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UNDER THE SEA: A map of the seafloor around the South Sandwich Islands has revealed undersea volcanoes. The peaks here are colored red, whereas the ocean depths are blue. Full-size image can be found here. Image: British Antarctic Survey
Iceland is known as the "land of ice and fire," but new findings suggest that the South Sandwich Islands in the southern Atlantic Ocean could easily take over that title. In addition to the 11 volcanic islands that make up this Antarctic archipelago, the British Antarctic Survey (BAS) recently discovered that 12 volcanoes lurk below the water's surface.*
Despite their icy environs, the South Sandwich Islands have fiery origins thanks to the volcanoes, some of which are still active. "Eruptions have been observed over the last century or so," says BAS scientist Philip Leat. In addition to a large eruption observed in 1956, a low-level eruption that started in 2002 lasted for six years.
Between these two visible surface volcanic reactions, an underwater volcano blew in 1962. Although nobody directly observed that activity, scientists discovered the eruption when large amounts of pumice, a volcanic rock so filled with gas bubbles that it floats on water, washed up on the shores of Antarctica, New Zealand and South America.
Because of the 1962 eruption, "we knew there were underwater volcanoes somewhere in the vicinity," Leat says. And when the BAS sent the RRS James Clark Ross on a seafloor survey around the South Sandwiches, using multi-beam sonar to map the area, the researchers found them. Sonar bounces sound waves off of objects and detects the echoes to determine the distance to those objects; using multiple sonar beams maps larger areas, so the James Clark Ross could cover a greater expanse of the ocean floor more quickly.
The research team announced at a July 13 poster session of the 11th International Symposium on Antarctic Earth Sciences that its survey discovered the presence of 12 active submarine volcanoes (some almost three kilometers high) and the remnants of more. The collapsed volcanoes had formed craters about five kilometers in width.
In addition to enhancing maps of the islands, the BAS survey may help scientists understand how undersea eruptions cause enormous reactions, such as tsunamis. The underwater volcanoes also create environments that are not only conducive to unique ecosystems, but also hold clues about an earlier era in Earth's history.
As a result of the volcanic activity around the South Sandwiches, molten rock lurks just under the seafloor. When ocean water leaks through cracks in the floor, it encounters that heat source, reacts, and spews back out as a mineral-rich, hot-water jet, creating a hydrothermal vent. Such vents, which are found in areas of the ocean where there is tectonic activity, create isolated hot-water ecosystems that are home to creatures very different from the more typical denizens of Antarctica's frigid waters.
"Above water it looks like a desert," says Andrew Clarke, a biologist formerly of BAS. "But below, it's far from it. There's a rich variety of life down there."
Although many of the species in Antarctic waters have adapted to live at near freezing temperatures, the denizens of hydrothermal vents have evolved to live comfortably in the heat. Water issues from a vent at temperatures that can exceed boiling. Moving away from a vent is a journey from one temperature extreme to the other, from boiling seas to ice water (think: a hot tub turned too high to a warm summer surf to just bearable New England waters in August to a winter swim at the same beach)—all within a range of several meters.
"Out of contact with light," Clarke says, "the whole system is driven by chemistry." The hot water emerging from the hydrothermal vents is rich in dissolved sulfur, which bacteria living around the vents oxidize to make energy, living off of chemical instead of solar energy. Meanwhile, larger predators such as crabs and shrimps feed off the bacteria.
"It's an ecosystem that builds up around the vents themselves," Leat says. The ecosystems of hydrothermal vents have been studied in oceans all over the world, from Samoa and Tonga to the Mid-Atlantic Ridge. Although there is a general set of organisms that tend to live around vents, each one supports a unique system. As Clarke says, "One of the interesting things is the animals growing around these things are quite different."
Although the map of the area has been released, BAS biologists are still studying the organisms living in the hydrothermal vents and will publish their results separately.
In addition to sulfur, the hot water also carries other minerals, such as the metals copper, lead, zinc and gold. When they drift to cooler water, they solidify and form deposits. Historically, many metal-bearing ores on land originated in a very similar environment to that of these hydrothermal vents. "Going back though history, there were huge numbers of these kinds of volcanoes," Leat says. Studying them can help us understand the process through which metals now inland gradually moved from the ocean to continental interiors.