Rock and Protalus Forms AKa¨a¨b, University of Oslo, Oslo, Norway

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Definition and Distribution conditions, and processes of debris reworking. If this layer is at least as thick as the local , that is, the seasonal thaw Perennially frozen debris in cold mountains may, under the depth under the specific local conditions, then positive ground force of gravity, form so-called rock glaciers, which often show temperatures during summer cannot reach down to the ground a shape similar to that of lava streams (Figures 1–7). Formerly, ice containing body. Thus, ground ice can persist rock glaciers were often thought to be a form of debris-covered for very long periods of time (Haeberli et al., 2006). glaciers, which led to the term ‘rock glaciers.’ Since then, a From the aforementioned principle, it becomes clear that number of fundamental differences between rock glaciers and the long-lasting persistence of the frozen core within a rock glaciers related, for example, to their thermal conditions and requires permafrost conditions. The difference between mass balance have been established. This has led to the use of rock glaciers, dead ice, or debris-covered glaciers can thus be the term ‘rock glaciers’ to distinguish their individual character described by the thermal state of the material. from that of glaciers (Barsch, 1996). This meaning of ‘rock Transformations between these conditions are possible. For glacier’ is used in this article, although the equivalent term instance, glacier ice under an insulating debris cover and lying ’’ is also established in the scientific literature. within a permafrost environment is subject to seasonal ice Rock glaciers occur in most cold mountains of Earth. These melt, though substantially damped, wherever the debris cover include the Andes, the Rocky Mountains, the European Alps, is thinner than the local seasonal thaw depth. The ice body will the Pyrenees, the Caucasus region, the Central Asian mountain continuously lose mass if not compensated by the advection of ranges, the Siberian mountain ranges, the Himalayas, the glacier ice. The resulting meltwater, and with it the contained New Zealand Alps, Greenland, Antarctica, and the Arctic and energy, can percolate into and warm the underlying ice body. Antarctic Islands (Barsch, 1996; Martin and Whalley, 1987). If the ice body contains sufficient debris, this material will Rock glaciers often occur in similar, though slightly colder and accumulate at the top of the , leading to a thickening drier climatic settings as glaciers (Brazier et al., 1998; Humlum, of the debris cover. When the thickness of the debris cover 1998). It is also speculated, on the basis of space imagery, that equals the local seasonal thaw depth, the underlying ice is rock glacier-like features exist on . Under suitable climatic protected against surface melt unless lateral processes such as and topographic conditions – that is, mean annual air tem- erosion occur. Thus, the debris within rock glaciers may stem peratures between roughly 2 and 6 C, total annual precip- from periglacial and glacial processes and sources, and the rock itation between 500 and 1500 mm, and sufficient debris glacier ice content may originate, for example, from refreezing production from rock walls (Etzelmu¨ller and Frauenfelder, rain and meltwater, glaciers, buried ice and snow patches, or 2009) – rock glaciers may be abundant, with their number combinations thereof (Haeberli and Vonder Mu¨hll, 1996; often exceeding that of glaciers in the same mountain range, Haeberli et al., 2006). Temporal transformations and spatial but with significantly smaller areas for individual rock glaciers. transitions between rock glaciers, debris-covered glaciers or dead ice, and ice-cored are often found in nature. Being closely related to permafrost conditions, rock glaciers indicate the existence of discontinuous or continuous moun- Thermal Conditions tain permafrost on a regional scale (Harris et al., 2009). The lower boundary of regional rock glacier distribution, however, A fundamental characteristic of rock glaciers is their thermal does not generally coincide exactly with the lower boundary of state (Haeberli, 2000; Haeberli et al., 2006). They consist of a permafrost distribution (Frauenfelder et al., 2008). Rock gla- perennially frozen mixture of debris and ice, that is, their long- ciers may end above the regional permafrost limit due to term existence requires permafrost conditions. Permafrost is topographic constraints, limited debris supply, or insufficient defined as lithospheric material with year-round negative time to fully develop. However, the coarse debris cover on top ground temperatures (Harris et al., 2009). A surface layer of of a rock glacier favors ground cooling due to enhanced heat depth ranging from several decimeters to meters thaws during transfer to the atmosphere, allowing rock glaciers to extend summer and is termed the ‘active layer.’ Below the ‘permafrost downslope below the regional permafrost limit. table’ – the lower boundary of the active layer – the tempera- tures in the ‘permafrost body’ remain negative down to the ‘permafrost base’ (Haeberli, 1985; Haeberli et al., 2006; Vonder Mu¨hll et al., 2003). Thus, the mixture of debris and Composition ice within the permafrost body remains frozen throughout the year, possibly over centuries and millennia if the thermal con- The composition of rock glaciers is investigated by boreholes ditions do not change substantially over time. and geophysical surveys, particularly geoseismics, direct cur- The surface layer of rock glaciers consists of debris with rent resistivity soundings, or georadar. So far, only a few bore- typical diameters ranging from several centimeters or decime- holes have been drilled through rock glaciers (Arenson et al., ters to meters, depending on the geological setting, weathering 2002; Haeberli et al., 1998, 2006; Harris et al., 2009; Hoelzle

535 536 PERMAFROST AND PERIGLACIAL FEATURES | Rock Glaciers and Protalus Forms

Figure 1 Active rock glacier in the Muragl valley, Upper Engadine, Figure 4 Inactive rock glaciers in the Upper Engadine, Swiss Alps. Swiss Alps. Photograph by R. Frauenfelder. Note the yellow color from lichen cover and the vegetated rock glacier fronts.

Figure 2 Active rock glacier Murtel, Upper Engadine, Swiss Alps. Figure 5 Relict rock glacier at the Julier Pass, Swiss Alps. Note the complete vegetation cover and the concave forms from ice loss.

Figure 3 Active rock glacier in the Suvretta valley, Upper Engadine, Figure 6 Typical ramp-type rock glaciers at Nordenskjo¨ldkysten, Swiss Alps. Svalbard. PERMAFROST AND PERIGLACIAL FEATURES | Rock Glaciers and Protalus Forms 537

Rock wall

2800 2700

Figure 7 Rock glacier in the Tien Shan. Photograph by S. Titkov. et al., 1998; Steig et al., 1998) and, therefore, the extent to –1 which their findings can be generalized is uncertain. All the 10 cm a drillings have revealed a surface layer of ice-free debris with a S thickness on the order of meters (active layer; Haeberli et al., WE 2006). The underlying layer, a few tens of meters thick, consists N of a frozen mixture of ice and debris with an ice content 20 m 100 m ranging from close to 100% to a few tens of percent. In rock glaciers where deep core drilling was conducted, a third layer of Figure 8 Surface displacement vectors on Murtel rock glacier, frozen coarse debris with little ice content was encountered. Swiss Alps, measured from air photographs of 1987 and 1996. This layer is believed to originate from surface blocks that had fallen over the rock glacier front and were overridden by the advancing rock glacier (see section ‘Kinematics and Advance’; limited by the deformation process itself, but rather by the Haeberli et al., 1998). available measurement technique (e.g., digital photogramme- In the rock glaciers with deep core drillings, a meter-thick try, global navigation satellite system, radar interferometry; layer of sand-rich ice was found within the second ice-rich layer, Lambiel and Delaloye, 2004; Strozzi et al., 2004). at depths between 15 and 30 m. The majority of the horizontal A coherent ice content within rock glaciers leads to the deformation within the rock glacier was found to be concen- lateral transfer of stresses and thus to a coherent velocity trated in these sandy ice layers (see section ‘Kinematics and field. Owing to effects such as lateral friction and variations Advance’; Arenson et al., 2002). in viscosity, the highest horizontal speeds on a rock glacier are Many geophysical soundings have confirmed the general usually found along its centerline. In the few available bore- layering and composition described earlier for other rock gla- holes through rock glaciers, the horizontal deformation was ciers (Berthling et al., 1998, 2000; Bucki et al., 2004; Isaksen concentrated in horizontal layers some decimeters to meters et al., 2000). In addition, some high-resolution soundings thick, rather than distributed throughout the body of the rock have revealed fine layers within the ice-rich subsurface layer. glacier, with a clear parabolic variation of horizontal speeds These microlayers were attributed to seasonal sequences of with depth such as found for glacier ice (Arenson et al., 2002). snow/ice and debris deposition at the rock glacier surface, and The basic concept of mass transport within a rock glacier their continuous incorporation and deformation within the can be derived from the characteristic thermal and kinematic creeping body (Berthling et al., 2000; Humlum et al., 2007). conditions: debris is deposited at the rock glacier surface, either directly from weathering processes at the headwall or from other secondary sources, in most cases presumably glaciers Kinematics and Advance and avalanches (Humlum, 1998; Humlum et al., 2007). Together with ice from different sources (see section ‘Thermal Besides their special thermal conditions, the second funda- Conditions’), this mass represents the mass input to the rock mental characteristic of rock glaciers is their mode of displace- glacier system. Following a basic law of fluid dynamics, the ment. Given sufficient terrain slope, the ice–debris mixture surface debris is transported downslope, with the highest creep within a rock glacier deforms under the force of gravity and speeds at the rock glacier surface. The advected surface material the stress of the vertical column. The resulting surface speeds then falls over the steep rock glacier front. The grain-size sort- on rock glaciers may be several meters per year (Ka¨a¨b et al., ing along the frontal slope occurring during this process leads 2003; Kaufmann and Ladsta¨dter, 2002; Figure 8). Rock glacier to the typical appearance of rock glacier fronts. The surface surface speed depends, among other things, on surface slope, debris that accumulates at the front is then overridden by the composition and internal structure, thickness of the ice-rich advancing rock glacier and again incorporated into its base, in body, and ground temperature (Haeberli, 1985). The observed a ‘caterpillar’ or ‘conveyor belt’ fashion (Ka¨a¨b and Reichmuth, minimum surface speeds of a few millimeters per year are not 2005; Figure 9). On account of their nonice sediment content 538 PERMAFROST AND PERIGLACIAL FEATURES | Rock Glaciers and Protalus Forms

Protalus rampart 1 Perennially frozen 2

Debris accumulation Ice-supersaturated permafrost frost heave steady-state creep

Grain-size sorting Permafrost Structured permafrost creep damped creep

Rock glacier – compression Rock glacier – extension 3 4 in on steep slope

Surface blocks from foot of scree slope

Blocks falling from surface Stiff basal layer

Evacuation

Figure 9 Development stages and structure of an idealized rock glacier. Particle-size sorting on scree slopes concentrates finer particles on the upper slope sections (1) that become supersaturated with ice and start to deform at higher rates than coarser particles, which are not ice-supersaturated, at the slope base (2). Further debris from the headwall accumulates preferentially on the resulting ramp (2) and rafts downslope above the creeping frozen body (3). This blocky upper layer is transported over the rock glacier front, overridden, and eventually reincorporated into the rock glacier as its basal layer (3), unless debris falls over a terrain ridge and thus leaves the rock glacier system (4).

and thermal conditions, rock glaciers, in contrast to glaciers, mass, usually down the steepest slopes. The ratios between cannot retreat. In the case of a total cessation of mass supply, a rock glacier length and width vary significantly, with values rock glacier will remain stagnant in its horizontal extent, but it both above and below 1. The latter ratio reflects a number of will not retreat. The main process of mass loss is the melt-out of internal conditions such as viscosity, and external factors such ice at the rock glacier front, where the thermally protecting as topography or mass supply. debris cover becomes thinner than the thaw depth due to Many rock glaciers show a characteristic surface morphol- frontal erosion. In addition, the frontal grain-size sorting ogy consisting of transverse and/or longitudinal ridges and leads to smaller grain sizes in the upper part of the front, furrows. This morphology, in addition to the overall shape, which reduces the insulation effect in comparison to the makes rock glaciers resemble lava streams. So far, little is known coarser debris at the rock glacier surface. Further processes of about the origin of the rock glacier surface morphology. mass loss of rock glaciers include evacuation of debris, erosion According to the main hypotheses, the formation of trans- and rockfall over steep terrain, debris flows, or fluvial and verse ridges and furrows on rock glaciers might be attributed to coastal erosion. Owing to the melt-out of ice, the potential (a) external factors, such as variations in debris input or cli- loss of nonice debris, and the decrease of horizontal speeds mate conditions; or (b) internal factors, such as the dynamic with depth, the advance rate of rock glaciers is significantly evolution of ridges under compressive flow, or disturbance smaller than their surface speed. Advance rates measured so far propagation from differential movement of discrete layers. In range from a few centimeters to decimeters per year. addition, it is unknown whether (c) transverse ridges and furrows represent dynamic structures, for example, forming from emergence under compressive flow regime (i.e., active Shape and Surface Morphology formation); or (d) if these forms are thermally derived due to, for instance, differential melt () or frost heave Rock glaciers can be up to several hundred meters wide and a processes (i.e., passive formation). It is important to note that few kilometers long. Their altitudinal extent may attain several these four elements of hypotheses do not necessarily contradict hundred meters, and their thickness may attain several tens of one another. One possible process might enhance the other, meters. Their overall shape and extent reflects the creep of the or a combination might be necessary for the formation of PERMAFROST AND PERIGLACIAL FEATURES | Rock Glaciers and Protalus Forms 539 microtopographic structures. A number of studies suggest that and the rock glacier margins, are elevated above the rock glacier compressive flow is a major process involved in the formation sections with formerly higher ice contents (Figure 5); thus, the of transverse ridges (Ka¨a¨b and Weber, 2004). original debris concentration of the active phase is still The high thermal inertia of rock glaciers allows these frozen reflected in the surface topography. The characteristic ridges bodies to continuously deform and preserves their topography on rock glaciers can often also be recognized on relict rock over centuries and millennia (Haeberli, 2000; Ka¨a¨b et al., glaciers. Similarly, the frontal zones, which in active rock gla- 2003). In addition, the debris content and the ice-free, blocky ciers contain less ice, become visible as terminal ridges. Relict active layer of rock glaciers contribute to the conservation of rock glaciers might be covered by dense vegetation, even trees. surface features even over periods of ice melt. The surface In some rock glaciers with clear relict characteristics, patchy topography of rock glaciers cumulatively reflects the dynamic permafrost and ground-ice occurrences and even gravitational history and thus, in a complex way, their present and past movements on the order of millimeters per year have been internal conditions and environment. found, pointing to continuous transitions between the stages of rock glacier activity (Strozzi et al., 2004). A number of studies have shown that the age of active Activity and Age rock glaciers is on the order of some to many millennia. The dating methods used range from surface dating techniques The degree of activity of rock glaciers is usually classified into (Frauenfelder et al., 2004; Laustella et al., 2003), 14C dating three different types: (i) active, (ii) inactive, and (iii) relict of organic material recovered from boreholes (Haeberli et al., (Ikeda and Matsuoka, 2002). It should, however, be noted 1999), flowline calculations (Frauenfelder et al., 2004; Ka¨a¨b that this classification is a theoretical concept. The transition et al., 1997, 1998), and lichen measurements (Haeberli et al., between the three stages is actually continuous. 1979; Sloan and Dyke, 1998) to paleoclimatic considerations Active rock glaciers have a coherent core of ice-rich frozen and numerical models (Frauenfelder et al., 2001, 2008). In the debris, the deformation of which leads to the rock glacier European Alps, today’s relict rock glaciers are believed to have creep, with rates of several centimeters to meters per year. The formed during the last Late Glacial period, though much older permafrost conditions of the body are intact, with perennially forms might exist at places where the climate conditions per- negative ground temperatures throughout the entire column, mitted continuous evolution, and glaciations did not interrupt except for the seasonally thawed active layer. The continuous this evolution. creep and related debris transport along the surface leads to the characteristic steep front with freshly exposed fines in the upper part and coarser debris in the lower part, vertically sorted Environmental Change and Climatic Significance by grain size due to debris fall along the front (see section ‘Kinematics and Advance’). The continuous deformation of Rock glaciers are intimately dependent on their geoecological the rock glacier surface, rotation and possibly related uplift of environment (Barsch, 1996). Thus, changes in these condi- individual blocks, and the outwash of fines toward the perma- tions have a potentially strong effect on the rock glacier system. frost table allow only a few specialized plant species to grow Millennia-scale climatic changes – for example, temperature and prevent lichens from reaching great age and size (Burga shifts such as between the Last Glacial Maximum, the Late et al., 2004; Haeberli et al., 1979; Sloan and Dyke, 1998). Glacial period, and the Holocene – affect, among others, the Inactive rock glaciers have no coherent ice core and thus no thermal conditions, and the production and availability of coherent velocity field due to the lack of stress transfer. They debris. An increase in ground temperatures, possibly above show no significant horizontal speed, except perhaps local the 0 C threshold, and/or a reduction of headwall weathering movement, for example, due to thermokarst processes, patches and rockfall onto underlying rock glaciers (Olyphant, 1987) of ice-rich ground, or erosion processes. Inactive rock glaciers are, for instance, most likely responsible for the decline in contain a thinner permafrost body and a thicker active layer activity of rock glaciers at the beginning of the Holocene. than active ones (see section ‘Environmental Change and Similarly, the atmospheric warming trends since the end of Climatic Significance’). They can be transitional forms between the Little Ice Age observed in most cold mountain regions active and relict rock glaciers. However, indications have also may be responsible for the recent decay of rock glaciers. In- been found that active rock glaciers might become inactive for deed, the altitudinal belts where active, inactive, and relict rock certain periods – for example, due to climatic changes – but glaciers are found suggest that temperature changes are an reactivate after environmental conditions become suitable important driver of changes in rock glacier activity (Frauen- again. Owing to their kinematic inactivity, inactive rock gla- felder et al., 2001, 2008). ciers or inactive parts of rock glacier complexes usually show Significantly less well investigated, but potentially of simi- more vegetation cover, and more and larger lichens compared lar importance to temperature, is the influence of long-term to active rock glaciers under a similar environmental setting. changes in precipitation regime. Related changes in mass sup- In particular, the frontal parts with fines exposed are preferred ply or ground temperature may also be due to spatiotemporal locations for the invasion of vegetation. Owing to the lack of changes in snow cover. Changes in the climatic regime may material supply toward and over the front, erosion is able to also indirectly favor or hinder rock glacier development. Thus, reduce the frontal slope. rock glaciers may be overridden by glaciers in times of climate No permafrost or ice is present within relict rock glaciers. change or rock glaciers may develop or recover at locations On account of this volume loss of excess ice, rock glacier previously occupied by glaciers (see section ‘Thermal Condi- sections with formerly higher debris content, such as ridges tions’; Ka¨a¨b and Kneisel, 2006; Maisch et al., 2003). 540 PERMAFROST AND PERIGLACIAL FEATURES | Rock Glaciers and Protalus Forms

As yet, too little is known about the exact climatic signifi- Berthling I, Etzelmu¨ller B, Eiken T, and Sollid JL (1998) Rock glaciers on Prins Karls cance of rock glaciers in order to use them reliably for paleo- Forland, Svalbard. I. Internal structure, flow velocity and morphology. Permafrost and Periglacial Processes 9: 135–145. climate reconstruction (Brazier et al., 1998; Frauenfelder et al., Berthling I, Etzelmu¨ller B, Isaksen K, and Sollid JL (2000) Rock glaciers on Prins Karls 2001; Humlum, 1998). The typical climate of modern active Forland. II. GPR soundings and the development of internal structures. Permafrost rock glaciers seems similar to the conditions found at the and Periglacial Processes 11: 357–369. equilibrium line altitude of modern glaciers, though it is some- Brazier V, Kirkbride MP, and Owens IF (1998) The relationship between climate and what drier. Thus, the altitudinal belts where active, inactive, rock glacier distribution in the Ben Ohau Range, New Zealand. Geografiska Annaler 80A: 193–205. and relict rock glaciers are found can be used as proxies for Bucki AK, Echelmeyer KA, and MacInnes S (2004) The thickness and internal structure paleoclimatic conditions that were suitable for active rock of Fireweed rock glacier, Alaska, USA, as determined by geophysical methods. glaciers to develop (Frauenfelder et al., 2001). Such reconstruc- Journal of 50: 67–75. tion is, however, substantially complicated by the fact that rock Burga C, Frauenfelder R, Ruffet J, Hoelzle M, and Ka¨a¨b A (2004) Vegetation on Alpine rock glacier surfaces: A contribution to abundance and dynamics on extreme plant glacier evolution depends not only on climatic conditions, but habitats. Flora 199: 505–515. also on debris supply and interactions with glaciation. 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