Rock Glaciers and Protalus Forms AKa¨a¨b, University of Oslo, Oslo, Norway ã 2013 Elsevier B.V. All rights reserved. Definition and Distribution conditions, and processes of debris reworking. If this layer is at least as thick as the local active layer, 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 permafrost 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 glacier 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 ’rock glacier’ 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 ice core, 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 Mars. 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 À2 and À6 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 moraines 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.