Introduction

Italy has been historically affected by diffuse gravity-related geological hazards (Abbate et al. 2021), and Sondrio province, with 41,666 documented events and a total landslide area of 1373 km2, is the most affected province in Lombardy and one of the most affected in terms of area in the whole of Italy (Trigila et al. 2018). Examples of the most devastating recent rock avalanches are the 1987 Val Pola landslide with a volume ca. 33 million m3 (Dramis et al. 1995), and the 2004 Punta Thurnwieser landslide with more than 1 million m3 (Pirulli 2009). More recently, just on the other side of the border between Italy and Switzerland, rock avalanches occurred in 2019 at Flüela Wisshorn with a volume of 0.25 million m3 (Cicoira et al. 2022), and in 2017 at Piz Cengalo with a volume of 3 million m3 (Walter et al. 2020). All of these rock avalanches have a common characteristic: they have a scar seated in degrading alpine permafrost. Permafrost degradation has been indicated as a triggering factor for many landslides as well as debris flows and rockfalls (Dramis et al. 1995; Hormes et al. 2008; Pirulli 2009; Walter et al. 2020; Ponti et al. 2021). Pre-historical landslides are recorded in the whole area. Many fans in the Valtellina that have mottled the landscape since the end of the LGM are dominated by landslides, such as the Migiondo and the Ponte in Valtellina fans (Guglielmin and Orombelli 2003; De Finis and Bini 2014). However, for ancient landslides, it is not easy to understand either the type of landslide or their triggering factors. In this paper we aim to study one of the larger old rock avalanche that could be related to permafrost degradation in the Central Alps, the Val Viola rock avalanche. In particular, we will focus on the following: (1) dating the Val Viola rock avalanche; (2) understanding the relationships between this rock avalanche and the paraglacial evolution and permafrost degradation in the Viola valley. To obtain these results, this study incorporates three essential approaches: 10Be dating (Lal 1991), Schmidt’s Hammer R values (Herzig 1951) and the Joint roughness coefficient (JRC) (Barton and Choubey 1977). In the European Alps, several investigations have employed cosmogenic 10Be dating to establish the chronology of landslide and rock avalanche events. These studies have played a pivotal role in creating a temporal framework for comprehending the history of such events in the region (Menounos et al. 2013). Ivy–Ochs et al. (2006) also utilized 10Be dating to estimate the recurrence intervals of landslides and rock avalanches in the Alps, providing crucial information for assessing long-term hazard potential and guiding land-use planning decisions (Ivy-Ochs et al. 2006). More recent research has shifted its focus towards dating events to explore potential triggering mechanisms that lead to landslides and rock avalanches (Fan et al. 2020).

Since the 1980s, Schmidt’s Hammer has found application in studies of glacial and periglacial environments, primarily for establishing relative ages of moraines (e.g., McCarroll 1989; Shakesby et al. 2006), rock glaciers, and deglaciated surfaces (e.g., Scapozza and Ramelli 2010; Guglielmin et al. 2012), as well as landslides and rock avalanches (e.g., Dawson et al. 1986; Clark and Wilson 2004). On the other hand, JRC has been used to assess surface roughness resulting from weathering, especially due to the differential erosion of minerals, which increases over time with exposure (McCarroll and Nesje 1996). These three methods have been widely used, but rarely compared each other in order to obtain data in complex situations. While SHED (Schmidt Hammer Exposure Dating) has been established as a good and effective method for dating landforms (Tomkins et al. 2018; Wilson et al. 2019), JRC has rarely been used in comparison to absolute dating or SHED (McCarroll and Nesje 1996).

The innovative integration of 10Be dating, Schmidt’s Hammer R values, and JRC analysis in this study will provide a comprehensive understanding of the age and dynamics of the Val Viola rock avalanche.

Study area

The study area is located in Upper Valtellina, Central Italian Alps (Fig. 1) in the upper part of the Viola Creek basin, a tributary to the Adda River, between 2120 and 3150 m a.s.l. of Corno Dosè. The main valley is ENE-WSW oriented and is crossed by two main tributary valleys: from North by the Cantone Valley and from South by the Val Cantone di Dosdè.

Fig. 1
figure 1

Reference map of site location (map data: Google, Maxar Technologies)

Full size image

Currently, glaciers are relegated to the upper part of the Val Cantone di Dosdè area, where a once big glacier has been shattered into ten glaciers covering an area of 1.88 km2 (Scotti et al. 2013). Evidence of at least four pre-LIA glacial advances have been found in the nearby Cantone Valley between 15 and 12 ka (Longhi et al. 2020). However, the main valley seems to have been deglaciated since at least ca. 13 ka (Scotti et al. 2017; Longhi et al. 2020). The climate is characterized by a continental regime: mean annual temperature is 1.9 °C and mean annual precipitation is ~ 1020 mm (Soncini et al. 2017).

Patches of discontinuous permafrost reach a lowest altitude of 2400 m a.s.l. on the north-facing slopes of the Corno Dosdè, while on the south-facing slopes of the Pizzo Bianco, it reaches a lowest altitude of 2600 m a.s.l. (Boeckli et al. 2012). Relict permafrost distribution, estimated by the distribution of climatically inactive rock glaciers, reach down to about 2000 m a.s.l. (Guglielmin and Siletto 2000). The lower boundary of permafrost reconstructed with soil micromorphology is at 2347 m a.s.l. around 12.5 ka (Longhi et al. 2021). Two different permafrost aggradation phases have been identified in the area between 11 and 9.7 ka and between 9.3 and 8 ka (Longhi et al. 2021).

The rock avalanche at Val Viola (Fig. 2) covers a surface area of around 1.1 km2 considering a partial coverage with debris cones in the southern part, but the volume cannot be estimated as the thickness of the avalanche is still unknown (Hormes et al. 2008). The body is cornered by the Viola Lake to the West at around 2300 m a.s.l. To the East, it reaches a lower height of approximately 2160 m a.s.l. The blocks in the rock avalanche consist of orthogneiss in the northernmost longitudinal zone, which is intersected by a paragneiss zone. The southwestern area is most represented by orthogneiss. Some mylonite blocks are sparsely observed in both the orthogneiss area and the paragneiss zone.

Fig. 2
figure 2

Main geological and geomorphological elements of the area: a rock avalanche body (Hormes et al. 2008 redrawn in brown); b lower boundary of relict permafrost (Guglielmin and Siletto 2000 redrawn as a pale violet line); c lower boundary of permafrost (Boeckli et al. 2012 redrawn as dark violet line); d morainic ridges (Hormes et al. 2008 redrawn as blue line; e degradation scarp (Hormes et al. 2008 redrawn as red line with rhombus); f orthogneiss; g paragneiss; h granites

Full size image

Methods

Geomorphological mapping and lake analysis

A field geomorphological survey at scale 1:5000 was carried out after photointerpretation of the available aerial photos (Regione Lombardia, volo TEM 1:10000). The direction of glacial striae on rochees moutoneé was recorded with a compass. The bathymetry of Viola Lake was interpolated by depth measurements taken in a grid with a spacing of 10 m for a total of 52 points. The interpolation was then performed using the Kriging algorithm in the software Surfer 13.0.

To date the lake, the sediment-rich SW sector of the lake at ca 4 m of depth was cored with a Russian type corer. The basal centimeter of the lacustrine sediment core was sampled, sealed in aluminum foil, and frozen at −20 °C. The sample was kept frozen until being sent to Beta Analytic Laboratories for radiocarbon dating. After pre-treatment, samples for radiocarbon dating were prepared for AMS by converting them into graphite. Calibrated ages were calculated with the software OxCal 4.2 using the INTCAL13 14C dataset (Reiner et al. 2013). Radiocarbon age data are reported as conventional radiocarbon years BP (14C yr BP) and as calibrated age ranges with a 2σ error (95.4%) (cal. yr BP; relative to AD 1950).

Cosmogenic sampling and analysis

Cosmogenic nuclides are produced in the upper surface of exposed rocks as a result of the bombardment of Earth’s surface by cosmic rays (Lal 1991). The accumulation of cosmogenic nuclides can therefore be used to determine the time elapsed since a rock surface was exposed (Nishiizumi et al. 1989; Lal 1991; Gosse and Phillips 2001). A total of 5 large boulders (dimension > 1 × 1.5 m) were sampled in the rock avalanche. For sampling, we selected tall boulders with flat tops in order to exclude the effects of slope processes or prolonged snow cover. Three orthogneiss and two paragneiss boulders were sampled (Fig. 3). For each site, we recorded the sample location and elevation with a handheld GPS, the site’s exposure geometry with a Brunton compass, and the sample thickness (Gosse and Phillips 2001). We calculated production rates of 10Be for each sample following the method implemented by the online exposure age calculator described by Balco et al. (2008) and subsequently updated in version 3. First, we determine the surface 10Be production rate due to spallation and muons for each sample using the Stone (2000) scaling scheme and the sea-level, high-latitude reference value of 4.01 at/g-qz/yr from Borchers et al. (2016). Then we calculate the effective production rate of 10Be for each sample by scaling them for topographic shielding and sample thickness, assuming a density of 2.7 g/cm3 and an attenuation length of 160 g/cm2 (Gosse and Phillips 2001). Calculated production rates for each sample are shown in the supplemental table.

Fig. 3
figure 3

Distribution of the cosmogenic samples sites (yellow) and sample site from Hormes et al. 2008 (red)

Full size image

Quartz separation and purification were performed at Vanderbilt University, following an adapted protocol from Corbett et al. (2016), with the aim of obtaining about 10 g of pure quartz from each sample. Each sample underwent a series of mineral separations, including density separations with lithium heteropolytungstate (LST) and magnetic separations. Samples underwent repeated etches in 1–2% hydrofluoric acid to remove non-quartz minerals and lightly etch the quartz to remove any meteoric 10Be. Once pure quartz samples were prepared, the chemical extraction and preparation of beryllium pellets was performed at the PRIME lab at Purdue University. Ratios of 10Be/9Be were measured at the PRIME lab at Purdue University and were normalized to the 07KNSTD3110 standard (Nishiizumi et al. 2007). 10Be measurements were corrected for background using procedural blanks. Procedural blanks were run with samples and averaged 4.21 × 104 ± 4.49 × 104 atoms 10Be. This averaged 2.1% of the 10Be measured in the samples and was a maximum of 2.6% in one sample. The measured 10Be concentrations for each sample are shown in the supplemental table.

Schmidt Hammer

Since the 1980s, Schmidt’s Hammer has been used in studies on glacial and periglacial environments, mostly for relative ages of moraines (e.g., McCarroll 1989; Shakesby et al. 2006), of rock glaciers and deglaciated surfaces (e.g., Scapozza and Ramelli 2010; Guglielmin et al. 2012), and of landslides and rock avalanches (e.g., Dawson et al. 1986; Clark and Wilson 2004).

In this study, we used a Schmidt-Hammer type N with an impact pressure of 2.207 Nm, which is particularly suited for hard rock (Guglielmin et al. 2012). A total of 93 big boulders (called afterwards station) with no lichenic coverage and no evident fractures were tested in dry conditions. Fifty-six boulders were orthogneiss and 37 were paragneiss (Fig. 4). At every station, 25 Schmidt Hammer measurements were taken at different points (Guglielmin et al. 2012). We report the average of all 25 measurements as the R25 value and the average of the 5 highest measurements as the R5 value. We proceed by categorizing all the data into geomorphological units and calculating the average R25 and R5 values for all the boulders within each unit, along with their corresponding relative standard deviation.

Fig. 4
figure 4

Distribution of the Schmidt-Hammer station where green are orthogneiss boulders, and blue are paragneiss boulders

Full size image

Three blocks on orthogneiss were sampled both for cosmogenic dating and for Schmidt’s Hammer R values. A linear regression was performed between absolute ages obtained with cosmogenic dating and Schmidt’s Hammer R values (both R25 and R5 distinctively) in order to create a calibration curve.

Joint roughness coefficient

To evaluate the surface roughness that is due to weathering and, in particular, due to differential erosion of minerals that increases with the time of exposure (McCarroll and Nesje 1996), we use a very simple type of profilemeter (“Barton Comb”) proposed by Barton and Choubey (1977) to measure the roughness of joints to evaluate their strength to the sliding. The “Barton Comb” comprises a 15 cm line of 209 freely moving pins. The instrument is pressed against a rock surface and the profile transferred to millimeter graph paper in the field. With this method, a large sample of rock surface profiles can be collected quickly and easily. The method proposed by Barton and Choubey (1977) compares the recorded profiles with the 10 standard profiles associated to the numerical index named JRC (Joint roughness coefficient) that ranges between 0 and 20. High JRC values indicate that more erosion has happened, which would support the idea that the rock has been exposed for a longer period of time, while low JRC values indicate less erosion and a shorter exposure time. For every orthogneiss examined boulder, we recorded two perpendicular profiles. These profiles were then compared to the standard profiles, and for each boulder, we calculated the average JRC values. Subsequently, using the same geomorphological units as employed for Schmidt’s Hammer R values, we computed the data averages for boulders within the same unit.

Rock avalanche units definition

According to the geomorphological survey that enhances that the landslide seems divided in three different areas, the digital elevation model (DEM, (Lombardia 2015) with a resolution of 5 m) was also used to analyze the topographic characteristics of the rock avalanche through the calculation of the Topographic Position Index (TPI, Gallant and Wilson 2000) and the calculation of the elevation distribution respect the Val Viola Lake altitude (2267 m a.s.l.) as reference level.

In addition, considering also the lithological composition and the morphometric characteristics of the blocks, it was finally evaluated the possibility of dividing the rock avalanche in different units. In particular, all the blocks that could be considered enough stable after their deposition due to their mass (a axes longer than 1.5 m and b axes longer than 1.5 m) were measured and their lithology type was checked. Following the resulting partition of the landslide, we calculated the mean exposure age and the relative error for each unit (Bigot-Cormier et al. 2005; Claude et al. 2014; Cui et al. 2021). We also calculated the R25 and R5 (Longhi et al. 2020) of each of the units and the mean JRC (McCarroll and Nesje 1996).

Results

Geomorphological survey and lake bathymetry

The landscape, as shown in the geomorphological map in Fig. 5, of the area is primarily characterized by glacial and gravity action and only subordinately by fluvial and periglacial actions. Erosive subglacial features such as rochees moutonees are present on the north-facing slope of the Corno Dosdè, in many cases with striae in the WSW-ENE or SSW-NNE direction. Subglacial deposits (lodgment till) and ridges (Rogen moraines), as well as ablation till and morainic ridges of the Val Cantone and Val Cantone di Dosdè, are present at the NW and SE corners of the area. Poligenic debris cones as well as scree slopes are widespread at the base of the north-facing slope of the Corno Dosdè. A large delta occurs at the western side of the Viola Lake while earth hummocks and some solifluction lobes are widespread throughout the area. It is particularly important to note that the blocks and boulders of the Viola rock avalanche are not homogenously distributed. Indeed, there are three units composed of more than 95% of the same lithology: two of orthogneiss and one of paragneiss. In detail along the northern margin of the landslide is possible to identify a unit (Orthogneiss_1) in which 38 of the 39 checked blocks were orthogneiss, and as reported in Fig. 6, the a axes have a mean of 4 me, while on the southwestern corner of the landslide, the unit named Orthogneiss_2 is entirely composed by orthogneiss blocks that have a mean a axes of ca 3 m although with some size overlapping with the Orthogneiss_1 unit. Finally, in the southern part of the landslide a third unit (Paragneiss) with statistically significant smaller blocks (mean a axes of ca 1.5 m) and almost entirely composed by paragneiss (36 on 37). In addition, considering the analyses of the DEM, it is possible to see how the unit Orthogneiss_1 has a mean elevation of 6.7 ± 6.5 m above the Viola lake level, considerably and statistically higher than the Paragneiss unit (−11.2 ± 9.9 m). The unit Orthogneiss_2 (4.1 ± 3.6 m) is comparable with Orthogneiss_1 and statistically different with the Paragneiss unit. Moreover, also the TPI indicate as Orthogneiss_1 and Orthogneiss_2 are generally convex, while Paragneiss is concave. Despite the similarity of the two Orthogneiss units in terms of lithology, morphometry of the blocks, and topographical characteristics, the units are clearly separated from each other by the Paragneiss unit.

Fig. 5
figure 5

Geomorphological map of the area

Full size image
Fig. 6
figure 6

Box and whisker representation of the size of the a axes of the checked blocks for each unit (red line indicates the mean; solid line blocks the standard deviation, while dashed delimits the 25th and 75th percentile)

Full size image

Based on these results, we decided to sample for 10Be cosmogenic ages and to take the Schmidt Hammer and JRC measurements in three different units: Ortogneiss_1, Ortogneiss_2, and Paragneiss (Fig. 8). In addition, the boulders are in some cases oriented towards E/NE.

The bathymetry shown in Fig. 7 supports the notion that the lake is significantly excavated (9 m is the deepest point of the bottom of the lake) with an elongated shape along the WSW-ENE direction, the same orientation as the striae recorded on the rochees moutonees of the area.

Fig. 7
figure 7

Bathymetry of the Viola Lake, contour lines obtained with the kriging method, and sampling site (red dot)

Full size image

The calibrated age of the lake sediments at 95% of probability is 4855 cal BP according to Intcal 2013 (Reimer et al. 2013).

Cosmogenic ages

We collected three samples from the Orthogneiss_1 unit and two from the Paragneiss unit (Fig. 8). No boulders were sampled in unit Orthogneiss_2 because we could not find suitable boulders to date in those units. The oldest sampled boulder is 19-VV-01 with an age of 8.3 ± 0.3 ka from the Orthogneiss_1, while the youngest is 19-VV-08 with an age of 6.6 ± 0.3 ka (Table 1), which is from the Paragneiss unit. Following the previous partition of the landslide, we calculated the mean exposure age and the relative error for each unit (Bigot-Cormier et al. 2005; Claude et al. 2014; Cui et al. 2021). Before calculating, we examined the data with IQR (Albano et al. 2020) in order to determine if any of the values could have been an outlier: no outlier has been detected. The mean of Orthogneiss_1 unit is 7.7 ± 0.2 ka, while the mean of the Paragneiss unit resulted in 7.0 ± 0.2 ka (Table 1).

Fig. 8
figure 8

Distribution of the Schmidt-Hammer station where green are orthogneiss boulders, and blue are paragneiss boulders

Full size image
Table 1 Results of calculations about exposure age of single cosmogenic samples and relative units
Full size table

Schmidt-Hammer measurements and calibration

The 93 stations analyzed were distributed throughout the three geomorphological units in the rock avalanche (Fig. 7) as shown in Table 2. The number of stations in each unit depended on the availability of suitable big boulders and varied between 17 and 39.

Table 2 Main results of Schmidt’s hammer test
Full size table

The highest R values have been found in unit Orthogneiss_2 both in R25 (47.10) and in R5 (49.89). Unit Orthogneiss_1 has lower values than unit Orthogneiss_2 with R25 = 40.33 and R5 = 43.54. Unit Paragneiss has the lowest values both in R25 (36.91) and R5 (40.98), but it must be taken into account that has a different lithology than the other Units.

SHED (Schmidt Hammer exposure dating) has been established as a good and effective method for dating landforms (Tomkins et al. 2018; Wilson et al. 2019). The three exposure ages for the boulders in the unit Orthogneiss_1 (Table 3) have been correlated with their respective R25 values and R5 values to generate a calibration curve between R values and absolute ages. The correlation is much better with R25 value (R2 = 0.955) than with R5 (R2 = 0.841). For this reason, we prefer the following equation to correlate Schmidt Hammer R values with the exposure age:

Table 3 Data used in the calibration curve between absolute ages and Schmidt’s hammer R values
Full size table
$$\mathrm{Exposure\;Age}=-383.88*R25+\mathrm{23,040}$$
(1)

Using Eq. 1, we calculate an age for the Orthogneiss_2 unit of 5.0 ± 0.3 ka. Equation 1 can also be applied to Unit Orthogneiss_1 to evaluate its error in calculating the exposure age of known data. The age calculated with Eq. 1 resulted in an age of 7.6 ka for the Orthogneiss_1 unit, while the exposure age obtained by cosmogenic dating resulted in a mean age of 7.7 ± 0.2 ka. The calibration cannot be tested on the Paragneiss unit because of the difference in lithology between the units.

JRC results

The JRC has been calculated only on units Orthogneiss_1 and Orthogneiss_2 because the granular texture of the orthogneiss is more suitable for this method. The values are reported and compared with the R25 and R5 values of Schmidt Hammer in Table 4.

Table 4 Comparison of the R25, R5, and JRC on the two units of orthogneiss
Full size table

The results show that unit Orthogneiss_1 (JRC = 13.5) has a much greater JRC than Orthogneiss_2 (JRC = 9.24) therefore we can assume that the Orthogneiss_1 unit has been exposed longer than Orthogneiss 2. This is consistent with the results of the R25 and R5 and the cosmogenic dating. A good relationship (R2 = 0.9058) between R25 and JRC is also confirmed by the linear regression calculated using each R25 of Unit Orthogneiss_1 and Orthogneiss_2 and their relative JRC.

Discussion

Age of the rock avalanche units

The three units are not only separable on the basis of the geomorphological survey but they also show different exposure ages (Table 5), and therefore may be considered as three different rock avalanche events. More precisely, unit Orthogneiss_1, the northernmost and larger part of the rock avalanche, is dated with cosmogenic nuclides at 7.7 ± 0.2 ka, and the Orthogneiss_2 unit, which is much smaller and limited to the most southwestern area, is around 5.0 ± 0.3 ka, as determined with Schmidt’s Hammer R values. The JRC results also support this thesis, with a difference of 4.26 between the two orthogneiss units. The Paragneiss unit can be considered as a different event not only because the different lithology reflects a different source area with respect to the two orthogneiss units but also because even considering the errors of the cosmogenic dates, there are more than 200 years of delay for the Paragneiss unit (Orthogneiss_1 7.7 ± 0.2 ka, Paragneiss at 7.0 ± 0.2 ka).

Table 5 Comparison between measured and calculated data in each rock avalanche unit
Full size table

Hormes et al. (2008) did not recognize the complexity of the rock avalanche units and used only one cosmogenic date sampled in our Orthogneiss_1 unit (Fig. 9). The correction of the Hormes date made by Scotti et al. (2017), who recalculated the original date using the CRONUS-Earth online calculator (http://hess.ess.washington.edu/) version 2.3 with a different shielding effect, determined an age of 7.7 ± 0.2 ka, which matches the mean of our exposure ages for Orthogneiss_1 within 50 years.

Additionally, both Scotti et al. (2017) and Hormes et al. (2008) mapped the rock avalanche deposit area as being quite larger than the area we mapped. These authors included a large part of the south-west corner of the valley as the rock avalanche (gray area in Fig. 9). We mapped this area as bedrock outcrops, glacial material, debris flows, debris slope deposits, and limited alluvial deposits which are clearly not related to the rock avalanche. We also did not map two eastern lobes as rock avalanche material, but this is more debatable. In the case of the more southern lobe (yellow area in Fig. 9), we identified this as an area of sparse boulders that are more likely individual rockfalls, some of which are more recent than the younger events of rock avalanche. In the case of the northern lobe (orange area in Fig. 9), we mapped some of this as glacial material and as an area where part of the rock avalanche deposit has been re-mobilized downhill as demonstrated by the orientation of the boulders (Fig. 9).

Fig. 9
figure 9

Boulder orientation; cosmogenic exposure age dating point from Hormes et al. (2008) (red dot); cosmogenic exposure sample points from this work (yellow dots); extra areas of the rock avalanche body from Hormes et al. (2008); and unit subdivision

Full size image

The rock avalanche as paraglacial effect or permafrost degradation impact

The paraglacial evolution of the upper Viola Valley

During the Last Glacial Maximum, only the portion of the Corno Dosdè that rises over 2900 m a.s.l. was above the glacial limit (Bini et al. 2010) as shown in Fig. 10. This is different respect from what was reported by Hormes et al. (2008) that indicate the LGM trimline as variable between 2600 and 2850 m a.s.l. overlapping the major part of the escarpment scars mapped between 2350 and 2650 m. Then Viola Valley suffered a strong glacial recession that left the entire valley completely ice free around 12.6 ka, except for the cirque of Corno Dosdè (Longhi et al. 2020). Scotti et al. (2017) dated with the Schmidt Hammer a morainic ridge (that is probably a Rogen moraine) just below the Viola Hut at 11.3 ± 1.4 ka, which is west of the rock avalanche deposits. This indicates that at least 4.5–5 ka of paraglacial conditions passed before the first rock avalanche event, not so far from what Hormes et al. (2008) suggested (between 4 and 8.6 ka), and therefore we agreed with these authors that can reasonably it is unlikely that the triggering factor of these events was the stress caused by the melting of local valley glaciers (Augustinus 1995).

Fig. 10
figure 10

Extent of the last glacial maximum from “La Svizzera durante l’ultimo massimo glaciale (LGM) 1:500000”, Bundesamt für Landestopografie by Bini et al. (2010) modified. Legend: glacier in light blue, nunatak in brown

Full size image

The history of permafrost degradation and aggradation since the deglaciation of the upper Viola Valley

Reconstructing the paleo distribution of permafrost is not an easy activity, especially in the mountains, where the more common paleo-indicators of permafrost like ice-wedge casts and pingo scars are extremely rare or absent. Soil micromorphology can be a good tool to achieve this goal (Kovda et al. 2017; Van Vliet-Lano¨e and Fox 2018; Longhi et al. 2021). In Viola Valley, permafrost paleodistribution (Fig. 11) was reconstructed through soil micromorphology (Longhi et al. 2021), and from this, two different phases of permafrost aggradation have been identified in the area. The first phase is dated at 11–9.7 ka with a lower boundary of at least at 2564 m a.s.l., and the second is dated at 9.3–8 ka with a lower boundary of at least at 2525 m a.s.l. (Longhi et al. 2021).

Fig. 11
figure 11

Reconstructed permafrost paleodistribution from Longhi et al. (2021) (green line = 11–9.7 ka, orange line = 9.3–8 ka), the lower boundary of relict permafrost from Guglielmin and Siletto (2000) (purple line), degradation scarps (barred red line), and rock avalanche body (gray polygon)

Full size image

Moreover, studies indicate a warmer period in the Alps between 7.8 and 6.5 ka (Hormes et al. 2008; Ivy-Ochs et al. 2009; Leutscher et al. 2011). This warmer period is also suggested by chironomids studied in two Swiss lakes (Heiri et al. 2003). Therefore, considering that the rock avalanche events of Orthogneiss_1 and Paragneiss both occurred in this warmer period, it is likely that permafrost degradation could be one of, if not the most important triggering factor of these two events. Indeed, the possible scar of the rock avalanches lies above the lower permafrost limit suggested by the soils (2,525 m asl) (Longhi et al. 2021) as well as the lower boundary of relict (undated) permafrost (Guglielmin and Siletto 2000).

Moving to the third event (Orthogneiss_2), there is a small cooling event around 5.2 ka that likely induced a rapid expansion of local glaciers, which is supported by the finding of a mummified prehistoric man in the Ötztaler Alps (the Neolithic Iceman “Otzi”) dated at 5.3–5.0 ka (Bonani et al. 1994). However, this cooling event was not detected in Viola Valley (Longhi et al. 2020; Longhi et al. in prep). We propose that the warm and wet period of the Holocene Thermal Maximum that ended 5 ka (Gomes et al. 2020; Llano et al. 2020) may have triggered the third event of the rock avalanche, as it relates to permafrost degradation. It is interesting to note that the bottom of the lake is WSW-ENE, the same orientation as some of the striations found on roches moutonnées close to the pass area. However, the basal age of the lake is around 4.9 ka years Cal BP, which is younger than the Orthogneiss_2 unit. This suggests that even if the orientation is glacially related, the lake formed because of the damming from unit Orthogneiss_2, and this can be supported by the elevation of the block in Orthogneiss_2, that is 4.1 ± 3.6 m above the level of the lake.

Conclusion

Using a multiple-method approach, including cosmogenic dating, Schmidt’s Hammer R values, and JRC, it was possible to differentiate three different events which were previously considered only one big Val Viola rock avalanche (Hormes et al. 2008; Scotti et al. 2017). This fact is relevant in the reconstruction of the landscape evolution and useful for the evaluation of the future hazards forecast indeed here it is demonstrated that the rock avalanche activity was longer than 2.5 ka causing only at the end of this process to the damming of the Viola Creek and forming a lake instead that only one catastrophic event.

The sequence of the events started with the bigger one happened around 7.7 ± 0.2 ka that deposited unit Orthogneiss_1 followed by a second event around 7.0 ± 0.2 ka, (Paragneiss unit) and finally 2 ka after the last event around 5.0 ± 0.3 ka (unit Orthogneiss_2) that dammed the Viola Creek and permitted the formation of Viola Lake.

Because the main valley was affered but at least 4.5–5 ka of paraglacial conditions, it is unlikely that the triggering factor of these events was the stress caused by the melting of local valley glaciers. Therefore, permafrost that formed in this area down to 2525 m a.s.l. at 9.3–8 ka (Longhi et al. 2021) and degraded successively between 7.8 and 6.5 ka (Hormes et al. 2008; Ivy-Ochs et al. 2009; Leutscher et al. 2011), mainly due to an increase of the air temperature (Heiri et al. 2003), was likely the main triggering factor of the first two rock avalanches events between 7.8 and 6.9 ka as well as for the third event that happened during the warm and wet period of the Holocene Thermal Maximum (Gomes et al. 2020; Llano et al. 2020).