Saunders Island, South Sandwich Islands

Saunders Island was discovered in 1775 by James Cook from HMS Resolution[2] and is part of the South Sandwich Islands in the southern Atlantic Ocean at the eastern end of the Scotia Sea.[3] They lie about 2,000 kilometres (1,200 mi) north of Antarctica and about the same distance south-east of the Falklands.[4] The islands are very remote and volcanic eruptions tend to go unnoticed,[5] and bad weather and hazardous oceanic conditions make landings difficult.[6] Politically, they are part of the British Overseas Territory of South Georgia and the South Sandwich Islands,[7] and tourist boats occasionally land on Saunders Island[8] which is accessible through several boulder beaches.[9] The waters around the island are fished commercially.[10]

East of the South Sandwich Islands, the Atlantic Ocean subducts in the South Sandwich Trench under the South Sandwich Plate[3] at a rate of 65–78 millimetres per year (2.6–3.1 in/year).[7] During the past five million years, this subduction process has given rise to the South Sandwich volcanic arc. This volcanic arc includes the Protector Shoal,[4] Kemp Caldera and Adventure Caldera seamounts[11] and numerous small islands[3] extending over a distance of about 350 kilometres (220 mi),[6] from north to south these are Zavodovski Island, Visokoi Island, Leskov Island, Candlemas Island-Vindication Island, Saunders Island, Montagu Island, Bristol Island, Bellingshausen Island-Cook Island-Southern Thule.[4] The largest islands are Saunders Island, Montagu Island and Bristol Island,[3] and most volcanoes (except for Vindication[11]) have either fumarolic activity or historical eruptions. The islands are formed by layers of lava flows and pyroclastic rocks that form stratovolcanoes, their rocks have compositions ranging from basalt to basaltic andesite that define a calc-alkaline suite.[4]

Saunders Island has dimensions of about 8.5 by 5 kilometres (5.3 mi × 3.1 mi) with a crescent-like shape. On the eastern side of the island, 6 kilometres (3.7 mi) wide Cordelia Bay opens to the northeast between Nattriss Point to the east and Sombre Point to the north, with the Brothers Rocks 2 kilometres (1.2 mi) due north of Cordelia Bay[12][13] and reefs especially on its northern side.[14] The southern part of the crescent west of Nattriss Point is formed by the inactive craters of the Ashen Hills which form a distinct topographic elevation;[15] the northern part of the crescent is blunter,[16] forming the highly conspicuous Yellowstone Crags,[17] Blackstone Plain and Harper Point, the northernmost point of Saunders Island. Between these lies the ice-covered bulk of the island with the 990 metres (3,250 ft) high Mount Michael volcano. To its west lies the small Carey Point peninsula and the larger Ollivant Point peninsula to the northwest.[13] The coasts are formed by cliffs, including ice cliffs, with inset bays and bouldery or sandy beaches.[16]

About 80% of the island is covered by ice.[18] Rocky outcrops occur at the coasts, eroded pyroclastic cones on the eastern side and the Mount Michael volcano.[3] Mount Michael is a steep-sided ice-covered conical stratovolcano[19] with a summit crater. The elevation of the summit of Mount Michael is mostly given as 990 metres (3,250 ft);[3] other reported heights are 843 metres (2,766 ft)[1] and 890 metres (2,920 ft).[20] The width of the summit crater is not precisely known but may reach about 700 metres (2,300 ft)[21] or 500 metres (1,600 ft).[20] The summit crater is surrounded by a second crater, conspicuous to the south[17] and southeast,[19] which is filled by ice.[20] It may be a caldera,[19] or a somma volcano.[1] A glacier descends on Mount Michael's eastern side, and the ice is full of crevasses.[22]

Cordelia Bay is about 30–50 metres (98–164 ft)[23] deep and does not appear to be a collapse scar or crater.[12] At Ollivant Point, there is a bouldery beach with boulder ridges. The ocean swells are high enough to frequently submerge the beach, which is surmounted by beach ridges.[24] The Ashen Hills consist of a cluster of overlapping pyroclastic cones[25] and feature tuff cones, which are uncharacteristic of South Sandwich Islands volcanoes that mostly lack evidence of highly explosive eruptions.[13] There are four craters;[17] the southern one has been partially eroded away[26] and river erosion in the Ashen Hills has formed gullies[16] that expose deposits from base surges.[27]

The island lies on the southern and southwestern side of a larger shallow shelf. It extends to the north to form Saunders Bank, while there are traces of past sector collapses on the western side,[28] and of submarine eruptions around the island, and of gullies formed by submarine erosion on the eastern side.[29] The shelf may have formed through wave erosion during periods of low sea level; other islands in the South Sandwich Islands have similar shelves.[23] Saunders Bank may be an eroded island that subsided over time.[30] Two northeast-southwest trending rows of seamounts are found north and southwest of Saunders Island; the northern consists of 480-metre (1,570 ft) deep Minke Seamount, 344-metre (1,129 ft) deep Orca Seamount and 75-metre (246 ft) deep Humpback Seamount, the southern of Saunders Island itself, 276-metre (906 ft) deep Fin Seamount and 400-metre (1,300 ft) deep Southern Right Seamount.[31][28] Saunders Island reaches a width of 55 kilometres (34 mi) at 2,500 metres (8,200 ft) depth,[32] and several submerged ridges extend from it.[33]

Saunders Island has yielded basaltic rocks,[34] andesite is rare.[35] Basaltic andesite may be an older rock.[36] The basalts contain sometimes olivine;[37] phenocrysts include augite, hypersthene and plagioclase. Dolerite and gabbro have been found, and some volcanic rocks form palagonite.[38]