GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 115 which likely affects permafrost thickness even away from the pingos, the experiment is done in the vicinity of the Innerhytta (or Innerhytte) Pingo, a well-known giant pingo (Fig. 1). The study area. Pingos are oval dome-shaped hills which form in permafrost areas when the hydrostatic pressure of freezing groundwater causes the raising of frozen ground. They can reach even 90 m altitude and over 800 m of diameter. In open system pingos, artesian pressure builds up under the permafrost layer, and as the water rises and pushes up the overlying material, it freezes in a lens shape. Innerhytta (Inner hut) pingo is located about 16 km east of Longyearbyen (Spitzbergen, Svalbard Islands) in the valley bottom in Adventdalen, a valley about E-W oriented, ending into Adventfjorden, nearby Longyearbyen, on the Spitzbergen Island of Svalbard Archipelago (Fig. 1). It has been first studied in detail by Piper and Porritt (1966), who produced a topographical map. The pingo is 410 m wide in E-W direction and 200 m in N-S direction. The height above the valley floor is 28 m, above the recognized maximum Holocene marine limit (Ross et al. , 2005). It is developed within, and uplifts, Jurassic shales of the Agardhfjellet formation (Major et al., 2000). A mineralized spring generates a noticeable icing covering the pingo summit and southern flank in the late winter, before the active-layer summer thawing (Fig. 1). In the area, both GPR and resistivity survey were done (Ross et al. , 2005; 2007), and continuous temperature monitoring is on-going (Christiansen et al. , 2010). Ross et al. (2005), from GPR measurements, hypothesized that a southwards migration of the water uprising occurred, so that the actual pingo activity is limited to the area around its apex. According to them, furthermore, the NE part of the structure would be an erosional remnant of bedrock due to the fluvial incision of the overlying relief of Janssonhaugen. The results of the successive resistivity tomographic experiment, however, were not resolutive. They were, in fact, compatible both with the hypothesis of the existence of a ground-ice body at depth (Piper and Porritt, 1966), as well as with the bedrock remnant, without massive ice-body (Ross et al. , 2007). The Fig. 1 – Map of Adventdalen in Svalbard Archipelago, with location of Innerhytta Pingo (red ellipse), and the existing seismic lines (black lines) and boreholes (blue and red circles). Above: a view of the Innerhytta pingo.