing resistance the steep rock glacier front there are very few blocks (does not depend on the applied stress). originating from the active rock glacier. This is regarded as good geomorphological evidence that very few In the initial stage of rock glacier development blocks leave the front slope. The reason is probably the front will reach the minimum angle of shearing that there are few large blocks exposed on the steep resistance. Because of the loose state of debris in the frontal slope. active layer at the front of a rock glacier it is reasonable Debris is expected to melt out in the upper area of to assume that the front of an active rock glacier the front slope, and will be exposed to gravitational will be close to the minimum angle of shearing resist- processes. After rearrangements in the active layer these ance. Rapp (1960) and Chandler (1973) investigated sediments will be exposed to percolating water that the inclination of talus and rock glacier slopes in refreezes in the pores. It is not known if these processes Spitsbergen. Chandler (1973) suggested a critical value cause ice saturation or not. If air voids exist as the debris for angular rockfill material to be 39°– 40°. When a is overridden by the advancing rock glacier, the basal critical angle is reached the front of the rock glacier debris might compress due to increased normal stress. will adjust to this angle. This will cause melting of the Shearing of coarse material implies movements normal supersaturated upper part of the rock glacier because

841 debris will be removed from the upper front and accu- initial stage. This process could force the front to mulate further down the slope. move upward in a mature state. When the normal stress is low the debris that accu- (3) The basal debris layer will increase in thickness in mulates on the lower frontal slope will be pushed for- the initial stage until equilibrium is reached between ward by the rock glacier, and shearing will occur. As the mechanism described under (2) and compres- the front of the rock glacier progress, the normal stress sion and damped creep of the overridden material. on the debris will increase, which might cause com- The latter mechanisms will probably be more sig- pression and dilatancy. The shear stress at the base nificant when the basal debris layer gets thicker. relative to the normal stress will eventually reach a The important parameters of these processes are critical level known as the coefficient of friction. If the the normal stress and shear stress near the front and coefficient is assumed to be 0.8, corresponding to an mechanical properties of sediments accumulating angle shearing resistance of 40° and an applied sharing at the front (partly controlled by permafrost temp- force of 100 kPa, the critical normal stress will be erature). The details of these processes cannot be 125 kPa. An ice-rock mixture with an ice-content of assessed with the present knowledge of rock glacier 75% will have a typical density of 1350 kg/m3, which rheology. The complexity of these thermo-mechanical means that the critical normal stress will be obtained processes should not be underestimated. with an ice/rock overburden of approx. 10 m. When the critical normal stress is reached debris will start to accumulate at the base of the rock glacier under the 4 THE DEVELOPMENT OF frontal slope (see sketch in upper part of Figure 4). HIORTHFJELLET ROCK GLACIER After the initial stage the rock glacier develops as a two-layered system. Several scenarios can be sug- From the georadar profile it is apparent that layers gested for the further development. In the lower part interpreted at rockslide deposits experience rotation of Figure 4 three scenarios are suggested. and flexure in the accumulation area (Figure 3). Apart from the area close to the rockwall, limited shear (1) The rock glacier base progresses in a direction strain seems to occur. However, even limited shear controlled by the initial angle between bed slope strain could change the mechanical properties of the and the base of the rock glacier. The initial accu- original sediments. The differences in apparent resis- mulation of debris will cause vertical strain on the tivity values from the upper to the lower parts (Isaksen supersaturated layer, which means that the rock et al., 2000) could be explained by limited shear, bring- glacier will move forward at an angle slightly less ing the individual particles in closer contact. Work hard- than the bed slope. If this initial movement ening will eventually strengthen the material resulting progresses, the accumulation of debris will get in practically no shear strain in the upper part of the thicker and thicker as the accumulation of debris supersaturated layer a distance down glacier from the at the base is no longer directly controlled by the accumulation area. bed slope gradient. The model consists of a layer of a few meters (2) If the process described under (1) is kind of a self below the upper rigid part with more randomly dis- repeating mechanism (substituting the original tributed blocks in a matrix of ice, which might be bed with the accumulated debris layer), there modelled according to the flow law of ice (Figure 5). might be a tendency of increased accumulation of This is consistent with borehole measurements in the debris at the base of the rock glacier causing ver- Alps (Haeberli et al., 1998). The basal layer of the tical strain on the advancing rock glacier after the supersaturated rock glacier originates from the upper accumulation area. More randomly distributed blocks could be due to sorting in the accumulation process, or shear strain in the accumulation area could disturb the original layering. A rigid ice/rock mixture in the upper supersaturated layer implies a strong longitudinal stress coupling, which significant implications for the overall dynamics. The rigid layer will resist longitudinal compression. Maximum surface shear strain is calculated to 0.7 104 a1 in the mid-zone based on surface velocity measurements published by Isaksen et al. (1) (2) (3) (2000). The measured surface strain indicates a cumu- Figure 4. The upper figure is a sketch of the initial stage lative thickening of the mid area on the order of a of debris accumulation. Lower part – see text for details. few meters. This zone probably also have transverse

842 Figure 5. A conceptual model of of Hiorthfjellet rock glacier. expansion. In the top frontal area of the rock glacier The progress of the front will be controlled by the no significant surface strain are measured. A small extent of this debris layer. The process is discussed by positive strain might occur near the front as a response Barsch (1996) and Kääb et al. (1998) with respect to to the stress field of the steep front. estimates of rock glacier age. At Hiorthfjellet rock The longitudinal stress coupling is significant in the glacier the present front advance rate is probably discussion on how the rock glacier responds to changes about 2–3 cm pr. year based on a simple 2D calcula- in basal friction as the length of the rock glacier tion using a thickness of 35 m of the supersaturated increases. Assuming scenario 2 (Figure 4) a lowering layer at the front (75% ice content). of the basal gradient as the rock glacier progress will The suggested conceptual model opens the possi- reduce the overall basal shear stress. If accumulation bility that the basal debris layer gets progressively input was constant the dynamic response would be a thicker towards the front. If the model is generally gradually thicker rock glacier from the front upwards applicable in Svalbard, age estimates will be even to maintain the basal shear stress. more sensitive to the exact extent of the basal debris Based on this conceptual model the longitudinal layer. surface profile of the Hiorthfjellet rock glacier is Most of the Svalbard rock glaciers exhibit a interpreted as a combination of increasing thickness decreasing longitudinal surface gradient towards the of bed debris, a small longitudinal compression in the front. There are several examples of an inverted gradi- central zone and probably a transverse expansion in ent close to the steep front slope. There are at least the mid and front zone and a dynamic feedback to four suggestions on the origin of these “depressions”: maintain the overall basal shear stress. The suggested 1. Liestøl (1962) stresses the protalus rampart process conceptual model is consistent with all available field as most important (in situ accumulation). He does data from the Hiorthfjellet rock glacier. Additional not rule out deformation due to high ice content. field data will however be needed to actually validate 2. Swett et al. (1980) suggests a rotational movement the model. like a cirque glacier causing the depressions. 3. Humlum (1982) points out some shortcomings in Swett’s argumentation, and argues that the inner 5 DISCUSSION AND CONCLUSIONS depressions are mainly a degradation form. 4. Berthling et al. (1998) states that these depressions Accumulation of debris at the base of the front slope of are most likely flow-related features related to com- a rock glacier can simply be explained by conservation pressive and extending flow. of debris mass and shear strain in sediments supersaturated with ice. As long as the debris accumulates on The processes described by Liestøl (1962) and the front slope, and not in front of the rock glacier, Humlum (1982) have been verified by field observa- rock glaciers will advance as a two-layered system. tions. There are however several examples of Svalbard

843 rock glaciers where these processes are unlikely to (I): internal structure, flow velocity and morphology. control the overall surface morphology. This is dis- Permafrost and Periglacial Processes, 9: 135–145. cussed by Berthling et al. (1998), who argue that Chandler, R.J. 1973. The inclination of talus, arctic talus the depressions are caused by extending and compres- terraces and other slopes composed of granular mate- sive flow, adapting a conceptual model suggested by rials. The Journal of Geology, 81: 1–14. Haeberli, W. 1985. Creep of mountain permafrost: inter- Haeberli and Vonder Mühll (1996) for the Alps. The nal structure and flow of Alpine rock glaciers. magnitude of measured surface strain is not convinc- Versuchsanstalt für Wasserbau, Hydrologie und Glazi- ing with respect to the importance of this process for ologie, der Eidgenössischen Technischen Hochschule the Hiorthfjellet rock glacier. Zürich, Mitteilungen 77. The overall extending-compressive flow regime Haeberli, W. and Vonder Mühll, D. 1996. On the character- could explain a cumulative thickening of the supersat- istics and possible origins of ice in rock glacier urated part. However, a cumulative thickening reduces permafrost. Zeitschrift für Geomorphologie, Supple- the shear stress, resulting in a dynamic feedback caus- mentband, 104: 43–57. ing the rock glacier to grow thicker in the accumula- Haeberli, W., Hoelzle, M., Kääb, A., Keller, F., Vonder Mühll, D. and Wagner, S. 1998. Ten years after drilling tion area. A hypothesis is therefore launched stating through the permafrost of the active rock glacir Murtèl, that the low and sometimes inverted surface gradients Easterm Swiss Alps: Answered questions and new per- towards the front of the Svalbard rock glaciers could spectives. In: Proceeding 7th International Permafrost be controlled by an increasing accumulation of debris Conference, Yellowknife, Canada. 403–410. at the base and a strong longitudinal stress coupling. Haeberli, W. 2000. Modern Research Perspectives Relating The dynamic feedback will have the opposite effect, to Permafrost Creep and Rock Glaciers: A Discussion. causing a steeper longitudinal gradient and a gradually Permafrost and Periglacial Processes, 11: 290–293. thinner supersaturated layer towards the front. Consid- Hoelzle, M., Wagner, S., Kääb, A., Vonder Mühll, D. 1998. ering the previous suggestions on the origin of this Surface movement and internal deformation of ice- particular surface morphology, an increasing thickness rock mixtures within rock glaciers at Pontresina- Schafberg, Upper Engadin, Switzerland. In: Proceeding of the basal layer could explain that the front moves 7th International Permafrost Conference, Yellowknife, upward. The hypothesis remains to be tested with field Canada. 465–471. data of surface velocities near the front. Humlum, O. 1982. Rock glaciers in northern Spitsbergen: a discussion. Journal of Geology, 90: 214–218. Isaksen, K., Ødegård, R.S., Eiken, T. and Sollid, J.L. 2000. ACKNOWLEDGEMENTS Composition, Flow and Development of Two Tongue- Shaped Rock Glaciers in the Permafrost of Svalbard. The University Courses on Svalbard (UNIS) and the Permafrost and Periglacial Processes, 11: 241–257. Department of Physical Geography, University of Kääb, A., Haeberli, W. and Gudmundsson, G.H. 1998. Oslo, supported this study. Two anonymous reviewers Analysing the creep of mountain permafrost using high precision aerial photogrammetry: 25 years of and the editor gave constructive comments. The authors monitoring Gruben Rock Glacier, Swiss Alps. extend their thanks to the persons and institutions Permafrost and Periglacial Processes, 8: 409–426. mentioned. Liestøl, O. 1962. 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