Invited reviewGlaciation of alpine valleys: The glacier – debris-covered glacier – rock glacier continuum
Introduction
In the present interglacial climate, some alpine landscapes sport small scraps of ice glaciers whereas terminal moraines record considerably extended ice glaciers from the past. Given that the landscape records the integrated effects of glacial to interglacial conditions, we seek to develop a quantitative framework that accounts for the bestiary of ice bodies that includes the effects of rocky debris production and transport. For clarity throughout the text, we will distinguish among three types of ice bodies: (i) bare ice glaciers; (ii) debris-covered glaciers; and (iii) rock glaciers. By building on an established modeling literature for bare ice and debris-covered glaciers, we show how the mechanics of rock glaciers can be viewed as an end-member response for ice glacier models that incorporate avalanche and rockfall dynamics and insulation of an ice core via a rocky mantle.
Rock glaciers are perhaps the most frequently encountered cryospheric features in modern, mid-latitude mountain ranges aside from seasonal snow and perennial snowfields. Many valleys once occupied by bare ice glaciers now support rock glaciers (e.g., Fig. 1). The downvalley snouts of rock glaciers are always rubbly and often oversteepened and thereby pose hazards to the unknowing hiker. Water from melting of the ice within rock glaciers is produced after all the snow has melted and can be an important source of flow to alpine rivers (e.g., Williams et al., 2006; Leopold et al., 2015). Rock glacier meltwater is also chemically distinct and, thus, an important regulator of bioavailable nutrients and heavy metal concentrations in streams, wetlands, and lakes fed by rock glaciers (e.g., Thies et al., 2013; Fegel et al., 2016). Rock glaciers have received considerable attention in the last couple decades. They have been cataloged in several geographic areas, for example in the Sierra Nevada (Millar and Westfall, 2008; Millar et al., 2013), western USA (Fountain Rock Glacier inventory; Johnson et al., 2007; McCabe and Fountain, 2013), Tyrolean Alps (Krainer and Ribis, 2012), Italian Alps, and on the fringes of Greenland (Humlum, 2000). In each of these settings they number in the hundreds to more than a thousand. Rock glaciers, however, are difficult to study, as seeing into them with geophysical methods is difficult (e.g., Degenhardt Jr. and Giardino, 2003; Degenhardt et al., 2003). The rock glacier setting in Disko Island, western Greenland, has been especially well documented (e.g., Humlum, 1996, Humlum, 2000, and references therein).
We acknowledge that an enduring debate exists about the origin of rock glaciers. Potter Jr (1972) states that a rock glacier is a tongue-like or lobate body usually of angular boulders that resembles a small glacier, generally occurs in high mountainous (or dry polar) terrain, and usually has ridges, furrows, and sometimes lobes on its surface, and has a steep front at the angle of repose. This definition is inherently morphological and says nothing about the genesis of the form. For most of a century, the origins of rock glaciers have been debated (see Whalley, 1974; Clark et al., 1998; Barsch, 1996): some argue for a glacial origin for interior ice, whereas others argue for a periglacial origin. We refer the reader to summaries of the history of thought about rock glaciers. These all start with the early work of Capps Jr (1910) and of Wahrhaftig and Cox (1959). Summaries include the state of knowledge from a 1996 Chapman Conference on rock glaciers (Clark et al., 1998), a further review of the topic a decade later (Haeberli et al., 2006), followed by Berthling (2011). It is clear from these reviews that (i) glacigenic ice cores exist in rock glaciers that meet the descriptive criteria of Potter Jr (1972), and (ii) rock glaciers also form from ice-saturated permafrost in similar alpine settings.
Our intent is neither to feed nor to settle this debate about the origin of rock glaciers. We acknowledge that rock glaciers as creeping features can originate from periglacial (e.g., Haeberli et al., 1998, Haeberli, 2000, Haeberli, 2005; Arenson et al., 2002) or from glacial processes (e.g., Monnier and Kinnard, 2015). Indeed, more recent work, using geophysical methods, reveals that rock glaciers are often composite forms in which debris-covered glaciers override older permafrost bodies and coalesce (e.g., Lugon et al., 2004; Haeberli, 2005; Ribolini and Fabre, 2006; Berthling, 2011). This suggests that there is some equifinality in rock glacier form as they can arise from the coevolution of periglacial and glacial processes (e.g., Haeberli, 2005; see also Giardino, 1983; Giardino and Vitek, 1988).
In this contribution, we explore the end-member scenario in which a rock glacier is cored primarily by clean glacigenic ice born of snow sources in the upper reaches of a valley. The highest concentration of rock is on the surface, whereas the interiors display concentrations of rocks that are small enough that the rheology of the body can be treated as that of pure ice. While some geophysical studies have shown a mixture of rock and ice in rock glacier interiors, or layering of pure ice with ice-saturated debris (e.g., Hausmann et al., 2007; Ribolini et al., 2010; Monnier and Kinnard, 2013), our approach remains valuable as an end-member example of rock glacier behavior. We further confine our focus to rock glaciers that occupy the centerlines of valleys as opposed to valley sidewalls and hence are most analogous to pure ice glaciers. A rock glacier evolving from purely periglacial processes on valley sidewalls would represent another end-member. This end-member approach allows us to ask valuable questions that place bounds on the behavior of rock glaciers: How would a rock glacier evolve if we treat it is as purely ice-cored? Does the resulting evolution match what is observed in some alpine landscapes?
We are inspired in part by the fact that rock glaciers have been observed forming from debris-covered glaciers based on direct observation and the analysis of remote sensing imagery (Shroder et al., 2000; Monnier and Kinnard, 2015) and may continue to do so in a warming climate. Our simulations span millennia, which provides a novel perspective of rock glacier evolution yet to be explored. A similar model incorporating solely periglacial processes and spanning millennia would be a valuable complement to our study, even though the two studies may produce divergent scenarios over millennial timescales.
As the recipe for making ice-cored rock glaciers requires sufficient supply of ice and rock, they are closely related to debris-covered glaciers. The importance of debris in dampening glacial retreat under modern climate change (e.g., Scherler et al., 2011) has focused attention on explicitly accounting for debris production and transport on alpine glaciers. To this end, we seek to identify the ingredients required for the formation of ice-cored rock glaciers and under what circumstances debris-covered glaciers convert to rock glaciers. We ask how these rock-involved glaciers respond to climate change in the future and in the past, and how this response differs from that of bare ice glaciers.
Rock glaciers are found at elevations below those that can support true (i.e., bare ice) glaciers. In other words, the environmental equilibrium line altitude (ELA) is above the landscape (Fig. 2). Throughout this paper, we distinguish between the environmental ELA (which represents the regional altitude of zero mass balance in the absence of shading, avalanching, and debris-cover effects) and the effective ELA (which accounts for these locally important variables). Rock glaciers rely upon avalanches to enhance snow thickness locally and upon thick rock cover to reduce melt, both of which lower the effective ELA. That so many rock glaciers exist in the mountains today reflects the importance of wind and avalanches in redistributing snow. Avalanches require steep slopes in excess of the angle of repose. Such steep headwall and sidewall slopes were generated by repeated occupation of the landscape in glacial times by larger, bare ice glaciers that tend to flatten valley floors and steepen valley walls. As such, a need exists for models that can account for the transition from bare ice to debris-covered to rock glaciers to adequately capture these dynamics over timescales of landscape evolution.
We summarize the continuum of debris-involved ice masses in Fig. 2. The differences result from the details of the patterns of mass balance and debris delivery.
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Glacier: Consider first a case in which the ELA intersects the topography, resulting in a classic bare ice glacier with an accumulation area above the ELA (case A). In the absence of debris, many existing one-dimensional glacier models (as well as many flavors of two-dimensional models) can reproduce glacier response to climate and climate change. The position of the terminus is governed by the mass balance profile and the geometry of the valley (e.g., Anderson et al., 2006); the terminus is found at a distance downvalley that is a bit more than twice the downvalley distance to the intersection of the ELA with the landscape. When examining a glacier with uniform width and a linear environmental mass balance gradient, the accumulation area ratio (AAR), the ratio of the accumulation to the total area of the glacier, is near 0.6.
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Debris-covered glacier: If debris is added to the surface of this glacier (case B), a debris-covered glacier occurs. If the debris is added in the accumulation area (e.g., at or near the base of the headwall), it will travel through the glacier, downward in the accumulation area, and upward in the ablation area as suspended clasts in the ice. The glacier surface will be debris free until the point of emergence of debris in the ablation area. There the emerging debris reduces the rate of ablation, and the glacier will extend accordingly. The glaciers can extend to much greater distances downvalley than the ice-free counterparts, depending on the strength and location of the debris source (Anderson and Anderson, 2016). The AAR is commensurately much smaller and can be significantly below 0.5 (Anderson and Anderson, 2016; Scherler et al., 2011). If debris is added instead to the surface of the ablation zone, as it can be when major landslides deliver debris across the glacier surface (e.g., Shulmeister et al., 2009; Shugar and Clague, 2011; Shugar et al., 2012; Vacco et al., 2010), no englacial component of the debris path exists. Instead the debris simply advects at the surface speed and alters the rate of ablation in accord with the thickness of the deposit. Terminus advance may occur when the debris lobe advects to the terminal region of the glacier.
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Rock glacier: If the environmental ELA is above the land surface, no glacier should occur in the valley (case C). If, however, the headwall of the valley is steep enough, accumulated snow will avalanche toward the valley bottom and form an avalanche cone. If this avalanche cone is thick enough and melt rates are low enough that the cone can persist through the summer, year after year, the ice will become thick enough to deform and a small glacier will form. In the absence of any debris, this glacier would be small indeed, perhaps extending a couple times the length of the avalanche zone. But if significant debris is entrained in the avalanches from the head wall or as rockfall deposited onto the avalanche cone, then debris can insulate the cone leading to the formation of a rock glacier. Owing to the very low sub-debris rates of ablation and small accumulation area, the rock glacier may become exceptionally asymmetric, with a very small accumulation zone and a long ablation zone. The AAR can, therefore, be extraordinarily small, on order of a few percent (Anderson and Anderson, 2016).
In this contribution we use the model presented in Anderson and Anderson (2016), which is complex enough to accommodate all of these cases yet remains efficient enough to run over thousands of years. The key addition to this prior mode is that it incorporates an avalanche cone in the accumulation zone. We begin with the bare ice case and present the ice dynamics and the meteorologically determined elevational pattern of rates of melt and accumulation. After Anderson and Anderson (2016), we reintroduce debris deposition, debris advection, and the insulating effect of surface debris on glacier melt. We then show the case in which the ELA lies below the base of the headwall, producing a glacier with partial debris cover (i.e., a debris-covered glacier). Then deviating from Anderson and Anderson (2016), we present the case in which the environmental ELA lies above the topography. We introduce a snow avalanche cone and more nuanced debris delivery patterns, the combination of which leads to the formation of a rock glacier. In this case, the avalanche cone constitutes the accumulation area and the entire ablation zone is debris covered. Finally, we explore the evolution of a glacier as it transitions between debris-covered glacier and rock glacier states when subjected to climate change.
Section snippets
Mass balance and ice discharge
An alpine glacier forms when a valley receives on average more snowfall than can melt on an annual basis. Internal deformation of the ice and basal sliding transports ice from this accumulation zone to an ablation zone where the ice melts. The equilibrium line separates these zones, the elevation of which is the ELA. The meteorological setting may most compactly be summarized by a local net balance field b(z) [m/y], which is positive above the ELA and negative below it (Fig. 3).
At any location
Debris-covered glaciers
Here we address the role of debris on a glacier surface. As the damping of the rate of melt of underlying ice depends upon the thickness of the debris layer, we must track the evolution of the thickness of the debris layer, h(x), and capture its influence on the environmental rate of melt. Here we summarize the essential ingredients of the problem as presented in Anderson and Anderson, 2016, Anderson and Anderson, 2018.
Rock glaciers
The dominant differences between rock glaciers and the debris-covered glaciers described above are the restriction of the accumulation zone to an avalanche cone and the complete debris coverage of the ablation zone of rock glaciers. Rock glaciers occur where the environmental ELA lies above the topography. We have summarized the rock dynamics, ice dynamics, and the role of debris cover in altering the summer balance. As such, we focus here on capturing the essence of headwall processes in
Discussion
The observed hundreds of meter lengths, tens of meters thicknesses, and meters per year speeds of rock glaciers are well described by the model presented here. When debris is being shed from the headwall, the debris dampens rates of ablation and serves to extend bare ice glaciers significantly downvalley and generates a debris-covered glacier with partial debris cover. Rock glaciers occur when the ELA lies above the topography and when a headwall supplies sufficient quantities of snow
Conclusions
By combining expressions for the environmental mass balance for ice dynamics that control the evolution of pure ice glaciers with the flux of debris, the effects of debris on rate of ablation, and the trajectories of debris, we can model the full continuum of glaciers, including glacigenic rock glaciers. Debris derived from headwall erosion can significantly lengthen the ablation zone of glaciers, thereby reducing the accumulation area ratio (Anderson and Anderson, 2016). Rock glaciers
Acknowledgements
We gratefully acknowledge the Earth Lab initiative at the University of Colorado at Boulder for support. In addition, this work was funded by National Science Foundation (NSF) awards EAR-1552883 and EAR-1123855. We thank W. Haeberli and two anonymous reviewers for critical assessment of the manuscript, and chief-editor Richard Marston for careful editing of the final version.
Competing interests
The authors declare they have no competing interests in this research.
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2024, Quaternary Science ReviewsTowards a sediment transfer capacity index of rock glaciers: Examples from two catchments in South Tyrol, (Eastern Italian Alps)
2022, CatenaCitation Excerpt :Movement of rock glaciers is enabled both by the deformation of ice-cemented fine debris within their body as well as sliding along a distinct inner shear horizon (e.g. Arenson, 2002; Kenner et al., 2017). Berthling (2011) defined rock glaciers as landforms determined by permafrost conditions, acknowledging that they may be formed either under glacial or periglacial realms (e.g. Anderson et al., 2018; Monnier and Kinnard, 2015; Whalley, 2020; Whalley and Martin, 1992). Recently, also a paraglacial mechanism taking into consideration landscape relaxation processes after deglaciation was proposed for rock glaciers formation by Knight et al. (2019).