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 55 metres (180 ft) 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.
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.[11][12] Bisco Basin,[3] a 20-kilometre (12 mi) wide embayment, lies north of the ridge. It was originally interpreted as a caldera or a sector collapse scar,[13] but appears to be of tectonic origin. The Nimrod Basin is another tectonic depression.[14] A southeastward extension of the ridge joins Protector Shoal with Zavodovski Island.[13]
The ridge is topped by about seven distinct seamounts[2] that are referred to in the literature as:[3] Tula ("PS1"), Biscoe ("PS2"), "PS3", Protector Shoal ("PS4"), Endurance ("PS5"), JCR ("PS6") and Quest ("PS7").[15][9] The names refer to ships.[16] Another seamount, Scoresby, was discovered west of Quest.[17] The seamounts have the sizes of small stratovolcanoes, with heights of about 400–1,400 metres (1,300–4,600 ft), and rise to about a few hundred metres below sea level.[12] The shallowest point of the chain is Protector Shoal which reaches a depth of 55 metres (180 ft) below sea level.[2] Initially it was thought that "Protector Shoal" was a 27-metre (89 ft) deep seamount; subsequently the name was attributed to the shallowest seamount.[18] This seamount is cut by a 2.5-kilometre (1.6 mi) wide slump scar, formed by a collapse that descended to the south-southeast. Another slump is identified on the northwestern side.[12]
Northwest of Protector Shoal is the broad[3] Nimrod Bank, at 400–600 metres (1,300–2,000 ft) depth.[14] JCR also features a collapse scar.[12] Quest is cut by two 3 kilometres (1.9 mi) and 1.6 kilometres (0.99 mi) wide nested calderas that are breached to the southwest,[19] with a maximum depth of 340 metres (1,120 ft).[20] The caldera volume reaches about 0.4–0.9 cubic kilometres (0.096–0.216 cu mi).[21]Lava domes and lava flows, probably from monogenetic volcanoes, have been emplaced south of JCR[19] and west of Nimrod Bank.[3] 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.[22]
Geology
East of the South Sandwich Islands, the South American Platesubducts beneath the Sandwich Plate at a rate of about 67–79 millimetres per year (2.6–3.1 in/year),[23] 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 3-kilometre (1.9 mi) high volcanic piles.[24][25]
At the northern margin of the arc, the subducted oceanic crust is about 76 million years old.[25] 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.[13]
Pumices dredged from the southern flank of Protector Shoal[12] 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.[29] Melts derived from subducted sediments played no role in the formation of Protector Shoal magmas.[30]
Climate and ecology
The climate in the region is polar, with frequent snow and storms;[5] mean annual temperatures hover around 0 °C (32 °F).[31]Fishery takes place around the Protector Seamounts,[32] but has been restricted[33] by the 2012 establishment of the South Georgia and South Sandwich Islands marine protected area[20] and as of 2022[update] by a Benthic Closed Area that disallows all bottom fishing.[34]
Volcanic activity has formed pillow lavas[36] and lava flows, covered by tephra and material eroded from the volcanoes.[37] The Quest calderas could have formed during a submarine eruption resembling the 1962 eruption.[21] Tula, Biscoe and "PS3" to the east appear to be older centres,[12] and rocks from Quest have an aged appearance.[38] The Nimrod Bank is relatively more recent,[14] and the lack of dropstones in dredges and pumice samples implies that most of Protector Shoal is less than 10,000 years old.[27]
The 1962 eruption is the only recorded eruption.[11] Frequent shallow seismicity at Protector Shoal continued after the eruption, and may indicate renewed eruptions.[39] No evidence of bathymetric changes between 1962 and 1964 has been found.[40] Future eruptions could lead to the formation of an island.[41] There is present-day hydrothermal venting,[5] inferred from temperature anomalies during dives on to Quest caldera, Protector Shoal and Scoresby Seamount.[42] 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,[38] no vent sites have been identified.[36]
1962 eruption
On 14 March 1962 the helicopter of the HMS Protector (A146) encountered a pumice raft around Visokoi,[43] which had probably erupted a few days before.[13] The ship progressed through a denser raft with larger pumice blocks as it sailed north towards and past Zavodovski.[43] The raft consisted of many streams of pumice blocks[44] and covered an area exceeding 5,000 square kilometres (1,900 sq mi).[45] Pumice blocks reached sizes of 1.5 metres (4 ft 11 in). Similar pumices were dredged from the top of Protector Shoal.[13] The pumices have been classified in two groups, a white bubble-rich one and a grey bubble-poor one, with slightly distinct chemical compositions.[27] Larger pumice blocks smelled of sulfur when they were broken open.[46] The total volume of the pumice was about 0.6 cubic kilometres (0.14 cu mi), derived from about 0.2 cubic kilometres (0.048 cu mi) of magma, insufficient to cause caldera formation.[27] The eruption may have decimated the submarine fauna around Protector Shoal.[7]
An earthquake recorded at Protector Shoal on 5 March 1962[11] may or may not be associated with the eruption.[13] Its epicentre was located just north of the middle portion of the Protector ridge.[13] The occurrence of this earthquake, the position of the pumice raft, the presence of a shallow seamount[10] and the similarity between its rocks and the pumice led to the recognition that the pumice raft had originated at Protector Shoal.[47]
Large pumice blocks can be colonized by organisms and thus aid their spread.[63] Some pumices after stranding were floated again and thus redeposited,[54] others were picked up and swallowed by birds, being found in their stomachs.[64]
^"Protector Seamounts". Global Volcanism Program. Smithsonian Institution. 2013. Archived from the original on 30 November 2023. Retrieved 19 January 2024.
