posted on 2018-01-24, 12:03authored byJ. L. Smellie, S. Rocchi, J. S. Johnson, G. Di Vincenzo, J. M. Schaefer
The remains of a small volcanic centre are preserved on a thin bedrock ridge at Harrow Peaks, northern Victoria Land, Antarctica. The outcrop is interpreted as a monogenetic tuff cone relict formed by a hydrovolcanic (phreatomagmatic) eruption of mafic magma at 642 ± 20 ka (by 40 Ar- 39 Ar), corresponding to the peak of the Marine Isotope Stage 16 (MIS16) glacial. Although extensively dissected and strewn with glacial erratics, the outcrop shows no evidence for erosion by ice. From interpretation of the lithofacies and eruptive mechanisms, the weight of the evidence suggests that eruptions took place under a cold-based (frozen-bed) ice sheet. This is the first time that a tuff cone erupted under cold ice has been described. The most distinctive feature of the lithofacies is the dominance of massive lapilli tuff rich in fine ash matrix and abraded lapilli. The lack of stratification is probably due to repeated eruption through a conduit blasted through the ice covering the vent. The ice thickness is uncertain but it might have been as little as 100 m and the preserved tephra accumulated mainly as a crater (or ice conduit) infill. The remainder of the tuff cone edifice was probably deposited supraglacially and underwent destruction by ice advection and, particularly, collapse during a younger interglacial. Dating using 10 Be cosmogenic exposure of granitoid basement erratics indicates that the erratics are unrelated to the eruptive period. The 10 Be ages suggest that the volcanic outcrop was most recently exposed by ice decay at c. 20.8 ± 0.8 ka (MIS2) and the associated ice was thicker than at 642 ka and probably polythermal rather than cold-based, which is normally assumed for the period.
The authors gratefully acknowledge the support of the Programma Nazionale di Recherche in Antartide (PNRA) of Italy, which provided all of the logistics involved and supported the research. We are also grateful to the British Antarctic Survey for supporting the acquisition of cosmogenic surface exposure ages; to the pilots Mark Read, Giles de Garnham and Bob McElhinney, and ground crews of Helicopters New Zealand who flew us so safely during the fieldwork; and Alberto Della Rovere and Giuseppe De Rossi, Base Commanders at the Italian Mario Zuchelli Station, and all the station support personnel who made us very welcome during our stay in 2005-06 and 2011-12, when the fieldwork for this study took place.
Bulletin of Volcanology, 2018, 80:12
/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/School of Geography, Geology and the Environment
VoR (Version of Record)
Bulletin of Volcanology
Springer Verlag (Germany) for International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI)