Bridgeman Island (South Shetland Islands)

Bridgeman Island is one of the South Shetland Islands in Antarctica. It is an almost circular, volcanic island marked by steep sides, measuring with a maximum elevation of high, lying east of King George Island.

It is surrounded by steep cliffs and consists mostly of lava flows and scoria, subdivided in two geological units. Bridgeman Island is the emergent portion of a larger volcano in the Bransfield Strait, which contains numerous volcanoes including Deception Island and numerous seamounts. The island formed during the Pleistocene; there is evidence of an eruption in 1821 that generated a probable tuff cone west of Bridgeman Island, which has now disappeared.

Geology and geomorphology

Regional

Antarctica features numerous volcanoes, 12 of which (e.g. Mount Melbourne, Mount Erebus, Mount Berlin, Deception Island) are known to be active or potentially active. A peak in volcanic activity took place during the Jurassic, when the ancient continent Gondwana broke up.Template:Sfn

Bridgeman Island lies in the eastern Bransfield Strait,Template:Sfn away from King George Island.Template:Sfn The strait separates the South Shetland Islands (including King George Island) to the northwest from the Antarctic Peninsula to the southeast. There are a number of mostly submarine volcanoes; from northeast to southwest these include: Gibbs Rise, Spanish Rise, G Ridge, Bridgeman Rise (with Bridgeman Island), Hook Ridge, Orca volcano, Three Sisters, Edifice A, Deception Island (with the eponymous island) and Sail Rock with the eponymous island.Template:SfnTemplate:Sfn The volcanoes produce mainly basaltic rocks with a characteristic sodium-rich composition.Template:Sfn Apart from discrete volcanoes, hydrothermal phenomena, high heat flow and earthquakes occur in the Bransfield Strait.Template:Sfn The line Deception Island-Bridgeman Island coincides with the axis of the Bransfield rift.Template:Sfn The origin of Bransfield Strait took place during the last 4 million years and is linked to the cessation of subduction off the Antarctic Peninsula, although the exact mechanism through which the strait opened is unclear.Template:Sfn

Local

Bridgeman Island is slightly elongated in north–southeast direction, measuring . It is surrounded on all sides by high cliffs,Template:Sfn especially on the almost vertical western side, and lacks beaches and raised beaches except for a narrow gravelly beach on the eastern side. The inner parts of the island are flatTemplate:Sfn and undulating; the highest peak reaches an elevation of .Template:Sfn The terrain was often covered with snow.Template:Sfn A small islet lies off its northern tip.Template:Sfn It has been described as a remnant of a stratovolcano.Template:Sfn

Bridgeman Island consists entirely of volcanic rocks and lacks volcanic landforms.Template:Sfn The rocks are subdivided in three units, the southeastern Lower Unit consisting of thin lava flows and scoria, the central Upper Unit formed by scoria and thick lavas, and the western unit that is formed by fragments from both other units. Below sea level, Bridgeman Island extends to a depth of , forming the Bridgeman Rise. The volcano is the second-most voluminous volcano of the Bransfield Strait,Template:Sfn with a peak-to-bottom height of .Template:Sfn It separates the deep basin of the Bransfield Strait into an eastern and central segment,Template:Sfn and its position may coincide with a boundary between segments of the Bransfield rift.Template:Sfn Around Bridgeman Island, there are submarine mounds and volcanic plugs that rise to a depth of below sea level;Template:Sfn otherwise, the local bathymetry is sparsely known.Template:Sfn Hydrothermal activity has been recorded between Deception Island and Bridgeman Island, as well as east of Bridgeman Island.Template:Sfn

The volcanic rocks of Bridgeman Island are formed by basaltic andesite and subalkaline basalt,Template:Sfn and there is one questionable occurrence of dacite.Template:Sfn These rocks define an aluminum-rich suiteTemplate:Sfn and share compositional traits with both mid-ocean ridge-derived and island arc-derived magmas.Template:Sfn Augite, diopside, olivine and plagioclase form phenocrysts.Template:SfnTemplate:Sfn The magmas of Bridgeman Island may have formed through the incomplete melting of spinel peridotite, with 10–20% of the resulting melt ending up as the magma.Template:Sfn Melts derived from the slabTemplate:Sfn and fractional crystallization may also have played a role.Template:Sfn Hydrothermal alteration has yielded secondary minerals such as calcite, chalcedony, hematite and limonite, giving the rocks a deep red colour.Template:Sfn

Name and research history

Bridgeman Island was discovered on 21 January 1820 by the mariner William Smith from the ,Template:Sfn and named presumably after Charles Orlando Bridgeman.Template:Sfn Other names include Yelena, as given by Bellingshausen, the misspelling BridgmanTemplate:Sfn and Burning Mount or Brians Isle, by captain Richard Sherratt.Template:Sfn

Eruption history

The island formed through Surtseyan eruptions and was apparently never significantly eroded by glaciers. The older volcanism is undated; a younger volcanic event occurred 63,000±25,000 years ago.Template:Sfn It is possible that either the eruptions occurred during times of low sea levels or that the island subsided over time, explaining why it was not significantly eroded by the sea.Template:Sfn The volcano may have produced tephra during the Holocene, which was identified in marine sediment cores.Template:Sfn

Historical activity and 1821 eruption

Bridgeman Island was the first volcano in Antarctica to be recognized as active and the first to be observed as erupting.Template:Sfn An eruption in 1821 was observed by numerous passing ships.Template:Sfn Until 2021, it was attributed to Penguin Island, which has young-looking volcanic featuresTemplate:Sfn but no clear evidence of recent activity.Template:Sfn From its discovery to 1835, the shape of the island did not change significantly,Template:Sfn although comparisons of sketches of the island during the 19th century with modern images imply that parts of the island disappeared after the 19th century.Template:Sfn

Based on observations from passing ships, it seems like the eruption commenced in January 1821 and ended at some point between 1853 and 1909.Template:Sfn From the observations, it appears that the eruption originated from what was probably a tuff ring on the southwestern side of the island. Repeated explosions were seen, which belched black smoke at irregular intervals as vents shut off and reopened.Template:Sfn The crater may have reached a width of .Template:Sfn Accounts mention that the smoke smelled of sulfur, dead penguins floating in the water,Template:Sfn and of boiling water and rock so hot that it could not be touched.Template:Sfn The tuff ring or vent on the southwestern end of the island may have significantly enlarged Bridgeman Island while it existed,Template:Sfn possibly extending it by more than .Template:Sfn It disappeared after the eruption, probably due to marine erosion,Template:Sfn as such shallow-water volcanoes are highly erodible.Template:Sfn

Later activity and present-day status

Dumont D'Urville reported fumaroles on the western side in 1838 and thought that part of the island must have collapsed in the past.Template:Sfn There are reports of eruptions in 1838, 1839 and 1850, sometimes attributed to Penguin Island instead;Template:Sfn reports of an eruption in 1909 are false.Template:Sfn Seismic swarms in the Bransfield Strait have been localized to Orca seamount and Bridgeman Island.Template:Sfn In 2022, acoustic anomalies were identified at a seamount southeast of Bridgeman IslandTemplate:Sfn and at a submarine depression on the same side of the island.Template:Sfn They may imply the occurrence of hydrothermal activity at Bridgeman Island.Template:Sfn However, no heated ground was reported on Bridgeman Island in 2013.Template:Sfn

Life

Lichens and mosses grow on the island,Template:Sfn which otherwise lacks vegetation.Template:Sfn Crustaceans occur in the neighbouring sea,Template:Sfn and snow petrels have been observed.Template:Sfn

See also

• Composite Antarctic Gazetteer
• List of Antarctic and sub-Antarctic islands
• List of Antarctic islands south of 60° S
• List of volcanoes in Antarctica
• SCAR
• Territorial claims in Antarctica

External links

• Website with a picture of Bridgeman Island
• Website with another picture of Bridgeman island Template:In lang
• Template:Cite gvp