## 4 Results
### Evolution of the Perimeter and Subsidence  
Observation of the evolution of the rock glacier surface was made using GNSS equipment on 68 points marked on the rocks along its perimeter. Similarly, sinking of the rock glacier was determined by using GNSS to observe 88 marked points on the rocks, for which one frontal and four transversal profiles were configured (Figure 5a). In addition to being observed by GNSS, some of these points were also checked using the total station, which revealed a degree of uncertainty between the two measurements of \(\pm\)3 cm [56].  
The beginning and end points of the profiles coincided with the points measured on the profiles of the rock glacier and, while no significant planimetric changes were found in the periodic observations (2004-2019), altimetric changes had taken place (Figure 5b). Variations in subsidence of the profiles of the rock glacier coincide with the values detected using the total station in its general dynamic (Figure 4) (Table 1).  
The analysis of the profiles reveals that the same volume loss does not occur in the centre of the glacier as in the areas close to the outline (Figure 5b). The topographic profiles in the frontal part of the rock glacier (profiles: P1 and P2) indicate that from 2004 to 2019 (a 15 year period) the mean loss of the underlying frozen mass was over 5.3 m (Figure 5b). Some points in the central and back areas of the rock glacier underwent greater subsidence (sinking by 7.8 m). The general evolution of the profiles indicates that the mean loss of elevation of the rock glacier was approximately 5.27 m which, extrapolated to the volume of permafrost loss in 2001-2019, was 20,105 m\({}^{3}\).  
In 2009, a geophysical study was made between the topographic profiles P4 and P5 to observe the thickness of the underlying ice and permafrost (Figure 5a). The technique used was electrical prospection using ABEM SAS 4000 equipment with a mean spacing between electrodes of 2.5 m [29]. The penetration of up to 40 m of the geophysical profile facilitated an estimation of the thickness of the frozen mass at around 10 m in 2009 (Figure 6). Depth with respect to the surface at which the ceiling of the relict ice glaciers and surrounding permafrost were detected was at \(-\)1.20 m at an altitude of 3108 m [30; 32].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Period** & **Direction** & **Horizontal** & **Subsidence** \\  & **Centesimal** & **Metrics** & **Metrics** \\  & **Graduation** & **Metrics** & **Metrics** \\ \hline August 2005–July 2006 & 166.4250 & 0.105 & \(-\)0.240 \\ July 2006–August 2006 & 187.0740 & 0.104 & \(-\)0.228 \\ August 2006–July 2007 & 208.9430 & 0.099 & \(-\)0.227 \\ July 2007–August 2007 & 198.8930 & 0.115 & \(-\)0.255 \\ August 2007–July 2008 & 180.7170 & 0.084 & \(-\)0.246 \\ July 2008–August 2008 & 175.0230 & 0.063 & \(-\)0.208 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Dynamic of steel rod 2 for 11 month periods (end of August-end of July) in consecutive years and periods of one month (August) [23].  
Figure 5: (a) Positions of the topographic profiles and the geophysical profile on the automatic cartography (Restitutor) of 2008. (b) Evolution of profile P5 between 2004 and 2019.  
Figure 6: Transversal electrical tomography of Corral del Veleta (August 2009). In blue, the thickness of the frozen bodies, with a mean value of 10 m. The rock glacier is situated over it [29].