South Sandwich Islands, the archipelago where earthquakes and volcanoes reveal a plate boundary

The South Sandwich Islands form an uninhabited volcanic arc in an area of high seismic activity in the South Atlantic (Wikimedia Commons)The South Sandwich Islands concentrate intense geological activity in the South Atlantic. The magnitude 5.1 earthquake recorded on November 1 once again put the focus on this arc of 11 uninhabited islands, where earthquakes and eruptions respond to a single process taking place beneath the ocean. The archipelago lies in a remote area, difficult to access due to the weather, sea ice, and waves. Its active volcanoes, its glaciated peaks, and one of the deepest ocean trenches on the planet make up a territory whose monitoring depends largely on satellite observations. For Argentina, the islands are part of the province of Tierra del Fuego, Antarctica, and South Atlantic Islands. The United Kingdom administers them together with South Georgia as a British overseas territory, within a situation that the Argentine state contests through its sovereignty claim. The South Sandwich Islands are located southeast of the South Georgia islands, between the South Atlantic and the subantarctic environment. The chain stretches north to south for more than 400 kilometers and comprises 11 main islands, in addition to seamounts. The British Antarctic Survey places the island group at the eastern end of the Scotia Sea. Zavodovski, Leskov, Visokoi, Candlemas, Vindication, Saunders, Montagu, Bristol, Bellingshausen, Cook, and Thule make up the main islands of the archipelago. The largest is Montagu, with about 110 square kilometers. Across much of the territory, volcanic slopes, rugged coasts, snow, and glaciers combine; during winter, sea ice can extend along the entire chain. The distance and weather conditions explain the lack of residents. There are no permanent settlements or urban infrastructure. Storms are frequent and cloud cover limits fieldwork, so a substantial part of the landscape's transformations is detected through optical and radar imagery. These tools make it possible to identify gas plumes, ash deposits, and changes in the glaciers. Without permanent settlements, the eleven islands are monitored from a distance to record their seismic and volcanic activity (Wikimedia Commons)The explanation for the high seismicity lies beneath the seabed. In front of the island arc, the South American Plate descends westward beneath the Scotia Plate and the South Sandwich microplate. This mechanism, known as subduction, advances at a speed of about 71 millimeters per year, according to the United States Geological Survey (USGS). The annual movement may seem small, but it accumulates stress in the rocks over long periods. When that energy is suddenly released, seismic movements of varying magnitude and depth occur. East of the archipelago lies the South Sandwich Trench, an underwater expression of the same tectonic process, whose deepest sections exceed 8,000 meters. The sequence of August 12, 2021 showed the seismic capacity of this zone. A magnitude 7.5 earthquake was followed, three minutes later, by another of magnitude 8.1. The USGS linked the larger event to a reverse fault in the subduction zone. During the following 24 hours, the agency located 61 aftershocks of magnitude 4.5 or greater. The episode also generated a tsunami that was recorded at great distances, although it caused no damage or casualties due to the absence of population on the islands. The United States National Oceanic and Atmospheric Administration (NOAA) reported that the waves reached coastal sea-level stations located up to 10,000 kilometers from the origin. The archipelago combines eleven volcanic islands, glaciers, and an ocean trench associated with the subduction of tectonic plates (Wikimedia Commons)Subduction is also behind the volcanoes. When the oceanic plate sinks, the water it carries alters the conditions of the mantle rocks and facilitates the generation of magma. That material rises through a thin oceanic crust and feeds the volcanoes that rise above sea level. On Zavodovski, the northernmost island of the group, rises Mount Curry. Between March and June 2016, emissions of ash and vapor were observed from its crater. Satellite images and reports from vessels showed dark deposits on its eastern flank. The Smithsonian Institution's Global Volcanism Program confirmed that eruptive period and noted that the ash covered between one-third and one-half of the island. Remote monitoring is decisive for documenting these episodes. Persistent clouds and the lack of inhabitants restrict on-site work, while satellites provide data on gases, ash, and changes on the surface. Each record makes it possible to follow the evolution of an archipelago where the movement of tectonic plates continues to transform the landscape.