- Some of the peaks rise 10,000 feet above the ocean floor.
- Volcanic islands above the sea surface were found by Captain Cook.
- Giant landslides from the sides of the volcanoes could trigger tsunamis.
In the first ever-survey of its kind, a chain of massive volcanoes that rise up to 1.86 miles were discovered lurking beneath Antarctic waters near the South Sandwich Islands in the remote Atlantic Ocean.
Captain Cook discovered the rugged islands in 1775 and wrote of the number of active volcanic islands peeking and erupting above the sea. "But the seafloor was completely unknown. It's never been surveyed at all," said Phil Leats of the British Antarctic Survey who led the new efforts.
"We mapped in great detail the area of the seafloor around these islands," he said.
The expedition's shipboard sonar scans reveal twelve new undersea volcanoes, some rising nearly two miles from the seafloor. Several of them have clearly been active in recent times. Others have collapsed, forming huge craters some three miles across.
"These are massive mountains on the seafloor," Leats said.
Given the presence of known volcanoes nearby, "It's not surprising, but it's very interesting," said Ian Dalziel of the University of Texas at Austin.
The volcanoes are a result of the South American continental plate sliding under the South Sandwich plate to the east, carrying water down into the deep interior of the earth. The water escapes upward, ultimately leading to eruptions of molten rock.
Strings of these volcanoes tend to occur in a line parallel to the plate boundary. They represent new continental crust that will grow, with more emerging from the sea over time, and one day smash into another continent.
"We have GPS data to show that the South Sandwich Islands are moving east very fast indeed with respect to Africa," Dalziel said. "It's a very active system."
The findings are exciting for geologists who hope to learn more about underwater eruptions and how new continental crust forms, but they are also a trove of discovery for biologists, because the vents of hot water spewing from the ocean floor create a novel environment for life.
"We know there's a whole different ecosystem of new life forms that exist around those hot areas," Leat said.
The findings also raise awareness of a hazard to humans. Such volcanoes often have unstable slopes, and if the entire side becomes unstable it can slip down in a sudden rush, generating a tsunami.
"This is quite well known," Leat said "Clearly this has happened in this area. We can see the scars. We can see the quite large slump deposits, which must have caused quite large tsunamis, so clearly it's an area where this kind of hazard does exist."
As remote as the South Sandwich Islands may be, the 2004 Indian Ocean tsunami showed that tsunamis can have devastating consequences great distances away from their origins. The west coast of Africa would be particularly vulnerable to a tsunami triggered by the newly found volcanoes, Dalziel said.