Seal Nunataks

The Seal Nunataks are a cluster of sixteen volcanic cones to the east of Cape Fairweather, on the Nordenskjöld Coast of Graham Land, within the British Antarctic Territory. They rise up to about 1,050 feet above the Larsen Ice Shelf.

The nunataks from east to west are:

• Christensen Nunatak
• Oceana Nunatak (on Robertson Island),
• Pollux Nunatak,
• Castor Nunatak,
• Larsen Nunatak,
• Murdoch Nunatak,
• Arctowski Nunatak,
• Hertha Nunatak,
• Gray Nunatak,
• Donald Nunatak,
• Åkerlundh Nunatak,
• Bruce Nunatak,
• Dallmann Nunatak,
• Evensen Nunatak,
• Bull Nunatak and
• Pedersen Nunatak

The nunataks were roughly charted by Carl Anton Larsen on 11 December 1893, and named Seløerne. Later charts mark them as Sel Öerne or 'Seal Islands'. Other charted names incluse Sar-Inseln ('sea bream islands'), löwen-Insel and löwen-Inseln ('(sea) lion island(s)'), 'Sea-Lion Islands', Robben-Insel ('seal island'), Dirk Gerritszarkipelagen, 'Dirk Gerritz Archipelago', Archipel des Phoques' ('archipelago of the seals') and others.

The Seal Nunataks have been described as separate volcanic vents of ages ranging from Miocene to Pleistocene. There are unconfirmed reports of Holocene volcanic activity.

Geography and geomorphology

The Seal Nunataks are part of Graham Land and were embedded within the Larsen Ice Shelf until its northern margin collapsed between 1986 and 1996, rising from about 1,640 feet below sea level.

When Carl Anton Larsen discovered the features in December 1893, he also identified them as volcanoes and named them Seal Islands.

The Seal Nunataks are part of a volcanic province in West Antarctica which extends over the Antarctic Peninsula, Marie Byrd Land and Ross Island. In the Antarctic Peninsula, Alexander Island, James Ross Island and the Seal Nunataks form this volcanic province. Alexander Island was active between 48 and 18 million years ago. James Ross Island lies about 95 miles to the north-east of the Seal Nunataks.

The nunataks consist of ridges with lengths of less than half a mile to three and a half miles flanked with scree deposits and occasionally by primary volcanic features. According to Otto Nordenskjöld in 1901-1903, Christensen Nunatak features a volcanic crater. Castor, Christensen and Hertha are the only vents which show evidence of subaerial activity in form of lava flows; elsewhere the nunataks are constructed by hyaloclastite and pillow lavas which form beneath ice. The volcanoes appear to align along northwest-striking lines.

Composition

The Seal Nunataks consist of basalt, basalt which contains clinopyroxene, olivine and plagioclase. Xenoliths consisting of lherzolite and spinel have been found at Seal Nunataks. Their geochemistry implies that the magmas are primitive mantle-derived melts which underwent only a little fractional crystallization.

Eruptive history

Potassium-argon dating has yielded ages between 4 million years ago and "recent" for the rocks in the volcanic field, but the reliability of the dates decreases the younger they are. Pillow lavas are usually about 1.5 million years old while subaerial activity occurred 700,000 years ago. The youngest dates were obtained on Donald and Gray, the dates being less than 200,000 years ago. Glacial erratics on some of the nunataks imply that they were covered with glaciers in the past.

The discoverer Captain Larsen observed volcanic activity on Christensen Nunatak and Lindenberg Island, but the reliability of these reports is considered to be unclear. Fumaroles have been observed on Christensen, Dallman and Murdoch, and eruptions were reported on the last two in 1982. This activity consisted of the possible formation of lava flows and a pyroclastic cone, respectively.

Location

• Location map: 65°2’56"S, 60°15’16"W

References

• Gazetteer and Map of The British Antarctic Territory: Seal Nunataks
• Hole, M. J. (1990-02-01). "Geochemical evolution of Pliocene-Recent post-subduction alkalic basalts from Seal Nunataks, Antarctic Peninsula". Journal of Volcanology and Geothermal Research 40 (2): 149–167. doi:10.1016/0377-0273(90)90118-Y. Bibcode: 1990JVGR...40..149H. 
• Oliver, R. L.; James, P. R.; Jago, J. B. (1983). Antarctic Earth Science. Cambridge University Press. ISBN 9780521258364. https://books.google.com/books?id=KE3wCs13g7IC&q=%22seal+nunataks%22&pg=PA334. 
• Smellie, J. L.; Hole, M. J. (1997-07-01). "Products and processes in Pliocene–Recent, subaqueous to emergent volcanism in the Antarctic Peninsula: examples of englacial Surtseyan volcano construction". Bulletin of Volcanology 58 (8): 628–646. doi:10.1007/s004450050167. SSN 0258-8900. Bibcode: 1997BVol...58..628S. 
• LeMasurier, W. E., ed (1990). Volcanoes of the Antarctic Plate and Southern Oceans. American Geophysical Union. ISBN 978-0-87590-172-5.