Alexander Island
| Alexander Island | |
.jpg) Mountains on Alexander Island | |
|---|---|
| Location | |
| Location: | 71°0’0"S, 70°0’0"W |
| Area: | 16,700 square miles |
| Highest point: | Mount Stephenson |
| Data | |
| Population: | 0 |
Alexander Island lies along the whole west coast of Palmer Land in the British Antarctic Territory. It is the largest island of Antarctica, and indeed the largest uninhabited island in the world after Devon Island in the Canadian Arctic.
The island lies in the Bellingshausen Sea west of Palmer Land, from which it is separated by Marguerite Bay and George VI Sound. Physically though the George VI Ice Shelf entirely fills George VI Sound and connects Alexander Island to Palmer Land. The island partly surrounds Wilkins Sound, which lies to its west.[1]
Alexander Island is about 240 miles long in a north-south direction, 50 miles wide in the north, and 150 miles wide in the south, with an area of 16,700.00 square miles (2.08 times the size of Wales).
It is also known as Alexander I Island, Alexander I Land, Alexander Land, Alexander I Archipelago, and was named Zemlja Alexandra I by its discoverer, Fabian Gottlieb von Bellinghausen , the Russian naval explorer.
History
Alexander Island was discovered on 28 January 1821 by a Russian expedition under Fabian Gottlieb von Bellingshausen, who named it Alexander I Land for the reigning Tsar Alexander I of Russia. He believed it to be a part of the Antarctic mainland, and it was mapped as such until 1940.
The insular nature of Alexander I Land was proven in December 1940, by a two-person sledge party composed of Finn Ronne and Carl Eklund of the United States Antarctic Service. [2] from that point it was noted as "Alexander island".
In the 1950s, a British base was constructed as Fossil Bluff (Base KG).[3]
The island is within the British Antarctic Territory, a claim announced in Letters Patent of 1908 annexing to the Crown what was then known as the Falkland Islands Dependencies.
On the island today stands the British Fossil Bluff meteorological centre and refuelling base.[4]
Geography
The surface of Alexander Island is predominantly ice-covered. There exist some exposed nunataks and a few ice free areas, for example Ablation Point Massif, of significant size. The nunataks are the peaks of north-south trending mountain ranges and hills.
Mountains and hills include the Colbert, Havre, Lassus, Rouen, Sofia University, and Walton mountains; Staccato Peaks; Lully Foothills; Elgar Uplands; and Douglas Range. These mountains, peaks, hills, and uplands are surrounded by a permanent ice sheet, which consists of glaciers that flow off of Alexander Island.
The glaciers flow west into the Bach and Wilkins Ice Shelves and Bellingshausen Sea, and east into the George VI Ice Shelf. The George VI Ice Shelf is fed by both by outlet glaciers from the ice cap on Palmer Land and Alexander Island.[1][5]
Another notable feature of Alexander Island is Hodgson Lake. It is a former subglacial lake that has emerged from under an ice sheet that covered it. It is almost a mile and a half long by a mile wide, and has a 300-foot deep water column that lies sealed beneath a perennial lake ice cap 12 feet to 13 feet thick. The northern side of this lake is bounded by the Saturn Glacier, which flows east into George VI Sound, while the southern side of Hodgson Lake is bounded by the northern face of Citadel Bastion. During the Last Glacial Maximum, Hodgson Lake was covered by the ice sheet at least 1,540 feet thick. This ice sheet started thinning about 13,500 years ago. It retreated and left Hodgson Lake covered by perennial ice at some time before 11,000 years ago. This lake has been covered by perennial ice since that time.[6][7]
Geology
Except for a strip along its eastern edge of six to eighteen miles wide, the bedrock beneath the ice sheet, which covers Alexander Island, consists of a complex assemblage of metasedimentary and metavolcanic rocks. These rocks, which are locally overlain and intruded by igneous rocks, are known as the "Lemay Group." It consists of turbiditic sandstone (subgraywacke and arkose) interbedded with secondary siltstone, mudstone, tuff, volcaniclastic sedimentary rock, agglomerate, and conglomerate, which have all been metamorphosed to prehnite–pumpellyite to blueschist facies. These metasedimentary rocks contain poorly preserved ammonites, pelecypods, gastropods, radiolarians, and fragmentary plant fossils. The LeMay Group also contains north-south, fault-bound slices of accreted ocean floor, ocean islands and seamounts, which includes pillow basalts, volcaniclastic sedimentary rocks, and chert. The rocks, which comprise the LeMay Group are asymmetrically Fold (geology)|folded and thrust faulted to the west, contain chaotic zones, and exhibit abundant coeval soft-sediment deformation. Based upon biostratigraphy, the oldest known rocks found within the LeMay Group are Lower Jurassic and range up to late Lower Cretaceous. The oldest rocks within this group may date back to Permo-Carboniferous times (350–260 million years ago). The LeMay Group is interpreted to be an accretionary prism that was built out from the west (fore-arc) side of a long-lived magmatic arc, the roots of which are now exposed on the Antarctic Peninsula, between Early Jurassic and Paleocene times, when subduction ceased.[8][9][10]
The easternmost six to eighteen mile wide strip of Alexander Island is underlain by a four-mile thick sequence of highly fossiliferous, shallow marine to subaerial fluvial sandstone, mudstone, and conglomerate of the Fossil Bluff Group. The marine strata within the Fossil Bluff Group contain numerous ammonites, belemnites, gastropods, pelecypods, and other marine fossils. The upper fluvial sedimentary rocks of the Fossil Bluff Group contain fossil forests with standing trunks as tall as 18 feet, other plant fossils, and associated paleosols as found at Citadel Bastion. These sedimentary rocks are essentially unmetamorphosed and gently folded. They have been affected by strike-slip, normal, and thrust faulting at different times. As determined from the fossils found in it, the Fossil Bluff Group accumulated over a period of time ranging from Early Jurassic to mid-Cretaceous. The sedimentary rocks of the Fossil Bluff Group unconformably overlie and are faulted against the LeMay Group. Earth scientists argue that the Fossil Bluff Group accumulated within a fore-arc basin on the western side of a long-lived magmatic arc, the roots of which are now exposed along the Antarctic Peninsula.[8][9][10]
Two different groups of igneous rock have been found within Alexander Island. One group of igneous rocks consist of volcanic and plutonic rocks of Late Cretaceous to Paleocene age. They unconformably overlie and intrude metasedimentary and other rocks of the LeMay Group and the sedimentary rocks of the Fossil Bluff Group. Some of these volcanic rocks consist of distinctive magnesium-rich andesites, which were created by the subduction of a segmented ancient mid-ocean ridge about 80 million years ago. Finally, there are several small outcrops of Neogene alkaline volcanic rocks, which formed after the cessation of subduction.[9][10][11]
Pictures
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Satellite photo of Alexander Island -
Alexander Island Mountain ranges
.jpg)
References
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("Wikimedia Commons" has material about Alexander Island) |
- ↑ 1.0 1.1 Stewart, J. 'Antarctic An Encyclopedia' (McFarland & Company Inc, 2011), ISBN 9780786435906
- ↑ Siple, Paul: Obituary: Carl R. Eklund, 1909–1962 in The Arctic, Volume 16, issue 2 (Arctic Institute of North America, 1963), pages 147–148
- ↑ British Antarctic Survey – Our bases
- ↑ Mills, William (2003). Exploring Polar Frontiers: A Historical Encyclopedia (1 ed.). pp. 9. ISBN 1-57607-422-6. https://books.google.com/books?id=PYdBH4dOOM4C&lpg=PA9&dq=Fossil%20Bluff%20base&pg=PA9#v=onepage&q=Fossil%20Bluff%20base&f=false. Retrieved 20 January 2015.
- ↑ Smith, J.A., M.J. Bentley, D.A. Hodgson, and A.J.Cook (2007) George VI Ice Shelf: past history, present behaviour and potential mechanisms for future collapse. Antarctic Science 19(1):131–142.
- ↑ Hodgson D.A., S.J. Roberts, M.J. Bentley, J.A. Smith, J.S. Johnson, E. Verleyen, W. Vyverman, A.J. Hodson, M.J. Leng, A. Cziferszky, A.J. Fox, and D.C.W. Sanderson (2009) Exploring former subglacial Hodgson Lake, Antarctica Paper I. Quaternary Science Reviews. 28:23–24:2295–2309.
- ↑ Hodgson D.A., S.J. Roberts, M.J. Bentley, E.L. Carmichael, J.A. Smith, E. Verleyen, W. Vyverman, P. Geissler, M.J. Leng, and D.C.W. Sanderson (2009) Exploring former subglacial Hodgson Lake, Antarctica Paper II. Quaternary Science Reviews. 28:23–24:2310–2325.
- ↑ 8.0 8.1 Macdonald, D.I.M. and P.J. Butterworth (1990) The stratigraphy, setting and hydrocarbon potential of the Mesozoic sedimentary basins of the Antarctic Peninsula. in B. John, ed., pp. 101–125. Antarctica as an exploration frontier; hydrocarbon potential, geology, and hazards. AAPG Studies in Geology. vol. 31 American Association of Petroleum Geologists, Tulsa,Oklahoma.
- ↑ 9.0 9.1 9.2 Macdonald, D.I.M., P.A. Doubleday, and S.R.A. Kelly (1999) On the origin of fore-arc basins; new evidence of formation by rifting from the Jurassic of Alexander Island, Antarctica. Terra Nova. 11(4):186–193.
- ↑ 10.0 10.1 10.2 Vaughan, A.P.M., and B.C. Storey (2000) The eastern Palmer Land shear zone: a new terrane accretion model for the Mesozoic development of the Antarctic Peninsula. Journal of the Geological Society. 157(6):1243–1256.
- ↑ McCarron, J.J., and J.L. Smellie: 'Tectonic implications of fore-arc magmatism and generation of high-magnesian andesites: Alexander Island, Antarctica': Journal of the Geological Society. vol. 155(2): 269–280 (1998)