TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-12-2327-2018Rock glaciers in Daxue Shan, south-eastern Tibetan Plateau: an inventory,
their distribution, and their environmental controlsRock glaciers in Daxue ShanRanZezeranzeze@pku.edu.cnLiuGengnianKey Laboratory for Earth Surface Processes of the Ministry of Education,
College of Urban and Environmental Sciences, Peking University, Beijing,
100871 ChinaZeze Ran (ranzeze@pku.edu.cn)16July20181272327234031December201723January20182July20183July2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://tc.copernicus.org/articles/12/2327/2018/tc-12-2327-2018.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/12/2327/2018/tc-12-2327-2018.pdf
Rock glaciers are typical periglacial landforms. They can indicate the
existence of permafrost, and can also shed light on the regional
geomorphological and climatic conditions under which they may have developed.
This article provides the first rock glacier inventory of Daxue Shan,
south-eastern Tibetan Plateau. The inventory is based on analyses of Google
Earth imagery. In total, 295 rock glaciers were identified in Daxue Shan,
covering a total area of 55.70 km2 between the altitudes of 4300 and
4600 m above sea level. Supported by ArcGIS and SPSS software programmes, we
extracted and calculated morphometric parameters of these rock glaciers, and
analysed the characteristics of their spatial distribution within Daxue Shan.
Our inventory suggests that the lower altitudinal boundary for permafrost
across the eight aspects of observed slopes differs significantly and that
the lower altitudinal permafrost boundary is ∼ 104 m higher on western
than eastern-facing slopes. Moraine-type and talus-derived rock glaciers
exhibit mean gradients that are all concentrated within the 22–35∘
range. However, lobate rock glaciers (27–45∘) have a higher mean
gradient than tongue-shaped rock glaciers (22–35∘). Shady (i.e. N,
NE, and E) slopes appear related to the presence of moraine-type rock
glaciers, whereas sunny (i.e. W, SW, and S) slopes appear related to the
presence of talus-derived rock glaciers. Rock glaciers in Daxue Shan are more
concentrated within tertiary monzonitic granite, which is more sensitive than
other lithological components to the freeze–thaw process. Continuous
weathering of this substrate provides the ideal raw material for the rock
glacier development. These results show that environmental controls (i.e.
topographical, climatic, lithological factors) greatly affect the formation
and development of rock glaciers. This study provides important data for
exploring the relation between maritime periglacial environments and the
development of rock glaciers on the south-eastern Tibetan Plateau (TP). It
may also highlight the characteristics typical of rock glaciers found in a
maritime setting.
Introduction
The term “rock glacier” was first proposed by the American scholar
Capps (1910) when investigating Kennicott Glacier in Alaska. By
definition, rock glaciers consist of perennially frozen masses of ice and
debris that creep downslope under the weight of gravity (Haeberli, 1985;
Barsch, 1996; Haeberli et al., 2006). As many Himalayan rock glaciers develop
out of moraines, it is hard to distinguish where the moraine ends and the
rock glacier begins. The bodies of rock glaciers are similar to moraines in
that, as their ice mass moves over a pore ice surface, they do not sort
materials in relation to the thickness of the debris they contain.
Statistically, rock glaciers occupy extensive areas above the tree line in
the mountainous regions of the world (Haeberli, 1985). Indeed, there are
∼ 73 000 rock glaciers globally (Jones et al., 2018a), with
∼ 1000 active rock glaciers in the Swiss Alps alone. The freeze–thaw
process experienced by the ice masses within rock glaciers can exert a major
impact on the hydrological cycle (Azócar and Brenning, 2010; Jones et
al., 2018a, b), and creep of rock glaciers can significantly negatively
influence any infrastructure built on top. Rock glacier research may
therefore aid a more detailed and accurate understanding of the genesis of
periglacial geomorphology and of the relations between rock glaciers and
their local environments.
Over the last 20 years, with the rapid development of more advanced
Geographical Information Systems (GIS), remote sensing (RS), and statistical
techniques, rock glacier research has entered a new, accelerated phase. This
phase has included compilations of rock glacier inventories (e.g. Bolch and
Marchenko, 2009; Cremonese et al., 2011; Bolch and Gorbunov, 2014; Falaschi
et al., 2014; Colucci et al., 2016; Janke et al., 2017; Wang et al., 2017b;
Jones et al., 2018a), mapping of their spatial distributions and their
relations with environmental controls such as topography and climate (e.g.
Chueca, 1992; Brazier et al., 1998; Brenning, 2005; Janke, 2007; Johnson et
al., 2007; Kenner and Magnusson, 2017; Onaca et al., 2017; Jones et al.,
2018b), estimations of the distribution of permafrost based on rock glaciers
(e.g. Allen et al., 2008; Boeckli et al., 2012; Schmid et al., 2015; Sattler
et al., 2016), and investigations of rock glacier dynamics (e.g. Haeberli et
al., 2006; Liu et al., 2013; Müller et al., 2016; Wang et al., 2017b).
However, compared with ice glaciers, rock glaciers remain poorly described
and infrequently studied. One reason is because they are mixtures of rock
fragments of different sizes, are spectrally similar to their surroundings,
and therefore cannot easily be automatically mapped from optical RS data
(Brenning, 2009). In addition, both supraglacial debris (on the glacier) and
debris along the glacier margins originate from surrounding valley rock
(Jones et al., 2018b); thus their debris surfaces do not produce distinct
spectral signals. As a result, it is often difficult to distinguish relict
rock glaciers from inactive rock glaciers that still contain ice using
optical RS imagery (Millar and Westfall, 2008; Kenner and Magnusson, 2017).
Rock glacier research in China has, up to this point, focused principally on
the Tian Shan Mountains (Cui and Zhu, 1989; Qiu, 1993; Zhu et al., 1996; Wang
et al., 2017b), rather than on the rock glaciers of Daxue Shan on the
south-eastern margins of the TP. This region has been, and continues to be,
strongly uplifted and deformed due to the extrusion and collision of the
Indian and Eurasian continental plates since the beginning of the Quaternary,
and is therefore characterized by an extremely complex matrix of relations
between different environmental factors such as climate and geomorphology. It
is therefore of particular importance to study the environmental controls on
the rock glaciers here to better understand the complex geographical
environment, identify potential natural hazards, and aid environmental
planning and management. The purpose of this study was twofold: first, to
describe and complete a systematic inventory of the previously undocumented
rock glaciers in Daxue Shan; and second, to characterize their distribution
and environmental controls (i.e. climatic, topographical, and lithological
factors). Stemming from these goals, there is an analysis and discussion of
the mechanisms driving the formation, development, and spatial distribution
of the rock glaciers in relation to different environmental controls in this
maritime setting.
(a) The location of the study area in the permafrost zone
of the TP. The Permafrost Zonation Index (PZI) or a corresponding map colour
indicates to what degree permafrost exists only under the most favourable
conditions (yellow) or nearly everywhere (blue); the map was produced using
a temporal resolution of 30 arcsec (< 1 km) on a WGS84 lat–long grid
plotted in a projected coordinate system (Gruber, 2012). (b) The
geographical and topographical maps of the study area based on a spatial
resolution of 30 m using ASTER GDEM V2 software, as shown in the WGS84
coordinate system. (c) Image of the study area. Map data: Google,
Landsat/Copernicus.
Study area
The study area is situated in China's Sichuan Province between
29.956–30.573∘ N and 101.477–101.974∘ E (Fig. 1). To the
west is the uplifted eastern sector of the TP, and to the east are mountain
gorges, both of which are important geographical boundaries (Zhang et al.,
2017). The topography of Daxue Shan is characterized by the strong
downcutting of high-energy water courses, resulting in a great altitudinal
range (1349–7321 m a.s.l.). The region's climate is relatively warm and
humid, and is strongly influenced by a south-westerly monsoonal atmospheric
circulation (Wang et al., 2017a). East of Daxue Shan is a subtropical monsoon
climatic zone which is principally affected by the aforementioned
south-westerly monsoonal atmospheric circulation but also by a south-easterly
monsoonal atmospheric circulation, both of which transport abundant
precipitation to this region. West of Daxue Shan the subtropical monsoon and
continental plateau climatic zones intersect, producing a cold-temperate
climate as well as abundant precipitation. Geologically, Daxue Shan is
located where the Songpan, Chuandian, and South China tectonic blocks
intersect, with the Xianshuihe (Ganzi–Yushu) Fault passing to the north-west
of the region (Zhang, 2013).
MethodsRock glacier inventory, classification, and database
The availability of more powerful RS tools such as Google Earth has
transformed geomorphological fieldwork and has, on the whole, made the
recognition of landforms in remote and poorly accessible areas both fast and
easy (Slaymaker, 2001; Bolch, 2004; Kääb et al., 2005). This is
beneficial to the present study as Daxue Shan is remote and difficult to
access; therefore we compiled an inventory of the rock glaciers in this
region using high-resolution Google Earth satellite imagery (for the period
October 2014–January 2017). Google Earth contains the best freely available
imagery for detecting rock glaciers across large spatial areas, and it has
been previously used for rock glacier identification in the Bolivian Andes
(Rangecroft et al., 2014) and the Hindu Kush Himalayan region (Schmid et al.,
2015).
Rock glaciers are characterized by distinct flow features and structural
patterns. Transversal or longitudinal flow features (ridges and furrows) are
common on rock glaciers due to the deformation of their internal ice
structures (Clark et al., 1998; Humlum, 2000; Haeberli et al., 2006;
Berthling, 2011). Many rock glaciers also exhibit structural patterns such as
steep frontal slopes and side slopes with swollen bodies. Due to the constant
supply of talus or debris, the surface textures of rock glaciers are usually
different from those of the surrounding slopes. Depending on their mobility
and permafrost presence, rock glaciers are usually divided into active,
inactive, and relict types (Sattler et al., 2016). In general, the presence
of ice within active and inactive rock glaciers is indicated by a steep
(> 35∘) frontal slope (Ikeda and Matsuoka, 2002), a well-developed
flow-like morphology defined by sets of parallel and curved ridges separated
by long v-shaped furrows (Barsch, 1996; Roer and Nyenhuis, 2007), and an
absence or sparse occurrence of vegetation (Onaca et al., 2013). Inactive
rock glaciers also contain ice but are immobile. In contrast, relict rock
glaciers, characterised by surface collapse features as a result of
permafrost degradation, have gentler frontal and marginal slopes and are
often vegetated (Wahrhaftig and Cox, 1959; Haeberli, 1985; Scotti et al.,
2013). Based on these criteria, we visually examined the Google Earth images
and identified any potential rock glaciers. We then mapped the rock glaciers
in the study region, using the Advanced Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) Global Digital Elevation Model version 2 (GDEM
V2) data set (to within a horizontal accuracy of 30 m) and the Google Earth
imagery as guides, marking the geographical location of each identified rock
glacier and delineating its outline.
The topographical characteristics of the rock glaciers identified in the
inventory were extracted from the GDEM V2 data set and attributed to each rock
glacier in a GIS environment. These characteristics were both qualitative and
quantitative and included each rock glacier's geographical location (i.e.
the coordinates of its centre), type as determined using dynamic, genetic,
and geometric criteria (moraine-talus; tongue-lobate), aspect, mean gradient
of slope (∘), area (km2), centerline length (m), average width
(m), and average altitude (m a.s.l.). A geological layer (Li et al., 1999)
was added to the GIS so that the relevant class of bedrock could be
attributed to each rock glacier.
Example images of different types of rock glaciers in Daxue Shan:
(a) moraine-type and tongue-shaped rock glaciers
(30.332767∘ N, 101.707756∘ E) (30 January 2017),
(b) moraine-type and lobate rock glaciers (30.217147∘ N,
101.791585∘ E) (15 November 2015), (c) talus-derived and
tongue-shaped rock glaciers (30.067066∘ N, 101.819432∘ E)
(21 October 2014), and (d) talus-derived and lobate rock glaciers
(30.127825∘ N, 101.812158∘ E) (21 October 2014). The red
lines show the outlines of the rock glaciers; the blue arrows indicate the
direction of flow of the rock glaciers. Map data: Google, CNES/Airbus.
Based on the main source of the mass input of debris into each rock glacier
and its subsequent transport downslope, we subdivided rock glaciers into two
distinct categories: talus-derived rock glaciers developing below talus
slopes and moraine-type rock glaciers evolving mainly from glaciogenic
materials (Lilleøren and Etzelmüller, 2016; Onaca et al., 2017)
(Fig. 2). In terms of their planar geometry, the length / width ratio was used
to distinguish between lobate- (length / width ratio < 1) and
tongue-shaped (length / width ratio > 1) rock glaciers (Fig. 2)
(Giardino and Vick, 1987; Martin, 1987; Barsch, 1996; Guglielmin and
Smiraglia, 1998; Onaca et al., 2017). The overall aspect of each rock glacier
was manually derived according to the main direction of flow. For spatial
analysis, these aspects were then recoded into 8 orientation classes.
Spatial distribution of rock glaciers vs. (a) elevation
(ASTER GDEM V2) and (b) Gruber's (2012) Permafrost Zonation Index
(PZI) in Daxue Shan. The green area represents the fringe of uncertainty.
However, due to the lack of data regarding the flow behaviour of rock
glaciers, it remains to be determined whether these landforms are currently
active, or whether they represent inactive rock glaciers. In addition, some
aspects of digitisation were challenging based on a visual interpretation of
remotely sensed imagery alone and thus the mapped rock glaciers are
inherently associated with some spatial uncertainty (Sattler et al., 2016;
Jones et al., 2018b). Consequently, some rock glaciers may not be correctly
delineated as delimitation of the upper boundary of rock glaciers through
geomorphic mapping is arbitrary (Krainer and Ribis, 2012), and delineation of
individual polygons where multiple rock glaciers coalesce into a single body
is inherently subjective (Scotti et al., 2013; Schmid et al., 2015).
Moreover, several complex landforms delineated as rock glaciers may be
landslide deposits or relict rock glaciers. Therefore, future research may
benefit from integration of additional data sources and further in situ
observations that could be used to constrain methods for rock glacier
identification using remote sensing and digital elevation data. Further, use
of a higher-resolution DEM paired with in situ climate data sets would likely
produce a more accurate representation of the distribution of the rock
glaciers in Daxue Shan. Due to the limitations imposed by the 30 m spatial
resolution and the uncertainties inherent in any visual identification, we
may have failed to identify all the rock glaciers in the study area. As a
result of these uncertainties, we chose to examine ranges of values during
our statistical analyses.
KMO and Bartlett's test.
Kaiser–Meyer–Olkin measure of sampling adequacy 0.387Bartlett's test of sphericityApprox. χ21216.315df28Sig.0.000
Note: df is degree of freedom, and Sig. is
significant level.
Spatial and statistical analyses
When there is collinearity between the variables, principal component
analysis (PCA) can used to determine the relationships between them (White
and Copland, 2015; Ran, 2017). However, in this study, we performed the
Kaiser–Mayer–Olkin (KMO) and Bartlett's tests to examine the suitability of
the data for factor analysis and we found a KMO value of 0.387 (Table 1),
which indicates that the original variables are not suitable for PCA because
there is weak collinearity (KMO < 0.5) between them. Therefore, we
retained the original variable information, which allows for convenient
interpretation and calculation (not too many dimensions), without
dimensionality reduction.
We assigned the eight geographical and topographical variables (i.e.
latitude, longitude, rock glacier area, length, width, altitude, mean
gradient, and aspect) for each of the rock glaciers to an eight-dimensional
random variable (i.e. X1, X2, X3 … X8). A
correlation coefficient ρij (i, j= 1, 2 … 8) of
Xi and Xj was introduced into the correlation matrix of the random
dimensional vector as an 8-order matrix for each element and was denoted
by R:
R=ρ11ρ12⋯ρ18ρ21ρ22⋯ρ28⋮⋮⋮⋮ρ81ρ82⋯ρ88,ρij=cov(Xi,Xj)DXiDXj,covXi,Xj=EXi-EXi⋅Xj-EXj
The diagonal element of the correlation matrix was 1, and the correlation
matrix itself was symmetrical. We performed the statistical analysis using
SPSS20® software. Correlations between the
topographical variables were then evaluated using Pearson correlation
coefficients at a corresponding 0.05 level of significance.
Statistics for the 295 rock glaciers found in Daxue Shan.
Number ofRG areaAltitudeLengthWidthGradient ofMAFRG typelandforms(km2)(m a.s.l.)(m)(m)slope (∘)(m a.s.l.)Moraine14628.11450179323528.454385Talus14927.59444280522830.054321Tongue27952.87447082921128.894347Lobate162.83449127558235.694447MTRG13926.86449681721827.964377MLRG71.25459230356438.294539TTRG14026.01444484120429.814317TLRG91.58441225359533.674376All RG29555.70447179923129.264352
Note: RG is rock glaciers, MTRG is moraine-type and
tongue-shaped rock glaciers, MLRG is moraine-type and lobate rock
glaciers, TTRG is talus-derived and tongue-shaped rock glaciers,
TLRG is talus-derived and lobate rock glaciers, and MAF is minimum
altitude of rock glacier front. Altitude of rock glacier, altitude of rock
glacier front, length, width, and gradient of slope are all mean values.
Results
In total, 295 rock glaciers were identified in Daxue Shan (Fig. 3), covering
an area of 55.70 km2 (Table 2). Of these, 50.51 % were
talus-derived rock glaciers, the other 49.49 % were moraine-type rock
glaciers. Most (94.58 %) of the rock glaciers were tongue-shaped and
the rest were lobate-shaped.
The rock glaciers are concentrated at altitudes between 4300 and
4600 m a.s.l., with a mean altitude of 4471 m a.s.l. (Fig. 4a).
Moraine-type rock glaciers are mainly concentrated between 4400 and
4600 m a.s.l., and talus-derived rock glaciers between 4300 and
4550 m a.s.l. In terms of general morphology, both tongue-shaped and
lobate-shaped rock glaciers are mainly located between 4350 and
4600 m a.s.l. (Fig. 4a). The upper boundaries for the vast majority of rock
glacier types were ∼ 4600 m a.s.l., because at higher altitudes there
are often ice glaciers. Figure 4b shows the ranges in areas covered by
different types of rock glaciers. Apart from a few outliers, the area of most
rock glacier types area is < 0.3 km2, and in this regard there is no
clear difference between rock glacier types. Figure 4c shows the ranges in
the mean gradients of the slopes of different types of rock glaciers.
Moraine-type and talus-derived rock glaciers exhibit mean gradients that are
concentrated within the 22–35∘ range. However, tongue-shaped and
lobate-shaped rock glaciers display a difference in mean gradient.
Tongue-shaped rock glaciers have slopes with mean gradients which are
concentrated in the 22–35∘ range, whereas those of lobate rock
glaciers fall within the 27–45∘ range. This means that the upper
(∼ 10∘) and lower (∼ 5∘) slopes of tongue-shaped
rock glaciers are both shallower than for lobate rock glaciers. Figure 4d
displays the ranges in the lengths of different types of rock glaciers.
Moraine-type and talus-type rock glaciers have similar lengths
(500–1000 m), but when the rock glaciers are categorized by shape,
tongue-shaped ones (500–1000 m) are generally longer than lobate-shaped
rock glaciers (200–400 m).
Box plots illustrating the distributional characteristics of rock
glaciers in Daxue Shan: (a) average altitude (m a.s.l.),
(b) area (km2), (c) range in the gradient of the slope
(∘), and (d) length (m). Box plots represent 25–75 % of
all values, the caps at the ends of the vertical lines represent 10–90 %
of values, and the line in the centre of each box indicates the median value.
The numbers of each rock glacier type are in brackets in the legend.
Figure 5 shows rock glacier abundance vs. aspect. Our data set revealed that,
apart from south-facing (5.44 %), south-east-facing (3.06 %), and
north-east-facing (20.75 %) slopes, the rock glaciers are fairly evenly
distributed on slopes with the remaining five aspects that each account for
∼ 15 % of the total. Moraine-type rock glaciers are most often
north-east-facing (30.34 %) or north-facing (20 %), whereas
talus-derived rock glaciers are most often south-west-facing (22.82 %) or
west-facing (17.45 %). Lobate rock glaciers tend
to be rare on south-facing (6.25 %) and south-east-facing (0 %)
slopes but are more commonly on north-facing, north-west-facing, and
east-facing, with each aspect accounting for ∼ 18.75 % of the
total. We compared all our results and discovered that shady (i.e. N, NE, and
E) slopes have more moraine-type rock glaciers, and sunny (i.e. W, SW, and S)
slopes have more derived from talus. North-facing (i.e. N, NW, and NE) slopes
also seem to be more favourable for the formation of lobate rock glaciers
than do south-facing (i.e. SW, S, and SE) ones (Fig. 5), possibly in relation
to the abundance of debris-producing steep rock walls on north faces. In
contrast to other regions (Lilleøren and Etzelmüller, 2016; Onaca et
al., 2017), we found that in Daxue Shan both moraine-type and talus-derived
rock glaciers have developed in the monzogranitic areas, and that rock
glaciers and monzonitic granite exhibit a high spatial correlation and
interdependence (Fig. 6).
Table 3 shows the results of the correlation analysis. Positive correlations
between rock glacier latitude, altitude, and length, and a negative
correlation between altitude and mean slope were significant
(p value ≤ 0.05). Negative correlations between latitude and
longitude, longitude and altitude, length and mean slope, and area and mean
slope, and positive correlations of area with length and width were highly
significant (p value ≤ 0.01).
Discussion
The spatial distribution and dynamics of rock glaciers are especially
dependent on the local topography and climate (Springman et al., 2012;
Delaloye et al., 2013). Lithology also exerts considerable control on the
rock glaciers (Onaca et al., 2017). Analysing local environmental factors
(i.e. climatic, topographical, and lithological factors) is therefore crucial
to obtaining an understanding of the formation, development, and spatial
distribution of rock glaciers.
Analysis of the abundances of different rock glacier types vs.
aspect. The number of rock glaciers for each aspect on each of the four radar
plots is shown as a percentage (%). Note: RG is rock glaciers,
MTRG is moraine-type and tongue-shaped rock glaciers, MLRG is
moraine-type and lobate rock glaciers, TTRG is talus-derived and
tongue-shaped rock glaciers, and TLRG is talus-derived and lobate rock
glaciers.
Note: correlations in bold are significant. * indicates a
p value ≤ 0.05. ** indicates a p value ≤ 0.01.
Topographical controls on rock
glaciers
The significantly positive correlations between rock glacier latitude,
altitude, and length (Table 3) are locally determined by the topographical
characteristics. With an increase in latitude from the south to the north,
the high-altitude rock glaciers increase in number, and flow further
downvalley than those at low altitude. The altitudes of the mountains and
rock glacier lengths increase with latitude, whereas air temperatures
decrease, which implies that the northern sector of Daxue Shan has an
environment that is more conducive to the formation of rock glaciers and
other periglacial landforms. Likewise, the significantly negative correlation
between latitude and longitude (Table 3) indicates that local environmental
factors dominantly control rock glacier distribution despite a general
increase in relief in the north-east of the Daxue Shan region compared to the
south-west (Fig. 3a). There is also a significantly negative correlation
between longitude and altitude (Table 3), as lower altitude areas to the east
are less conducive to the development of rock glaciers where warmer and more
humid conditions are common. The negative correlation that exists between
rock glacier length and mean gradient of slope is likely because the shortest
rock glaciers are the talus-derived variety, and these have usually developed
in steep topographical environments. Rock glacier area and mean gradient of
slope, and altitude and mean slope are significantly negatively correlated
(Table 3), likely because larger and high-altitude rock glaciers are mostly
concentrated on gentle slopes that may be more conducive to their
development. In summary, the topography of Daxue Shan is an important
environmental control on the formation, development, and spatial distribution
of the region's rock glaciers.
The mean altitude of a rock glacier's front (MAF) has often been taken to be
a good approximation of the lower boundary of the discontinuous permafrost
zone (i.e. Scotti et al., 2013). We found a substantial altitudinal
difference between the lower permafrost boundaries identified on the
above-mentioned eight aspects. For example, permafrost was assumed to be
probable above 4300 m a.s.l. on east-facing slopes, and above
4403 m a.s.l. on west-facing slopes. The mean lower permafrost boundary was
calculated as occurring at 4352 m a.s.l. (derived from a mean value of
4315 m a.s.l. for east-facing slopes, and 4419 m a.s.l. for west-facing
slopes). The mean lower permafrost boundary on east-facing (shady) slopes
would therefore be ∼ 104 m lower than that of west-facing (sunny)
slopes (Fig. 7).
The rock glaciers of Daxue Shan superimposed on the local
lithologic–geologic environment. Stratigraphic data from Li et
al. (1999).
Minimum altitudinal rock glacier fronts (MAF) for all eight aspects,
along with the overall mean. These values are taken to represent the lower
boundaries of the potential permafrost extent in the Daxue Shan region (bars
indicate standard errors of the mean). Daxue Shan lies along an approximately
NW–SE axis; therefore we used this as the boundary separating east-facing
(i.e. N, NE, E) shady slopes from west-facing (i.e. S, SW, W) sunny
slopes.
Climatographs for the (a) Kangding (2615.7 m a.s.l.,
30.03∘ N, 101.58∘ E), (b) Daofu
(2957.2 m a.s.l., 30.59∘ N, 101.07∘ E),
(c) Danba (1949.7 m a.s.l., 30.53∘ N,
101.53∘ E), and (d) Ganzi (3393.5 m a.s.l.,
31.37∘ N, 100∘ E) meteorological stations. Data sources:
Meteorological Data Centre of the China Meteorological Administration
(http://data.cma.cn/, last access: 2 June 2017, calculated for the
period 1981–2010).
In addition, the formation and development of the Daxue Shan rock glaciers
are also strongly influenced by the landforms created by glacial erosion and
deposition. The south-eastern margins of the TP are in a region of Quaternary
glaciation which has been, and continues to be, strongly affected by
monsoonal atmospheric circulations (Owen et al., 2005). This region possesses
numerous ancient glacial relics and abundant landforms created by glacial
erosion and deposition (Li and Yao, 1987). We found that the distribution of
rock glaciers is in close association with ice glaciers, as the upper
boundaries for rock glaciers were ∼ 4600 m a.s.l., and at higher
altitude there are often ice glaciers. In the context of global warming, it
is widely accepted that the majority of glaciers on the Tibetan Plateau (TP)
and its surroundings have experienced accelerated reduction (Bolch et al.,
2012; Yao et al., 2012). The rate of glacier decline in Daxue Shan was
-0.25± 0.20 % year-1 during 1990–2014 (Wang et al.,
2017a), with some ice glaciers transforming to rock glaciers. Glacial
depositional landforms (e.g. moraine ridges) are highly conducive to the
formation and development of moraine-type rock glaciers. Moraine ridges or
moraines left after the retreat of the ancient glaciers can provide
significant quantities of boulders, erratic blocks, debris, sand, and ground
ice. In the process of downslope movement, rock glaciers can incorporate old
moraine material as well as the debris from both sides of the moraine ridge.
Glacial erosional landforms in particular have a close relation with the
formation and development of talus-derived rock glaciers. Ice structures,
snow layers, and moraines within glaciers collapse from time to time,
supplying talus to the feet of mountains. As a result of the freeze–thaw
process and the effect of gravity, talus creep then forms rock glaciers.
Climatic controls on rock glaciers
The west-facing slopes of Daxue Shan lie in the intersection between a
sub-frigid monsoonal zone and a continental plateau climatic zone and therefore
experience a cold-temperate climate. At the Daofu meteorological station
(2957.2 m a.s.l.), mean annual precipitation (MAP) is ∼ 613.5 mm,
and mean annual temperature (MAT) is ∼ 8.14 ∘C (Fig. 8b).
Based on an adiabatic rate of 0.65 ∘C/100 m, we estimated the MAT
at 4311 m a.s.l. (i.e. the lower permafrost boundary) to be
∼-0.66 ∘C. The east-facing slopes of Daxue Shan are principally
affected by a subtropical monsoonal climatic environment and
by a south-westerly monsoonal atmospheric circulation but also by
a south-westerly monsoonal atmospheric circulation. East-facing slopes
therefore experience high levels of precipitation (snowfall). MAP at the
Kangding meteorological station (2615.7 m a.s.l.) reaches 858.3 mm, and
MAT is ∼ 7.29 ∘C (Fig. 8a). We calculated the MAT at
4352 m a.s.l. (i.e. the lower permafrost boundary) to be
∼-4.00 ∘C. Here, the freeze–thaw process would be frequent
(Fig. 8), meaning that the climatic environment would provide temperature and
precipitation conditions that are highly favourable to the formation and development of
rock glaciers.
The distribution of rock glaciers in Daxue Shan is in general agreement with
Gruber's (2012) global Permafrost Zonation Index (PZI) map, but many rock
glaciers are situated within the PZI fringe of uncertainty (Fig. 3b).
However, the PZI is strictly controlled by temperature that decreases with
increasing altitude; thus our results indicate the importance of the local
climatic controls on development of rock glaciers and thus permafrost. In
addition, compared with the distribution of ice glaciers in Daxue Shan, the
distribution of rock glaciers also has characteristic small differences
between the south and north, owing to a north–south corridor effect for
water and heat transport and diffusion through the longitudinal gorges (Wang
et al., 2017a).
Lithological controls on rock glaciers
Lithology is a critical control for the supply of talus to ice and
rock glacier surfaces (Haeberli et al., 2006). Figure 6 shows that the major
exposed strata in the Daxue Shan region are composed of Tertiary monzonitic
granite, consistent with the NW–SE trending Xianshuihe Fault. The
surrounding mountains in this area generally consist of biotite-muscovite
granite that intruded 16–13 Ma ago (Roger et al., 1995). Also located in
this region is the tectonically important Zheduotang Fault, which runs
through the Zheduo Valley, and is one of the most active fault systems on the
TP's margins (Allen et al., 1991). It can be seen from Fig. 6 that the
distribution of rock masses along the Xianshuihe Fault in the Daxue Shan
region is clearly controlled by this NW–SE left-lateral strike-slip fault.
The Tertiary monzogranites are clearly highly conducive to the formation and
development of rock glaciers. This is consistent with the findings of Onaca
et al. (2017) in the southern Carpathian Mountains. According to Popescu et
al. (2015), rock glaciers located in granitic and granodioritic massifs are
composed of larger clasts compared with those found in metamorphic massifs.
Thus, the higher porosity of the substrata in granitic and granodioritic
massifs allows for significant cooling beneath the bouldery mantle because
the denser cold air is trapped between the large boulders (Balch, 1900). The
lithological and mineralogical characteristics which accompany the high
porosity of tertiary monzogranites are therefore more favourable to the
formation and development of local rock glaciers than are other lithologies.
In addition, rock glacier formation is also controlled by slope and
sedimentation rates contributing debris to the landforms (Müller et al.,
2016). There are large sources of sediment and sediment storages in Daxue
Shan, which are controlled by the processes occurring within this setting
(Müller et al., 2014). An abundance of steep rock walls and deepened
valley sides provides catchment areas for rock glacier development and, when
combined with intense monsoonal precipitation and tectonic activity, drives
sediment transport processes and rock glacier development in Daxue Shan.
Several researchers (e.g. Cui and Zhu, 1989; Zhu, 1992; Zhu et al., 1992;
Liu et al., 1995; Bolch and Gorbunov, 2014) have previously identified
hundreds of rock glaciers in the northern Tian Shan Mountains. They found that
most of the identified rock glaciers were tongue-shaped and were located at
altitudes between 3300 and 3900 m a.s.l. on north-facing slopes. Most rock
glaciers in Daxue Shan are also tongue-shaped. However, the altitudes at and
the aspects on which these rock glaciers are found to differ between the Daxue
and the Tian Shan mountain ranges. First, in terms of altitude, the rock
glaciers of Daxue Shan are located between 4300 and 4600 m a.s.l., which is higher
than the Tian Shan rock glaciers by approximately 700–1000 m. It would be
reasonable to assume, therefore, that the rock glaciers located in lower
latitudes are more likely to be found at higher altitudes. Second, in terms
of aspect, the rock glaciers of Daxue Shan are more evenly distributed across
all eight above-mentioned aspects than are the rock glaciers of the Tian Shan
Mountains. This could be explained by several factors, including the
differences in overall altitude as well as the orientation of the main
massif of each mountain range. Daxue Shan lies along an approximately NW–SE
axis, whereas the Tian Shan Mountains are roughly W–E in orientation. Rock
glaciers are therefore less commonly found on the east- and west-facing
slopes of the Tian Shan. The effect of solar radiation is stronger on the
south-facing slopes of the Tian Shan Mountains than on its north-facing ones,
meaning that conditions on these south-facing slopes are less conducive to
the development of rock glaciers; most of the range's rock glaciers are
therefore found on its north-facing slopes. Furthermore, when higher
altitudes are reached, all aspects experience lower air temperatures,
resulting in a lessening of the impact caused by the difference between air
temperature and solar radiation exposure; this phenomenon is similar to that
found in Daxue Shan and explains why rock glaciers there are fairly evenly
distributed on all eight aspects. However, when altitudes are lower, the
impact of solar radiation combined with warmer air temperatures is greater,
particularly on south-facing slopes; both temperature and solar radiation are
lower on shady north-facing slopes, however, explaining the predominance of
north-facing rock glaciers in the Tian Shan Mountains.
Conclusions
Rock glaciers are widespread in Daxue Shan, and of these, tongue-shaped rock
glaciers cover the largest area. The occurrence and characteristics of these
rock glaciers can mostly be explained by local environmental controls (i.e.
climatic, topographical, and lithological factors).
In total, 295 rock glaciers were identified in Daxue Shan, covering a total
area of 55.70 km2. The altitudes at which moraine-type rock glaciers
are found (i.e. 4400–4600 m a.s.l.) are at least 50–100 m higher than
for talus-derived rock glaciers (i.e. 4300–4550 m a.s.l.), although the
upper altitudinal limit for both these types of rock glacier is
∼ 4600 m a.s.l. At higher altitudes there are often ice glaciers.
Except for a few outliers, the area of each type of rock glacier is no
greater than 0.3 km2. There is no significant difference between
moraine-type and talus-derived rock glaciers in terms of the mean slope
gradients (i.e. they are all clustered within the 22–35∘ range),
but the upper and lower mean slope gradients of tongue-shaped rock glaciers
are ∼ 10 and ∼ 5∘ lower than for lobate rock glaciers,
respectively. In terms of length, moraine- and talus-derived rock glaciers
have similar lengths (∼ 500–1000 m), but according to shape,
lobate-shaped rock glaciers are distinctly shorter than tongue-shaped by
∼ 300–600 m. We found shady (i.e. N, NE, and E) slopes more conducive
to the presence of moraine-type rock glaciers than sunny (i.e. W, SW, and S)
ones that appear more conducive to the presence of talus-derived rock
glaciers. In addition, north-facing (i.e. N, NW, and NE) slopes appeared more
favourable to the formation of lobate rock glaciers than south-facing ones (i.e.
SW, S, and SE). The mean regional lowest altitudinal limit of rock
glaciers is 4352 m a.s.l., an altitude which was taken to indicate the
local permafrost's mean lower boundary. On east-facing slopes, the
permafrost's lower boundary can therefore reasonably be assumed to be
∼ 104 m lower than on west-facing slopes.
The correlation matrix of rock glacier variables indicates that the
formation of rock glaciers is closely related to local environmental
conditions. The local climatic environment leads to a frequent freeze–thaw
process within these rock glaciers, a process which is also beneficial to
their formation and development. Tertiary monzonitic granite, with its large
clastic and highly porous characteristics, is more sensitive than other
lithological components to the freeze–thaw process, and continuous
weathering of this monzogranite substratum thus provides the ideal raw
material for the rock glaciers of Daxue Shan.
The data associated with this article can be found in the
Supplement. These data include maps of the most important areas described in
this article, as well as a tabulation of the parameters of the rock glaciers
found in Daxue Shan.
The supplement related to this article is available online at: https://doi.org/10.5194/tc-12-2327-2018-supplement.
ZR and GL designed the research. ZR performed the analysis and wrote the paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was funded by the National Natural Science Foundation of China
(grant nos. 41230743 and 41371082). We thank the editor and three reviewers
for detailed and constructive suggestions for revisions to the
manuscript. Edited by: Peter
Morse Reviewed by: Tobias Bolch, Luke Copland, and Johann
Müller
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