Three glacier–rock glacier transitional landforms in the central
Andes of Chile are investigated over the last decades in order to highlight
and question the significance of their landscape and flow dynamics.
Historical (1955–2000) aerial photos and contemporary (> 2000)
Geoeye satellite images were used together with common processing operations,
including imagery orthorectification, digital elevation model generation, and
image feature tracking. At each site, the rock glacier morphology area,
thermokarst area, elevation changes, and horizontal surface displacements
were mapped. The evolution of the landforms over the study period is
remarkable, with rapid landscape changes, particularly an expansion of rock
glacier morphology areas. Elevation changes were heterogeneous, especially in
debris-covered glacier areas with large heaving or lowering up to more than
Glacier–rock glacier interactions related to Holocene glacier fluctuations (e.g. Haeberli, 2005) and the current evolution of small debris-covered glaciers having survived to the post-Little Ice Age (LIA) warming (e.g. Bosson and Lambiel, 2016) are important issues in high-mountain studies. They may provide key insights into the mechanisms of rock glacier development (Dusik et al., 2015) and of cryosphere stability and resilience against climate changes; the latter topic is of societal importance in arid–semiarid mountain areas, where the potential permanence of underground solid water resources subsequent to deglaciation may constitute a non-negligible water resource (e.g. Rangecroft et al., 2013).
The most striking geomorphological expression of glacier–rock glacier interactions is large glacier–rock glacier transitional landforms, which are assemblages of debris-covered glaciers in their upper part and rock glaciers in their lower part (e.g. Kääb et al., 1997; Krainer and Mostler, 2000; Ribolini et al., 2007; Monnier et al., 2014; Janke et al., 2015). Here, it is important to recall and highlight the differences between both types of landforms (Nakawo et al., 2000; Kääb and Weber, 2004; Haeberli et al., 2006; Degenhardt, 2009; Benn and Evans, 2010; Berthling, 2011; Cogley et al., 2011). Rock glaciers are perennially frozen homo- or heterogeneous ice–rock mixtures covered with a continuous and several-metre-thick ice-free debris layer that thaws every summer (known as the permafrost “active layer”); rock glacier movement is governed by gravity-driven permafrost creep. Debris-covered glaciers are glaciers covered with a thin (no more than several decimetres thick) and generally discontinuous debris layer; debris-covered glaciers movement is governed by gravity-driven ice creep and sometimes basal slip in response to a mass balance gradient; debris-covered glaciers do not require permafrost conditions. Rock glaciers and debris-covered glaciers exhibit distinct morphologies that are of critical importance in the surface energy balance and subsurface heat transfer. On their surface, rock glaciers exhibit “the whole spectrum of forms created by cohesive flows” (Barsch, 1992, p. 176) of “lava-stream-like …viscous material” (Haeberli, 1985, p. 92). These features vary for each case and study area; according to our field surveys in the Andes, they can be grouped into three main types: small-scale (< 1 m high) ripples or undulations resulting from deformations in the active debris layer moving together with the underlying perennially frozen core, medium-scale (1–5 m high) ridge-and-furrow assemblages resulting from the compression of the whole ice-debris mixture, and large-scale (5–20 m thick and > 100 m long) superimposed flow lobes upon which the first two feature types may naturally appear. Hereafter, we will simply refer to these features as “cohesive flow-evocative features”. Contrarily, debris-covered glaciers are characterized by a chaotic distribution of features evocating surface instability such as hummocks, collapses, crevasses, meandering furrows, and thermokarst depressions and pounds. As a consequence, on rock glaciers the large- and fine-scale surface topography is rather smooth and convex, whereas on debris-covered glaciers it is rather rough and concave. Another morphological difference is the presence of ice visible from the surface: whereas ice is generally invisible from the surface of rock glaciers, it is frequently exposed on debris-covered glaciers due to the discontinuity of the debris cover or the occurrence of the aforementioned morphological features. Finally, and correlatively, over pluri-annual to pluri-decadal periods the morphology of well-developed rock glaciers is quite stable (besides cases of climate warming-related destabilizations, the geometry of surface features evolves but their overall pattern remains the same), while debris-covered glacier morphology is characterized by instability (surface features rapidly appear and disappear).
According to the literature at least three types of glacier–rock glacier
interactions can be distinguished:
The readvance(s) and superimposition/embedding of glaciers or debris-covered
glaciers onto/into rock glaciers, with related geomorphological and thermal
consequences (Lugon et al., 2004; Haeberli, 2005; Kääb and Kneisel,
2006; Ribolini et al., 2007, 2010; Bodin et al., 2010; Monnier et al., 2011,
2014; Dusik et al., 2015). This is the sensu stricto significance of “glacier–rock
glacier relationships” (Haeberli, 2005) as defined by what has been called
the “permafrost school” in reference to the long-term “rock glacier
controversy” (see Berthling, 2011). The continuous derivation of a rock glacier from a debris-covered glacier by
evolution of the surface morphology (see above) together with the
conservation and creep of a massive and continuous core of glacier ice
(e.g. Potter, 1972; Johnson, 1980; Whalley and Martin, 1992; Potter et al.,
1998; Humlum, 2000). This process was not initially called a “glacier–rock
glacier relationship”; this view is indeed held by what has been called the
“continuum school”, in opposition to the permafrost school (Berthling, 2011).
Nevertheless, such a phenomenon does belong, literarily, to the domain of
glacier–rock glacier interactions. The transformation of a debris-covered glacier into a rock glacier not only
by the evolution of the surface morphology but also by the evolution of the
inner structure, i.e. the transformation of the debris-covered continuous
ice body into a perennially frozen ice–rock mixture by addition from the
surface of debris and periglacial ice and fragmenting of the initial glacier
ice core. This has been described as an alternative to the dichotomous
debate between the permafrost school and continuum school (Monnier and
Kinnard, 2015); such phenomenon has been described as achievable over a
human-life or historical timescale (Schroder et al., 2000; Monnier and Kinnard,
2015; Seppi et al., 2015).
In the present study, we aim to provide insights into the aforementioned
issue using the variety of glacier–rock glacier transitional landforms
encountered in the semiarid Andes of Chile and Argentina. These landforms
have shown a particularly rapid evolution over the last decades which allow
studying glacier–rock glacier interactions on an historical timescale.
Three landforms with distinct morphologies have been chosen in the central
Andes of Chile in an attempt to diagnose their geomorphological
significance, especially in terms of glacier–rock glacier interactions and
cryosphere persistence in the current climatic context. To this purpose,
this study makes use of aerial and satellite imagery and remote sensing
techniques in order to document the morphological and dynamical evolution of
the studied landforms over a pluri-decadal time span.
We studied three glacier–rock glacier transitional landforms in the
central Andes of Chile, respectively named Navarro, Presenteseracae, and Las
Tetas (Fig. 1). Navarro and Presenteseracae are located in the Navarro
Valley, in the upper Aconcagua River catchment (33
Location of the study sites. Drainage network, which reflects the variations in climatic–hydrologic conditions along the Chilean territory, is shown in blue.
The upper Navarro Valley belongs to the Juncal River catchment and Juncal
Natural Park, which are part of the upper Aconcagua River catchment, in the
Valparaíso region of Chile (32
Geomorphological legend shared for all subsequent figures.
Map of the Navarro Valley. See Fig. 2 for legend. The background of the map is the 2014 Geoeye image draped over the Geoeye DEM (see the “Material and methods” section). Elevation contours are derived from the Geoeye DEM and the contour interval is 20 m. The boundary between the Navarro's western and eastern units is indicated with a dashed white line. The red circle indicates the location described in the text where morainic crests and rock glacier lobes are superimposed. Note also the decayed (D) rock glacier lobes in the area between Navarro and Presenteseracae.
Navarro fills the major part of the upper Navarro Valley floor between
Photos of the lower
Navarro is divided between an eastern and a western unit, with the two being
separated by a central series of aligned morainic crests (Fig. 3). The
eastern unit, which is located in the more shadowed northeastern part of
Navarro Valley, is
Monnier and Kinnard (2015) provided an empirical model of permafrost probability based on logistical regression for the upper Aconcagua River catchment. According to this model, Navarro may be in a permafrost state. The permafrost probability is close to 1 in the upper parts; nevertheless, there is a marked decreasing gradient in permafrost probability from 0.9 to 0.7 between the central part and the terminus of the western unit (Fig. 3).
Presenteseracae is a small (
Las Tetas is located in the Colorado Valley, which is the uppermost part of
the Elqui River valley, in the Norte Chico region of Chile (30
Las Tetas is a
Map of the Las Tetas landform. See Fig. 2 for legend. The background of the map is the 2012 Geoeye image draped over the Geoeye DEM (see the “Material and methods” section).
We acquired historical (prior to 2000) aerial photos and contemporary (after 2000) satellite images for the three study sites. Stereo pairs of aerial photos were inspected, selected, and scanned at the Geographic and Military Institute (IGM) of Chile. Scanning was configured in order to yield a ground resolution of 1 m. At Las Tetas, photos from 1978 and 2000 were selected; at Navarro and Presenteseracae, photos from 1955 and 2000 were selected. A stereo pair of Geoeye satellite images was also acquired for each site. The Geoeye imagery was acquired on 23 March 2012 and 14 February 2014 at Las Tetas and Navarro Valley, respectively, as panchromatic image stereo pairs (0.5 m of resolution) along with four bands in the near-infrared, red, green, and blue spectra (2 m of resolution).
Orthoimages, orthophotos, and altimetric information were generated from the
data. The first step involved building a digital elevation model (DEM) from
the stereo pair of Geoeye satellite images. The Geoeye images were
triangulated using a rational polynomial camera (RPC) model supplied by the
data provider. The exterior orientation was constrained using one or two
(according to the site) ground control points (GCPs) acquired with a
differential GPS system in the field in 2014 over bedrock outcrops visible
on the images. Sets of three-dimensional (3-D) points were extracted
automatically using standard procedures of digital photogrammetry
(Kääb, 2005) and edited manually to remove errors. A 2
The Geoeye images were pan-sharpened and orthorectified using the Geoeye DEM. The aerial photos were then orthorectified using the corresponding DEMs, except when no reliable DEM could be obtained (as for 1955 at Navarro); in that case, the Geoeye DEM was used. The orthorectification was constrained by the internal camera information, tie points, and GCPs extracted during the process. The accuracy of the orthorectification was estimated using the GCPs. The root mean square error (RMSE) corresponding to the sets of GCPs at the different times is displayed in Table 1. The ground resolution of the orthophotos was then resampled at 0.5 m in order to equal that of the Geoeye products.
Errors generated during the aerial photo processing. The ground root mean square error (RMSE) relates to sets of ground control points (GCPs) extracted from the Geoeye orthoimage and used for the orthorectification of the aerial photos.
The altimetric information was used to calculate the elevation changes of
the landforms between the different dates, after removal of the vertical
bias. The total elevation change was further converted into annual rates of
elevation change. As outlined by Lambiel and Delaloye (2004), elevation
changes at the surface of rock glaciers may be explained by several and
possibly concomitant factors: (i) downslope movement of the landform and
advection of local topographic features, (ii) extensive or compressive flow,
and (iii) melting or aggradation of internal ice. Therefore, it is difficult
to unambiguously interpret elevation changes. Studying the Muragl rock
glacier (Swiss Alps), Kääb and Vollmer (2000) highlighted how mass
advection caused subtle elevation changes (between
Uncertainty related to the measurement of annual elevation changes. Reported
uncertainties correspond to 1 and 2-standard deviation (
The geomorphology of each landform was carefully interpreted from the orthoimages and orthophotos. First, we located and mapped the boundary between debris-covered and rock glacier morphology, according to the detailed criteria of differentiation presented in the Introduction. The thermokarst area was also monitored over time by mapping the thermokarst depressions at the surface of the landforms as polygonal shapes, and their total area was calculated. Salient and recently appeared features such as cohesive flow-evocative ridges on Presenteseracae and cracks on Las Tetas were also mapped.
We used image feature tracking in order to measure horizontal displacements
at the surface of the landforms. Computer-programmed image feature tracking
is a sub-pixel precision photogrammetric technique that has been widely used
for studying the kinematics of glaciers, rock glaciers, and other mass
movements. We followed the principles and guidelines provided by
Kääb and Vollmer (2000), Kääb (2005), Wangensteen et al. (2006),
Debella-Gilo and Kääb (2011), and Heid and Kääb (2012).
We used ImGRAFT, which is an open-source image feature tracking
toolbox for MATLAB (Messerli and Grinsted, 2015) using two orthoimages (from
spaceborne, airborne, or terrestrial sensors) of the same area and
resolution but at different times. All the orthoimages were pre-processed in
order to enhance their contrast. Two template matching methods were tested:
normalized cross-correlation (NCC) and orientation correlation (OC). The NCC
method was found to yield more consistent results at the different sites and
was thus used for this study. NCC gives an estimate of the similarity of
image intensity values between matching entities in the orthoimage at time 1
(
Sizes of search template and search window used for the image feature tracking.
Results from feature tracking generally need to be filtered, especially when
dealing with old orthophotos (Wangensteen et al., 2006). In this study, the
following filtering procedure was followed. (1) We excluded displacements
smaller than the orthorectification error (RMSE, Table 1). (2) We excluded
displacements exhibiting a signal-to-noise ratio (SNR) < 2 (as
recommended by Messerli and Grinsted, 2015); SNR is the ratio between the
maximum NCC coefficient and the average of the NCC coefficient's absolute
values in the search window, and can be used as an indicator of the “noise”
in the results. (3) A directional filter was applied in order to eliminate
vectors diverging excessively from one another, based on Heid and
Kääb (2012). For that purpose, the mean displacements in the
Whereas all vectors obtained after filtering were mapped (see “Results and interpretations” and
related figures), the final displacement statistics were calculated after
removing upslope-pointing vectors (vectors deviating from more than
Summary statistics of horizontal displacements detected on the landform
surfaces (see text for further details). The mean displacement (
Sequence of orthophotos obtained for Navarro. The base of the landform front that could be reliably identified is indicated in colour (blue, magenta, and orange line in 1955, 2000, and 2014, respectively). At each date the boundary between debris-covered and rock glacier morphology is depicted with a red line (dotted in 1955, dashed in 2000, continuous in 2014).
The methods used in this study first allowed to obtain series of images depicting at first sight conspicuous landscape evolutions: Figs. 6, 7, and 8 show the orthophotos and orthoimages obtained at each site together with the delineated boundary between debris-covered and rock glacier morphology areas and the front slope base at each time. These figures highlight how the landforms' landscape has changed over both historical (before 2000) and contemporary (after 2000) periods. Thermokarst areas could be easily mapped and calculated, except in 2000 at Las Tetas.
Sequence of orthophotos obtained for Presenteseracae. The base of the landform front that could be reliably identified is indicated in colour (blue, magenta, and orange line in 1955, 2000, and 2014, respectively). Note how the rock glacier morphology developed since 2000. In the southern part of the landform, it is nevertheless less well defined and more unstable; it is conspicuously cut by a central furrow and exhibits a few areas of bare ice over which debris slumps may occur. In the northern part of the landform, the rock glacier morphology is more developed; there is neither remaining bare ice area nor evidence of debris cover instability and sliding.
Reliable DEMs and related maps of elevation changes were obtained for the 2000–2014 period at Navarro (Fig. 10) and Presenteseracae (Fig. 12), and for both the 1978–2000 and 2000–2012 periods at Las Tetas (Figs. 13 and 14, respectively). However, and as mentioned in the “Material and methods” section, no reliable and complete DEM could be obtained for the Navarro Valley in 1955, which explained the lack of elevation change measurements at Navarro and Presenteseracae.
The efficiency of the image feature tracking method varied according to the
sites and periods but, on the whole, provided valuable information (Figs. 9–14 and Table 4).
Filtering led to keeping between 12 and 38 % of the
measured horizontal displacements according to the site and period (Table 4).
The order of magnitude of the mean horizontal displacements is 0.50–1 m yr
Continued.
The interpretation of the main geomorphological evolution, elevation changes, and horizontal displacement patterns is summarized for each individual landform in Table 5a, b, and c, respectively, and the results are discussed jointly in the following section.
The three cases studied have distinct significance in terms of glacier–rock glacier relationships and cryosphere persistence under ongoing climate change. Our results lead us to consider the following issues: (i) initial development of the landforms, (ii) differences between debris-covered and rock glacier areas, and (iii) current and future evolution of the landforms.
Sequence of orthophotos obtained for Las Tetas. The base of the landform front that could be reliably identified is indicated in colour (blue, magenta, and orange line in 1978, 2000, and 2012, respectively). At each date the boundary between debris-covered and rock glacier morphology is depicted with a red line (dotted in 1978, dashed in 2000, and continuous in 2012).
Horizontal displacements at the surface of Navarro between 1955 and 2000. The boundary between debris-covered and rock glacier morphology is depicted with a dotted red line in 1955 and with a dashed red line in 2014. Note that moraine crests and thermokarst depressions in 2000 are indicated. The background of the map is the 2000 orthophoto.
Horizontal displacements and elevation changes at the surface of Navarro between 2000 and 2014. The boundary between debris-covered and rock glacier morphology is depicted with a dashed red line in 2000 and with a continuous red line in 2014. Note that moraine crests and thermokarst depressions in 2014 are indicated. The background of the map is the 2014 Geoeye image.
Horizontal displacements at the surface of Presenteseracae between 1955 and 2000. The position of the base of the front at the two dates is indicated with dashed lines, as in Fig. 7; push moraine ridges in the upper part are also indicated. The background of the map is the 2000 orthophoto.
Horizontal displacements and elevation changes at the surface of Presenteseracae between 2000 and 2014. The position of the base of the front at the two dates is indicated with dashed lines, as in Fig. 7; the boundary between rock glacier and debris-covered glacier features and push moraine ridges in the upper part are indicated. The background of the map is the 2014 Geoeye image.
Horizontal displacements and elevation changes at the surface of Las Tetas between 1978 and 2000. The boundary between debris-covered and rock glacier morphology is depicted with a dotted red line in 1978 and with a dashed red line in 2000. Thermokarst depressions in 1978 are indicated. Thermokarst areas could not be accurately and reliably delineated on the 2000 orthophoto and are hence not mapped. The background of the map is the 2000 orthophoto.
Horizontal displacements and altitudinal changes at the surface of Las Tetas between 2000 and 2012. The boundary between debris-covered and rock glacier morphology is depicted with a dashed red line in 2000 and with a continuous red line in 2012. Note that thermokarst depressions in 2012 are indicated. The background of the map is the 2012 Geoeye image.
Navarro and Las Tetas are composite landforms with a debris-covered glacier in their upper part and a rock glacier in their lower part. Considering the clear spatial organizations of surface features and the strong morphological boundaries, in particular the way the debris-covered glacier embeds into the rock glacier in the Navarro's western unit (Fig. 3) and the abrupt transition at Las Tetas (Fig. 5), these landforms most probably result from the (re)advance(s) of glaciers onto or in the back of pre-existing rock glaciers. Many other examples of such development of glacier–rock glacier assemblages have been studied and reported in the literature (Lugon et al., 2004; Haeberli, 2005; Kääb and Kneisel, 2006; Ribolini et al., 2007, 2010; Bodin et al., 2010; Monnier et al., 2011, 2014; Dusik et al., 2015). In the central part of the Navarro's western unit, the elevated lateral margins exhibit cohesive flow-evocative ridges, which probably resulted from the lateral compression exerted by the glacier during its advance (“composite ridges” of the glaciological terminology; Benn and Evans, 2010, p. 492). Also, the boundary between the debris-covered and rock glacier morphologies in 1955 (Fig. 6) gives a minimum indication of the lowest advance of the debris-covered glaciers onto the rock glaciers. However, the origin and age of the rock glaciers located in the lower part of the landforms are almost impossible to assess. Nonetheless, considering the context, they may have developed following several glacier advances and moraine deposition phases, suggesting the idea of a cycle in the landform development (see section “Study sites” and the red circle in Fig. 3). Such a development has led to the rock glacier being cut off from the main rock debris sources (i.e. the rock walls up-valley), resulting in the rock glacier being dependent on the ability of the debris-covered glacier to provide material (debris and ice) required for the sustainment of the rock glacier.
Presenteseracae is a completely distinct case. As studied by Monnier and
Kinnard (2015) and the present work, in 1955 Presenteseracae was a
debris-covered glacier and is now a debris-covered glacier transforming into
a rock glacier. The initial development phase or, in this case, the
“glacier–rock glacier transformation” has been occurring over the last
decades. In less than 20 years, the surface debris cover spread over almost
all of the northern part; a front appeared at the terminus, and
cohesive-flow evocative ridges appeared in the lower part, perpendicular
to flow vectors (Figs. 7, 11, and 12). The latter ridges may be related to
emergent, debris-rich shear planes (Monnier and Kinnard, 2015) bent by the
landform movement. Displacement speeds were high (> 1 m yr
Our study basically relied on the landscape differentiation between debris-covered and rock glacier areas. The criteria enounced and discussed in the Introduction section have been used to distinguish and partition the surface morphology of the landforms studied. Our subsequent results show that, at Navarro and Las Tetas, debris-covered and rock glacier areas are characterized by contrasting patterns of horizontal displacements and elevation changes. Flow patterns in rock glacier areas are conspicuous and spatially coherent and express the cohesive extensive flow of the landform in the direction of the main longitudinal axis. Flow patterns in debris-covered glacier areas are either not detectable or, when detected, generally more chaotic. This low movement detection rate and chaotic organization of displacement patterns in debris-covered glacier areas can be explained by the inherently less cohesive mass flow and the unstable surface morphology resulting from the ablation of ice under a shallow debris layer. Elevation changes in debris-covered glacier areas have larger amplitudes and are spatially heterogeneous; in rock glacier areas, elevation changes are rather moderate and thus expressive of cohesive extensive flow. These different flow dynamics appear perfectly coherent with the definition of, and distinction made between, debris-covered and rock glaciers in the Introduction section.
All the landforms studied are characterized by a rapid landscape evolution over the last few decades. Changes occurred over the entire surface (Presenteseracae), in the contact/transition area between debris-covered and rock glaciers and in the debris-covered glacier area (Navarro), or even in both areas, though more subtly in the rock glacier area (Las Tetas). This continuum in surface evolution perhaps best illustrates the process of glacial–periglacial transition. To our knowledge, an important result of our study not previously reported is the observed upward progression of the rock glacier areas which proceeds at the expense of the debris-covered glaciers on such composite landforms. At Presenteseracae, over a time span of a few decades, the rock glacier morphology has grown from being inexistent to covering approximately half the landform surface. At Navarro, rock glacier areas have subtly (in the western unit) or considerably (in the eastern unit) expanded, until, in the latter case, covering most parts of the essentially debris-covered glacier morphology present initially. As a first-order consideration, topoclimatic conditions seem to play a key role in this differentiated evolution: Presenteseracae and the eastern unit of Navarro are located in more shadowed and thus colder sites (see Figs. 3 and 5).
The dynamical evolution correlates with the landscape evolution, to varying degrees according to the site. As stated in the Introduction, when areas with debris-covered glacier morphology evolve into areas with rock glacier morphology, changes occur in the surface energy balance and subsurface heat transfers, which is likely to result in changes in flow dynamics depending upon the topography and the topoclimatic context. The displacement speed of the three studied landforms has decreased over the study period, at least over the overlapping areas where movement was detected in the different periods (Tables 4, 5a, b, and c). Whereas in other areas of the world many studies have reported rock glaciers to be accelerating under the current climate warming trend (e.g. Roer et al., 2005, 2008; Delaloye et al., 2010; Kellerer-Pirklbauer and Kaufmann, 2012), the decreased velocity highlighted in this analysis suggests an increasing stabilization of the landforms as they evolved from debris-covered glacier bodies to rock glaciers. As the transition from debris-covered to rock glacier seems to be proceeding mainly from the terminus upward, the increasingly debris-rich, lower rock glaciers may exert an increasing buttressing force on the remaining debris-covered glacier upslope, causing a general deceleration of the landform.
At Las Tetas, however, increasing displacement speeds downslope and the
apparition of tension cracks in the lower rock glacier area during the
recent (2000–2012) period point towards a possible acceleration or even
destabilization of the landform terminus (Figs. 3, 8, and 14). Such
evolution may be related to the observed decrease in modelled permafrost
probability along the landform area (Fig. 5) and the climate evolution in
this region: Rabatel et al. (2011) reported a warming trend of 0.19
According to the results and interpretations presented for the Navarro's eastern part and Presenteseracae, rock glaciers can develop at the expense of debris-covered glaciers, by an upward progression of their morphology and correlative widespread development of cohesive mass flow. These are true cases of debris-covered glaciers evolving in rock glaciers (see Introduction: type iii). At Presenteseracae, however, the flow does not appear as strikingly cohesive as for the Navarro's western unit, possibly due to the smaller size of the landform as well as a steeper slope that may constitute a limiting dynamical parameter (Monnier and Kinnard, 2015). As these two landforms are located in favourable topoclimatic conditions, they should thus pursue their evolution towards rock glaciers. Despite the important insights presented by our study, it must be stressed that the evolution of the internal structure in response to morphological and dynamical changes at the surface remains unknown; it would require decades of borehole and geophysical survey monitoring to properly assess this. However, the transition may proceed by fragmentation of the glacier ice core and its mixing with debris and other types of ice (interstitial, intrusive) entrained from the surface. This is an alternative to the common and controverted model of the glacier ice-cored rock glacier where the evolution of the landform is controlled by the expansion and creep of a massive and continuous core of glacier ice (e.g. Potter, 1972; Whalley and Martin, 1992; Potter et al., 1998).
The Navarro's western unit and Las Tetas are more commonly known cases of assemblages that have formed and evolved in reaction to the superimposition/embedding of glaciers onto or in the back of rock glaciers and their subsequent dynamical interactions (see Introduction: type i). In both cases, the progression of the rock glacier at the expense of the debris-covered glacier is rather limited (Navarro's western unit) or null (Las Tetas). It is difficult to assert here whether the debris-covered glaciers are “pushing away” the rock glaciers or whether the latter are “pulling” the former; both processes probably occur (see also Sect. 5.3.2.). The dynamical links between both units certainly constitute a complex issue deserving more attention. Furthermore, as these whole landforms continue to advance, the rock glaciers could plausibly become entirely isolated from their main debris source in the upper cirques, while the increasingly warming conditions could cause the debris-covered glacier to become stagnant or disappear. Also, as the rock glaciers penetrate into areas with less favourable topoclimatic conditions, their future sustainment can be questioned.
We have used remote sensing techniques, including imagery orthorectification,
DEM comparisons, and image feature tracking, in order to depict and measure
the geomorphological evolution, elevation changes, and horizontal
displacements of three glacier–rock glacier transitional landforms in the
central Andes of Chile over a human-life timescale. Our study highlights
how, as climate changes and mountain landscapes and their related dynamics
shift, the glacial and periglacial realms can strongly interact. The
pluri-decadal landscape evolution at the three studied sites is noticeable:
rock glacier morphology areas expanded, as well as the movement detection
area in image feature tracking; thermokarst reduced; elevation changes
tended to become more homogenous; and the mean horizontal displacement decreased
and spatially coherent flow patterns enhanced. These overall results point
toward the geomorphological and dynamical expansion of rock glaciers.
However, the modalities and significance vary between sites. Navarro and Las
Tetas are composite landforms resulting from the alternation between glacier
(re)advance and rock glacier development phases; they currently exhibit an
upward progression of the rock glacier morphology with associated cohesive
mass flow and surface stabilization, or ice-loss-related downwasting and
surface destabilization features. Presenteseracae is a special case of small
debris-covered glacier that has evolved into a rock glacier during the last
decades, with the rock glacier morphology having mostly developed
We have furthermore provided new insights into the glacier–rock glacier transformation problem. Most of the common and previous glacier–rock glacier evolution models depicted a “continuum” process based on the preservation of an extensive core of buried glacier ice. However, our findings rather suggest that the transformation of a debris-covered glacier into a rock glacier may proceed from the upward progression of the rock glacier morphology at the expense of the debris-covered glacier, in association with an expanding cohesive mass flow regime and a probable fragmentation of the debris-covered glacier into an ice–rock mixture with distinct flow lobes. The highlighted importance of topoclimatic conditions and corresponding morphologic evolutions also supports the inclusion of the permafrost criterion within the rock glacier definition.
Aerial data were acquired in the framework of a partnership with the private Juncal Natural Park. The latter has imposed restrictions on the publications of direct aerial views of the park. At the moment, access to the original data set is therefore not possible.
The authors declare that they have no conflict of interest.
This study is part of the Project Fondecyt Regular no. 1130566 entitled “Glacier-rock glacier transitions in shifting mountain landscapes: peculiar highlights from the central Andes of Chile”. Fondecyt is the National Fund for Research and Technology in Chile. The authors want to thank Arzhan Surazakov, who performed the image processing, and Valentin Brunat, who was involved in the software handling and related data management in the framework of a master's thesis supported by the above-mentioned project. The authors also thank Andreas Kääb and Christophe Lambiel for their important help in improving this manuscript, as well as the associate editor for final edition and language corrections. Edited by: Arjen Stroeven Reviewed by: Christophe Lambiel and Andreas Kääb