Recent permafrost aggradation in Deception Island, Antarctica. Results of geoelectrical surveys in the area of Craters 70

A. CORREIA; VIEIRA G.; GOYANES, G.; DAVID, A.

Évora

Congreso; 4th European Conference on Permafrost (EUCOP); 2014

Universidade de Lisboa y Universidade de Évora

Deception Island is horse-shoe shaped stratovolcano with 15 km diameter with a large caldera that opens towards the southeast, forming a bay about 7 km wide. The maximum altitude is at Mount Pond (539 m asl). About 57% of the island area is covered by glaciers. In geological terms Deception Island is composed of volcano-sedimentary deposits, including pyroclastic flows and deposits, strombolian scoriae and lava, volcanic and hypo-volcanic indurated ashes, and phreatomagmatic deposits. Recent eruptions took place in the interior of the island in 1967, 1969, and 1970. Permafrost is widespread in the island but its characteristics are still poorly studied. In this study we present geoelectrical data collected in the Crater 70 area of Deception Island which was formed during the eruptions of 1970. The study area is located in the southern slope of a volcanic cone and the objective of the geoelectrical survey was to determine if there was permafrost aggradation after the eruptive event and to assess the thickness of the frozen body. Two electrical resistivity tomography (ERT) profiles and a vertical electrical sounding (VES) were done. Along the two ERT profiles thaw depth probing was also done to compare with the geoelectrical results. ERT profile 1 was done on a flat area at the base of the slope of the volcano cone; most of ERT profile 2 was done along the slope of the volcanic cone with its last quarter in the lower slope section; both ERT profiles were done along the same direction; the vertical electrical sounding was done in the lower part of the slope crossing the lower section of ERT profile 2. Along ERT profile 1 the top of the frozen ground varied from 50 to 70 cm and the electrical resistivity of the frozen ground was about 1,300 Ù.m. Along ERT profile 2 the top of the frozen ground varied from 40 to 70 cm and the electrical resistivity to the frozen ground was about 1,600 Ù.m. In both profiles the electrical resistivity increased with depth to values that vary from 4,000 to 7,400 Ù.m. Both ERTs indicate that the thickness of permafrost is about 10-12 m with a body of more resistive frozen material at intermediate depth; this result is also observed in the VES which indicates that the frozen layer is about 15 m thick with the higher electrical resistivities located at about 8 m depth. Such a higher resistivity is probably related to higher ice-content as has been observed in other areas in Deception Island.