Lonely Volcanoes of the South Sandwich Islands

You likely don’t get much more remote than the South Sandwich Islands. This chain of volcanic islands sits on the boundary of the Southern and Atlantic Oceans, closer to Antarctica or the southern tip of South America than pretty much any other landmass. They are one of the few island chains in the world that weren’t populated (to our knowledge) before European explorers and merchants found them starting in the mid 17th century. Yet, in our increasingly connected world, even remote volcanoes like those that stretch across the South Sandwich Islands are worth considering.

No British territory has more volcanoes than South Georgia and the South Sandwich Islands, the islands’ official designation. South Georgia Island is to the northwest of the South Sandwich Islands and isn’t volcanically active, but it might be the most famous in the territory. Sir Ernest Shackleton made a truly miraculous traverse of the island in 1916 after the near-tragedy of the Endurance Antarctic Expedition. Luckily for Shackleton, he knew that there were several whaling stations on the island and crawled into Stromness with Tom Crean and Frank Worsley.

Beyond this nearby claim to fame, the South Sandwich Islands only pop up in the history books thanks to their peripheral involvement in the Falklands War. Argentina has claimed the South Sandwich Islands and South Georgia as its territory since the late 1920s. Since Argentina’s ill-fated attempt to take the closer Falkland Islands from the United Kingdom in 1982, the South Sandwich Islands have been mainly home of a small population of fisherman and scientists with their families.

The islands that poke out of the ocean in the South Sandwich Islands have a mishmash of names that reflect the variety of people who discovered them. The northernmost have Russian names like Zavodovska, Visokoi and Leskov Islands. The southern islands are decidedly British, with names like Vindication, Candlemas, Saunders, Bristol, Cook, Thule Montagu and Bellingshausen Islands. (If you're wondering where the North Sandwich Islands might be, there aren't any, but the South Sandwich Islands were named as such to differentiate them from the Sandwich Islands in the Pacific Ocean ... a place now known as Hawai'i. Both were named for various Earls of Sandwich).

The arc-like orientation of the islands is due to the nature of their formation. As part of the South Atlantic Plate slides under the tiny Sandwich Plate, volcanic islands form as magma rises from the mantle in this subduction zone. The South Sandwich Islands are close a clone of the Lesser Antilles arc that includes islands like Martinique and Montserrat in both the length of the arc and the spacing between the islands. Both chains are what volcanologists call oceanic island arc.

The volcanoes in the South Sandwich Islands are like those in the Lesser Antilles as well, producing a mix of explosive eruptions and lava flows. However, unlike their more tropical cousins to the north, keeping track of what is happening in the South Sandwich Islands is much more of a challenge thanks to their extreme location and some of the unique weather that roars through the Straits of Magellan south of the Tierra del Fuego.

At least four volcanoes have erupted in the island chain since 2000. The most recent (and currently active) is Mount Michael on Saunders Island, while Mount Sourabaya on Bristol Island and Mount Curry on Zavodovski both erupted in 2016. To round out the quartet, Mount Belinda on Montagu Island erupted in 2007. However, an additional three volcanoes likely erupted since the islands were discovered: Southern Thule in 1975, the Protector Seamounts (volcanoes that have yet to emerge from the ocean) in 1962 and Candlemas in 1911.

These days, most of our knowledge of what is happening volcanically in the South Sandwich Islands comes from satellite observations. The weather and seas in the South Atlantic is notorious being being, well, terrible. Much of the year, the islands are covered in a cloak of clouds. Even when the weather breaks, the tops of the volcanoes can keep their shroud of fog thanks to their elevation and constant snow and ice fields.

The winds across the islands from the west combined with the clouds tend to produce spectacular von Kármán vortices and wave clouds that can run for hundreds of kilometers. They can sometimes be confused for volcanic plumes, but they are actually caused by the peaks of the volcanoes disrupting the prevailing winds, causing the air and clouds to rotate into the swirls. The occasional volcanic plume can be incorporated into these vortices as well, making identifying volcanic activity that much more difficult.

Luckily, the satellites that are watching the planet can “see” more than just what we can see. Thermal signals from active volcanic vents, lava domes and lava flows can be easily spotted in infrared. Satellites can use other detectors to measure sulfur dioxide in the atmosphere, another tell-tale clue that eruptions (or at least magmatic degassing) is happening.

One of the most spectacular images of eruptions in the South Sandwich Islands was the 2001-07 eruptions on Montagu Island. A long lava flow crept down the northeastern slopes to the sea and along the way, it melted its way through glaciers that coated Mount Belinda. You can see in the image how the lava flow is cracking the glacier (which is flowing ice) as it moves downslope. A lava lake may have been present at the summit during this period as well. Current images of the island still show that lava flow, partially covered in new snow and ice, but still prominent along with the lava delta it produced sticking out into the Atlantic Ocean.

More recently, the plumes from eruptions or degassing at Mount Michael on Sanders have been easy to spot on satellite images. A recent expedition confirmed that Michael is the home of a persistent lava lake as well. That would be an excellent source for a persistent heat signature and gas emissions from a volcano like Michael that has likely been erupting since the late 1990s.

The 1962 eruption of the Protector Seamount is interesting it erupted rhyolite, a composition of magma unlike erupts at volcanoes like Michael and Belinda. Not much is known about this eruption because it was submarine and only noticed with a Royal Navy ship encountered a rhyolitic pumice raft. However, finding rhyolite pumice betray the potential for more explosive activity in the South Sandwich Islands.

Volcanoes like those in the South Sandwich Islands have the capacity to produce large explosive eruptions. Just looking at the geography of Montagu Island betrays its violent past, with the outline of a caldera showing on its icy surface. However, our knowledge of the eruptive history of most of these volcanoes is sparse at best. Much of the material might be at the bottom of the South Atlantic Ocean or buried under the ice on the islands. Without that record of past activity, it is hard to say how often large explosive eruptions might have happened in the island chain.

The South Sandwich Islands are a great example of volcanoes that may have a long and lively history of volcanoes that no one on Earth knew about until the European Age of Exploration began. Even with these scattered visits over the last few centuries, it took the advent to reliable, high resolution satellite imaging of the Earth to really get a sense of what happens at these remote island volcanoes. The next step would be uncovering their past activity to understand just how potentially hazardous these volcanoes might be.