Geophysical prospecting on a complex rock glacier in the semi-arid Andes of Argentinia (Morenas Coloradas, Mendoza, Argentina) part of Vignettes:Vignette Collection
In the semi-arid high Andes of Argentina and Chile, rock glaciers are more frequent than glaciers. Rock glaciers are periglacial landforms composed of rock, ice, water and air formed by creeping debris on sloped terrain. Aridity and high solar radiation hamper the development of large glaciers, favor permafrost conditions and the generation of rock glaciers. Some of the world's largest rock glaciers can be found in this part of the Andes, often of greater extent than neighboring glaciers. Due to the rainfall scarcity, permafrost melt from rock glaciers and frozen slopes contributes significantly to river runoff. Without it, human settlement and intensive winegrowing in the dry foothills and forelands towards the east of the Andes would be impossible. Due to their significant size, rock glaciers are assumed to be important water stores for this part of the Andes and the foreland. However, little is known about their ice and water content. One of the few rock glaciers that have been studied in more detail already since more than 20 years is located in the valley of Morenas Coloradas, about 60 km west of the city of Mendoza, Argentina. We revisited the valley in 2008 in order to repeat measurement of the permafrost conditions using geophysics, previously performed by Barsch & King in 1989. The objective of our investigation is to detect permafrost occurrence and depth of the active layer, the upper surface layer that melts during summer. We compared today's situation with the situation 20 years ago and current data from borehole temperatures. The complex rock glacier in the Morenas Coloradas catchment is a transitional landform composed of a high altitude glaciated zone with a large debris covered glacier tongue where thermokarst phenomena, subsidence due to melt of solid ice, are observed. This area is followed by various active rock glacier lobes and subsequently, inactive and relict lobes in the lower sections. (Figures 1 and 2A). Rock glacier activity indicates the rate of periglacial creep and is a function of debris supply, ice content and slope. When a rock glacier doesn't show any signs of movement, it is termed inactive if ice content can still be expected or detected. Once the ice is gone, the landform collapses due to the loss of material and becomes a relict (or fossil) rock glacier. Morenas Coloradas rock glacier is about 5 km long and covers the entire width (300-600 m) of the upper valley at altitudes between 3300 and 4800 m asl. Lithology of the area is composed of Upper Carbon/Lower Triassic greywacke, vulcanite and granite. Mean annual air temperatures of 1.6 to 6.0 °C (surrounding stations at 3560 m, 2500 m a.s.l.) and an annual precipitation of approx. 500 mm/a characterizes the semiarid climate of the region. A striking difference of this rock glacier compared to ones from the European Alps, apart from the greater size, is the size of composing blocks at the surface. While many rock glaciers in the Alps contain blocks of up to several meters in length, the surface of Morenas Coloradas rock glacier is composed of finer material with block sizes not exceeding half a meter. Thus, the internal composition of rocks, air filled voids, water and ice differs from rock glaciers in the Alps as well. We applied geophysical methods to investigate the permafrost conditions at the rock glacier. Geophysical methods measure specific physical properties of the subsurface, delivering high resolution images of the underground. We used ground penetrating radar (GPR) and electric resistivity tomography(ERT) complementary on the rock glacier in February 2008. Permafrost conditions create high resistivity (>10 20 kOhm) of subsurface material that can be differentiated from unfrozen ground (< 10 kOhm) and solid ice (>1 mOhm). Distinct reflectors in radar images represent sharp boundaries between frozen and unfrozen parts of ground (Figure 4). In order to verify the geophysical results, temperature measurements within boreholes are required. A combination of different methods is helpful for data interpretation and supports a more detailed view on rock glacier interiors. Our results were cross checked with previous measurements and borehole temperature data at these locations. We carried out our measurement at three different locations that have been studied previously by Barsch and King (1989), where ground temperatures are monitored in boreholes today. These locations represent different lobes (frontal zones) of the rock glacier at different altitudinal levels (Balcon 1, 3550 m a.s.l.; Balcon 2, 3740 m a.s.l.). The uppermost locality was set within the area of thermokarst at 3810 m a.s.l. (Figure 2B). Both methods delivered similar results of the permafrost table location, confirming each other very well. The permafrost table is the lower limit of the unfrozen active layer. Subsurface resistivity increases between the lowest and the uppermost study sites and with depth (Figure 3). Depth of active layer decreases from 8 meters at Balcon 1 to 3 meters in the area of thermokarst as a function of temperature decrease with altitude. While the permafrost table is clearly detectable in the GPR images at Balcon 2 and the thermokarst area, reflectors at Balcon 1 hint at only local ice occurrences, like ice lenses. Comparing today's resistivity values with the situation 20 years ago indicates how the different parts of the landform have evolved. In the lower parts (Balcon 1), resistivity of the lower subsurface decreased in 20 years indicating, that the ice content has decreased (Table 1) as a probable reaction to increasing air temperatures in this area. The upper measurement sites don't show significant changes to previous records. Permafrost and rock glaciers are phenomena sensitive to changing climates and react to temperature rise by thaw and increase of the active layer thickness. These changes can be detected by geophysical techniques. The study exemplifies that rock glaciers in the Andes not only differ from other examples in the northern hemisphere, but may also play an important role for hydrology that needs to be investigated in order to predict impact of environmental change on mountain areas and their forelands.