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Scientists aren’t sure how or why the giant layer of rock formed, but it may be related to volcanic activity that ceased in the region about 31 million years ago. . | Credit: mtcurado/Getty Images
Move aside, Bermuda Triangle: the North Atlantic’s newest mystery lies beneath this enigmatic archipelago. Scientists have discovered a strange layer of rock, 20 kilometers (12.4 miles) thick, beneath the oceanic crust beneath Bermuda. This level of thickness has never been seen in any other similar layer around the world.
“Typically, you have the bottom of the oceanic crust, and then you would expect it to be the mantle,” said the study’s lead author. William Frazerseismologist at Carnegie Science in Washington DC “But in Bermuda, there’s another layer that’s placed under the crust, in the tectonic plate that Bermuda sits on.”
While the origin of this layer isn’t entirely clear, it may explain an ongoing mystery about Bermuda, Frazer told Live Science. The island sits on an oceanic ridge, where the oceanic crust is higher than its surroundings. But there is no evidence that any volcanic activity continues to create this bulge – the island’s last known volcanic eruption was 31 million years ago.
The discovery of the giant new “structure” suggests that the last eruption may have injected mantle rock into the crust, where it froze in place, creating something like a raft that raises the ocean floor by about 1,640 feet (500 meters).
Bermuda has long had a reputation for mystery, in large part due to the Bermuda Triangle, an area between the archipelago, Florida and Puerto Rico, where an unusual number of ships and planes have allegedly disappeared. (This reputation, however, was greatly exaggerated.) The real mystery, however, is why Bermuda’s ocean waves exist.
Island chains like Hawaii are thought to exist because of mantle hotspots, which are places in the mantle where hot material rises, creating volcanic activity. At the point where the hot spot meets the crust, the ocean floor often rises. But when tectonic movement slides the crust away from that hot spot, the ocean swell usually subsides.
Bermuda’s undulation has not diminished, despite 31 million years of volcanic inactivity there, Frazer said. There is some debate over what is happening in the mantle beneath the island, but there are no eruptions occurring at the surface.
Frazer and co-author of the study Jeffrey Parkprofessor of earth and planetary sciences at Yale University, used records from a seismic station in Bermuda of large distant earthquakes around the world to get a picture of the Earth about 50 km below Bermuda. They examined the places where the seismic waves from these earthquakes suddenly changed. This revealed the unusually thick layer of rock, which is less dense than the other rock around it.
Their findings were published Nov. 28 in the journal Geophysical Research Letters.
“There’s still this material that’s left over from the days of active volcanism under Bermuda that helps keep it potentially like this high-relief area in the Atlantic Ocean,” Sarah Mazzaa geologist at Smith College in Massachusetts who was not involved in the work, told Live Science.
Mazza’s own research into the volcanic history of Bermuda found that the types of lavas there are low in the mineral silicon, a sign that they come from carbon-rich rocks. Mazza’s examination of variations in zinc molecules in samples from Bermuda, published in September in the journal Geologydiscovered that this carbon comes from deep in the mantle. It was probably first pushed there when the supercontinent Pangea formed between 900 and 300 million years ago, Mazza said. This is different from what is seen in hotspot islands in the Pacific or Indian oceans, she added. This difference may be because the Atlantic, which opened up when Pangea broke up, is a young ocean compared to the Pacific or Indian oceans, which were at the edges of Pangea.
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“The fact that we’re in an area that was formerly the heart of the last supercontinent is, I think, part of the story of why this is unique,” she said.
Frazer is now examining other islands around the world to see if there are similar layers to the one found beneath Bermuda, or if the archipelago is truly unique.
“Understanding a place like Bermuda, which is an extreme location, is important to understanding places that are less extreme,” Frazer said, “and gives us an idea of what are the more normal processes that happen on Earth and what are the more extreme processes that happen.”