Protector Shoal

The South Sandwich Islands are a north-south trending arc-like chain of islands

South Sandwich Islands

Protector Shoal is the shallowest point of the Protector Seamounts, a group of submarine volcanoes in the Southern Ocean. They are part of the South Sandwich island arc, a volcanic arc that has given rise to the South Sandwich Islands. Protector Shoal reaches a depth of Template:Convert below sea level and is part of a larger group of seamounts that formed atop a larger ridge. Some of these seamounts bear traces of sector collapses, and one is capped by nested calderas.

The seamount erupted in 1962, probably during March, and produced a large pumice raft that was swept by ocean currents around Antarctica. Various islands near to or in the Southern Ocean, such as Australia and New Zealand, have had Protector Shoal pumice wash up on their coastlines. Pumice attributed to this eruption has been recovered as far as Hawaii. There have been no eruptions since, but there is ongoing seismicity and underwater hydrothermal venting.

Geography and geomorphology

The Protector Seamounts are a Template:Convert east-west trending ridgeTemplate:Sfn at Template:Convert depthTemplate:Sfn about Template:Convert northwest of Zavodovski Island.Template:Sfn They lie at the northern end of the South Sandwich Islands,Template:Sfn a Template:Convert long north-south trending chain of eleven islands at the eastern margin of the Scotia Sea.Template:Sfn From north to south they include Zavodovski Island, Leskov Island, Visokoi Island, Candlemas Island-Vindication Island, Saunders Island, Montagu Island, Bristol Island and Southern Thule (Bellingshausen Island-Cook Island-Thule Island);Template:SfnTemplate:Sfn they are small, lack vegetation and are heavily glaciated.Template:Sfn Submarine volcanoes lie at each end of the chain: Protector at the northern, Nelson and Kemp at the southern.Template:Sfn The seamount was already known before the 1962 eruption,Template:Sfn but received a name only after the eruption.Template:Sfn

The northern slopes of the common ridge have a low gradient and are cut by numerous faults associated with the subduction process. The southern slopes are less regular and steeper.Template:SfnTemplate:Sfn Bisco Basin,Template:Sfn a Template:Convert wide embayment, lies north of the ridge. It was originally interpreted as a caldera or a sector collapse scar,Template:Sfn but appears to be of tectonic origin. The Nimrod Basin is another tectonic depression.Template:Sfn A southeastward extension of the ridge joins Protector Shoal with Zavodovski Island.Template:Sfn

The ridge is topped by about seven distinct seamountsTemplate:Sfn that are referred to in the literature as:Template:Sfn Tula ("PS1"), Biscoe ("PS2"), "PS3", Protector Shoal ("PS4"), Endurance ("PS5"), JCR ("PS6") and Quest ("PS7").Template:SfnTemplate:Sfn The names refer to ships.Template:Sfn Another seamount, Scoresby, was discovered west of Quest.Template:Sfn The seamounts have the sizes of small stratovolcanoes, with heights of about Template:Convert, and rise to about a few hundred metres below sea level.Template:Sfn The shallowest point of the chain is Protector Shoal which reaches a depth of Template:Convert below sea level.Template:Sfn Initially it was thought that "Protector Shoal" was a Template:Convert deep seamount; subsequently the name was attributed to the shallowest seamount.Template:Sfn This seamount is cut by a Template:Convert wide slump scar, formed by a collapse that descended to the south-southeast. Another slump is identified on the northwestern side.Template:Sfn

Northwest of Protector Shoal is the broadTemplate:Sfn Nimrod Bank, at Template:Convert depth.Template:Sfn JCR also features a collapse scar.Template:Sfn Quest is cut by two Template:Convert and Template:Convert wide nested calderas that are breached to the southwest,Template:Sfn with a maximum depth of Template:Convert.Template:Sfn The caldera volume reaches about Template:Convert.Template:Sfn Lava domes and lava flows, probably from monogenetic volcanoes, have been emplaced south of JCRTemplate:Sfn and west of Nimrod Bank.Template:Sfn Dives on to Quest and Protector Shoal have found a seafloor covered with gravel, sediments, talus and volcanic blocks that reach metre sizes. Some areas are sandy, others blocky or gravelly and there are rocky outcrops at Protector and Scoresby.Template:Sfn

Geology

East of the South Sandwich Islands, the South American Plate subducts beneath the Sandwich Plate at a rate of about Template:Convert,Template:Sfn increasing to the south. This subduction is responsible for the volcanism of the South Sandwich arc. The oceanic crust under the arc is young, only about 8–10 million years, and formed on the Scotia spreading ridge west of the South Sandwich Islands. It is a young volcanic arc which produces mainly basaltic rocks, forming Template:Convert high volcanic piles.Template:SfnTemplate:Sfn

At the northern margin of the arc, the subducted oceanic crust is about 76 million years old.Template:Sfn Below Protector Shoal, the downgoing South America slab has an east-west trending tear. Fluids ascending through the tear could be enhancing melt production under the seamount.Template:Sfn

Composition

Protector Shoal has erupted rhyolite or rhyodacite, with andesite inclusions.Template:Sfn This is the only known occurrence of rhyolite in the South Sandwich Islands.Template:Sfn Phenocrysts include amphibole, augite, diopside, ilmenite, orthopyroxene, plagioclase, quartz and titanium-containing magnetite.Template:Sfn The rocks define a potassium-poor tholeiitic suite, common on the South Sandwich Islands.Template:Sfn

Pumices dredged from the southern flank of Protector ShoalTemplate:Sfn appear to fall into four distinct groups, based on trace element composition, which do not appear to be derived from each other. They may have all formed through partial melting or fractional crystallization of crustal rocks underlying Protector Shoal. The crustal rocks are most likely the product of island arc volcanism rather than spreading at the Scotia Ridge, although the latter possibility cannot be ruled out.Template:Sfn Melts derived from subducted sediments played no role in the formation of Protector Shoal magmas.Template:Sfn

Climate and ecology

The climate in the region is polar, with frequent snow and storms;Template:Sfn mean annual temperatures hover around Template:Convert.Template:Sfn Fishery takes place around the Protector Seamounts,Template:Sfn but has been restrictedTemplate:Sfn by the 2012 establishment of the South Georgia and South Sandwich Islands marine protected areaTemplate:Sfn and Template:As of by a Benthic Closed Area that disallows all bottom fishing.Template:Sfn

Quest, Scoresby and Protector Shoal have been investigated during dives. Bryozoans, crinoids, various species of octocorals, serpulid worms, sponges and starfish colonize exposed rocks. Anemones, brittle stars and some sponges live on top or inside of other animals. Holothurians, rattails, sea urchins, shrimps and spider crabs complete the fauna.Template:Sfn Above Template:Convert depth macroalgae form dense stands on Protector Shoal.Template:Sfn

Eruption history

Volcanic activity has formed pillow lavasTemplate:Sfn and lava flows, covered by tephra and material eroded from the volcanoes.Template:Sfn The Quest calderas could have formed during a submarine eruption resembling the 1962 eruption.Template:Sfn Tula, Biscoe and "PS3" to the east appear to be older centres,Template:Sfn and rocks from Quest have an aged appearance.Template:Sfn The Nimrod Bank is relatively more recent,Template:Sfn and the lack of dropstones in dredges and pumice samples implies that most of Protector Shoal is less than 10,000 years old.Template:Sfn

The 1962 eruption is the only recorded eruption.Template:Sfn Frequent shallow seismicity at Protector Shoal continued after the eruption, and may indicate renewed eruptions.Template:Sfn No evidence of bathymetric changes between 1962 and 1964 has been found.Template:Sfn Future eruptions could lead to the formation of an island.Template:Sfn There is present-day hydrothermal venting,Template:Sfn inferred from temperature anomalies during dives on to Quest caldera, Protector Shoal and Scoresby Seamount.Template:Sfn In Quest caldera, microbial mats colonize the surroundings of vent chimneys on the caldera rim, but there is no distinct vent-associated fauna. Thermal anomalies at Protector are concentrated in a small depression in the collapse scar,Template:Sfn no vent sites have been identified.Template:Sfn

1962 eruption

On 14 March 1962 the helicopter of the HMS Protector (A146) encountered a pumice raft around Visokoi,Template:Sfn which had probably erupted a few days before.Template:Sfn The ship progressed through a denser raft with larger pumice blocks as it sailed north towards and past Zavodovski.Template:Sfn The raft consisted of many streams of pumice blocksTemplate:Sfn and covered an area exceeding Template:Convert.Template:Sfn Pumice blocks reached sizes of Template:Convert. Similar pumices were dredged from the top of Protector Shoal.Template:Sfn The pumices have been classified in two groups, a white bubble-rich one and a grey bubble-poor one, with slightly distinct chemical compositions.Template:Sfn Larger pumice blocks smelled of sulfur when they were broken open.Template:Sfn The total volume of the pumice was about Template:Convert, derived from about Template:Convert of magma, insufficient to cause caldera formation.Template:Sfn The eruption may have decimated the submarine fauna around Protector Shoal.Template:Sfn

An earthquake recorded at Protector Shoal on 5 March 1962Template:Sfn may or may not be associated with the eruption.Template:Sfn Its epicentre was located just north of the middle portion of the Protector ridge.Template:Sfn The occurrence of this earthquake, the position of the pumice raft, the presence of a shallow seamountTemplate:Sfn and the similarity between its rocks and the pumice led to the recognition that the pumice raft had originated at Protector Shoal.Template:Sfn

The pumice was carried around Antarctica by the Antarctic Circumpolar Current (ACC)Template:Sfn for distances exceeding Template:Convert,Template:Sfn and from there into the South Atlantic, South Pacific and South Indian Ocean.Template:Sfn Wind aided in the movement of the larger blocks, which thus advanced more quickly.Template:Sfn The pumice reached New Zealand in 1963Template:Sfn or 1964,Template:Sfn Tasmania in 1963Template:Sfn or 1964, VictoriaTemplate:Sfn and Western Australia in 1964, Southern Australia in 1965.Template:Sfn The pumice persisted in the Southern Ocean for more than five yearsTemplate:Sfn and eventually spread worldwide.Template:Sfn Some pumices were captured by the Humboldt Current,Template:Sfn getting washed up on Juan Fernandez Islands in 1965,Template:Sfn and further into the North Equatorial Current, eventually arriving in Hawaii.Template:Sfn A sample of Hawaiian pumice demonstrated that almost 72% of Hawaiian pumices come from Protector Shoal,Template:Sfn which was attributed to a sampling issue as Christmas Island has very few Protector Shoal pumices.Template:Sfn Other localities with tentative findings of Protector Shoal pumices are Bouvet,Template:Sfn the Falklands, Heard Island, Kerguelen, Macquarie Island, Marion Island and South Georgia.Template:SfnTemplate:SfnTemplate:Sfn Pumice samples derived from Protector Shoal have been recovered from Livingstone Island in the South Shetland Islands and Rio Bueno on Tierra del Fuego Template:Sfn at the tip of South America,Template:Sfn where they arrived after being carried around Antarctica by the ACC,Template:Sfn and are a conspicuous feature of the beaches there.Template:Sfn

Large pumice blocks can be colonized by organisms and thus aid their spread.Template:Sfn Some pumices after stranding were floated again and thus redeposited,Template:Sfn others were picked up and swallowed by birds, being found in their stomachs.Template:Sfn

See also

• List of volcanoes in South Sandwich Islands

External links

• Template:Cite gvp