The island is rarely visited owing to the remote location, and there is only sparse vegetation. Penguins and seabirds live along the coasts. Before an eruption in 2001, which continued for several years and formed a lava delta on the northern coast, it was not known to have been active during the Holocene (the past 12,000 years).
With dimensions of 10 by 12 kilometres (6.2 mi × 7.5 mi), Montagu is the largest of the South Sandwich Islands.[2] It consists of an eroded[8] mostly ice-covered shield volcano.[9] The island has a roughly rectangular outline, with a promontory on the southeastern corner. There rises the 900 metres (3,000 ft) high[10] steep-sided Mount Oceanite with a 270 metres (890 ft) wide and 100 metres (330 ft) deep summit crater,[5] and a landslide scar to the east.[11]Capes on the island include the northern Poncet Point which formed during the 2001–2007 eruption,[12] northeastern Leeson Point, eastern Mathias Point just east of Mount Oceanite, Allen Point southeast of Mount Oceanite, southern Scarlett Point, southwestern Horsburgh Point and Hollow Point and northwestern Borley Point. Several small islets lie around the island, including Longlow Rock just off the western coast.[5] Rocky outcrops are limited to coastal cliffs,[5] which alternate with ice falls.[13]
A 6 kilometres (3.7 mi) wide ice-filled caldera lies on Montagu;[5] it is the largest known caldera of the South Sandwich Islands[2] and formed from explosive eruptions.[8] The caldera rim is breached to the northwest, where a tidewater glacier exits the caldera to reach the sea. Several secondary cones have formed inside and around the caldera, including Mount Belinda[5] which is 1,125 metres (3,691 ft)[8] or 1,295 metres (4,249 ft) high.[14] The name has also been applied to the a peak on the southern caldera rim, which is the high point of Montagu Island[15] at 1,370 metres (4,490 ft).[2] Some parasitic cones rise above the cliffs.[16]
The submarine structure of Montagu Island is characterized by a largely regular, oval edifice with the pointy end to the west.[17] A shallow shelf, which probably formed through erosion during glacial sea level lowstands, surrounds the island with widths of 14 kilometres (8.7 mi) to the west.[18] There lies the 6 by 4.8 kilometres (3.7 mi × 3.0 mi) Longlow Bank at less than 600 metres (2,000 ft) depth. At 2,000 metres (6,600 ft) depth Montagu Island is about 62 kilometres (39 mi) deep. Several ridges and landslide scars extend and cut into the edifice.[19] A smaller submarine volcanic edifice merges into the western end of the Montagu Island volcano.[17] About 70 kilometres (43 mi) east-southeast of Montagu lies Montagu Bank,[20] a seamount in the forearc with a steep eastern slope.[21] It is much older than the main volcanic arc; potassium-argon dating has yielded ages of 32–28 million years.[22]
The few outcrops indicate that Montagu Island is formed by alternating layers of lava flows, scoria, tuffs and volcanic ash.[16] Rocks are mostly basalt and basaltic andesite,[5] defining a potassium-poor tholeiitic suite,[23] and have a relatively uniform composition. Clinopyroxene and olivine-containing lavas close to Allen Point are defined as oceanites;[24] Mount Oceanite is named after these rocks.[25]Diopside occurs also inclusions in other volcanic rocks.[26] Sediments subducted into the trench may play a role in the formation of Montagu magmas.[27]
A rock from Horsburgh Point yielded an age of 1,000,000 ± 300,000 years by potassium-argon dating.[33] Before 1990, there was no indication of any historical or Holocene activity at Montagu.[34] Other seabirds and seals occasionally come ashore on the island.[35] This probably reflects its remote location and extensive ice cover.[5] Landing parties likewise did not document any heated ground or fumarole activity.[29] Eruptions of low intensity may have taken place between 1995 and 1997.[2]
Pyroclastic cone formed by the 2001–2007 eruption, with melt pits, tephra-covered ice and a moat formed by ice melting[36]
Satellite images observed an eruption at Mount Belinda, commencing in September or October 2001[37] and ending in 2007,[38] although heat anomalies from the cooling of the flow continued into 2010.[15] The eruption was intermittently observed by satellites and passing ships during the following two years. Initially it consisted of the emission of tephra, volcanic plumes[2] and a 600 metres (2,000 ft) long lava flow.[39] A lava lake might have formed during the course of the eruption.[40] In August 2003, a larger lava flow was observed, which extended down the northern flank of Mount Belinda over a length of 2 kilometres (1.2 mi), cutting down into the ice.[41] The eruption peaked in 2005, when ash emissions built a 500 metres (1,600 ft) wide cinder cone on Mount Belinda, with a 150 metres (490 ft) wide crater. A lava flow reached the sea on the northern coast, forming a 800 by 500 metres (2,600 ft × 1,600 ft) wide lava delta[42] later named Poncet Point.[12] Steam columns rose from the site where lava entered the seawater, and a lagoon filled with hot water. Additional lava flows were emitted on the summit,[42] which developed a number of glacial-volcanic landforms such as melt pits formed by lava bombs.[36] The eruption led to the pronounced melting and disruption of the ice cap,[43] with one glacier advancing into the sea.[44]
This eruption was discovered and followed almost exclusively through satellite imaging,[45] which also recorded sulfur dioxide emissions[46] at a rate of 180 tons per day while the eruption was ongoing.[8]
"Poncet Point". British Antarctic Survey. 18 December 2013. Retrieved 25 September 2023.
Convey, P.; Greenslade, P.; Pugh, P. J. A. (April 2000). "The terrestrial micro-arthropod fauna of the South Sandwich Islands". Journal of Natural History. 34 (4): 597–609. doi:10.1080/002229300299462. S2CID85820204.
Gressitt, J. Linsley, ed. (1967). Entomology of Antarctica. Antarctic Research Series. Vol. 10. Washington, D. C.: American Geophysical Union. doi:10.1029/ar010.
