The Bermuda Triangle has been the source of tall tales for decades, but deep in the North Atlantic Ocean lies a real mystery that has always left geologists baffled. The archipelago of Bermuda sits atop a portion of oceanic crust that swells out, rising above the surrounding crust. This swell itself is not such an unusual occurrence, since the feature often stems from continued volcanic activity in other parts of the world. The catch is, Bermuda hasn’t been volcanically active for the last 30 million to 35 million years.
A new study published in Geophysical Research Letters may have just cracked the case of this mysterious undersea swell. In the study, researchers inspected seismic data to find that an approximately 12-mile (20-kilometer) thick layer of rock hides below the oceanic crust. This layer may have arisen back when Bermuda experienced its last eruption, but shockingly, the swell beneath the island hasn’t disappeared after all these years.
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The Birth of Volcanic Islands
One way that volcanic islands emerge is when mantle plumes originating from Earth’s core-mantle boundary break through the oceanic crust. Magma that rises from deep within the Earth forms the plumes, columns of molten, buoyant rock that go on to serve as the foundation of seamounts (underwater mountains) and volcanic island chains.
Hawai’i and the Galapagos Islands, for example, are volcanic island chains that were created by hotspot volcanism; in this process, a tectonic plate glides over a stationary mantle “hotspot” as it releases magma from the ocean floor, building island after island like a volcanic assembly line, according to the British Geological Survey.
Material from mantle plumes also causes parts of the oceanic crust to swell, although this feature usually dissipates as a tectonic plate moves the crust.
Bermuda sits atop a swell, but unlike Hawai’i or the Galapagos Islands, it isn’t volcanically active and lacks a mantle plume. This has presented quite a mystery for geologists, who have attempted to theorize how the swell has maintained its shape without active volcanism.
Solving the Mystery of the Swell
In the new study, researchers addressed this mystery by examining seismic data from distant earthquakes recorded at a seismic station on Bermuda. Seeing how the waves from the earthquakes behaved differently beneath the island, they were able to find evidence of a thick layer of rock below the oceanic crust that is less dense than the surrounding mantle material.
This rock layer, the researchers say, may be a remnant of volcanic activity that occurred when the island was created. At approximately 12 miles (20 km) thick, the layer is estimated to be twice as thick as other instances of underplating, which occur in areas where mantle material becomes trapped within the crust or beneath the Moho (the boundary between the crust and the mantle).
Because the layer is less dense than the rest of the mantle, it provides the swell with buoyancy, allowing it to maintain its shape in the absence of heat or volcanic activity. The researchers also say that the layer likely extends out, similar to the underplating beneath the Hawaiian islands.
“In the case of Bermuda, underplating may extend beyond the volcanic edifice and have a bowl-like shape. If underplating extends beyond the edifice, we consider ∼200 km [124 miles] to be an extreme upper limit on the lateral extent of underplating and consider underplating as far as 50 to 100 km [31 to 32 miles] from the island to be more reasonable,” the researchers wrote in the study.
Bermuda’s Rising Sea Level
While Bermuda — which consists of 181 islands — doesn’t have to worry about active volcanoes, its residents have to deal with the mounting consequences of climate change.
According to the National Oceanic and Atmospheric Administration, the relative sea level trend for Bermuda is 2.18 millimeters per year based on monthly mean sea level data from 1932 to 2024. This is equivalent to a change of 0.72 feet in 100 years. While this may not seem like a lot, a small change like this can still pose a threat for Bermuda, where over 20 percent of the land mass sits at elevations less than 6.5 feet, according to the Bermuda Institute of Ocean Sciences and Arizona State University.
Read More: What Is the Scientific Mystery Behind the Bermuda Triangle?
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