Step aside, the Bermuda Triangle: The North Atlantic’s newest mystery lies beneath this enigmatic archipelago. Scientists have discovered a strange rock formation 20 kilometers thick beneath the oceanic crust of Bermuda. This level of thickness has never been seen in other similar formations around the world.
“Normally, you would expect the bottom of the ocean to be oceanic crust, and then the mantle,” said study lead author William Fraser, a seismologist at Carnegie Science in Washington, D.C. “But in Bermuda, there is another layer that lies below the crust within the tectonic plate that Bermuda sits on.”
The origin of this layer is not entirely clear, but it may explain an ongoing mystery about Bermuda, Fraser told Live Science. The island rests on an ocean swell, and the oceanic crust is higher than the surrounding area. But there is no evidence of ongoing volcanic activity causing the swells, and the last known volcanic eruption on the island was 31 million years ago.
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Bermuda has long had a mysterious reputation, largely due to its Bermuda Triangle (an archipelago between Florida and Puerto Rico), where an unusual number of ships and aircraft have gone missing. (However, this reputation is greatly exaggerated.) But the real mystery is why the Bermuda Sea swell exists.
Archipelagoes such as Hawaii are thought to exist thanks to hotspots in the mantle. Hotspots are places in the mantle where hot material rises and causes volcanic activity. Where hotspots meet the Earth’s crust, the ocean floor often rises. But when tectonic movements cause the Earth’s crust to slide away from its hot spot, ocean swells typically subside.
Fraser said Bermuda has not had volcanic activity for 31 million years, but the swell has not subsided. Although there is debate about what is happening in the mantle beneath the island, no eruptions have occurred at the surface.
Fraser and study co-author Geoffrey Park, a professor of earth and planetary science at Yale University, used records from a seismic observatory on the island of distant large earthquakes around the world to image the Earth up to about 50 kilometers (about 31 miles) below Bermuda’s surface. They looked at where the seismic waves from these earthquakes suddenly changed. This revealed an unusually thick layer of rock that was less dense than other surrounding rocks.
Their findings were published in the journal Geophysical Research Letters on November 28th.
Geologist Sarah Mazza of Smith College in Massachusetts, who was not involved in the study, told Live Science that “there is still material left over from a period of active volcanism under the sea floor of Bermuda, which potentially helps maintain this high-relief region of the Atlantic Ocean.”
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Mazza’s own research into Bermuda’s volcanic history found that the lava types there were low in silica minerals, indicating that they were made from carbon-rich rocks. Mazza’s study of variations in zinc molecules in samples from the Bermuda Islands, published in September in the journal Geology, found that this carbon comes from deep in the mantle. Mazza said it was likely first pushed into the supercontinent Pangea when it formed between 900 million and 300 million years ago. This is different from what is seen on hotspot islands in the Pacific and Indian Oceans, she added. This difference is thought to be because the Atlantic Ocean, which opened when Pangea split, is a younger ocean than the Pacific and Indian Oceans that were at the edge of Pangea.
“I think the fact that we’re in an area that was previously the center of the last supercontinent is part of the explanation of why this continent is unique,” she said.
Fraser is currently investigating other islands around the world to see if geological formations similar to those found in Bermuda exist, or if the archipelago is truly unique.
“Understanding extreme places like Bermuda is important in understanding less extreme places, and can help us understand what are the more normal processes that occur on Earth and what are the more extreme processes,” Fraser said.
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