Researchers have imaged an unprecedented ~20 km (12.4 mile) thick, low‑density rock layer beneath Bermuda’s oceanic crust that may represent frozen mantle material emplaced during volcanism about 31 million years ago. Seismic data from a Bermuda station reveal discontinuities down to ~50 km that define this unusually thick layer, which could buoy the seafloor by ~500 m. The study (Frazer & Park) was published Nov. 28 in Geophysical Research Letters, and independent work ties Bermuda’s unusual lava chemistry to deep, carbon‑rich mantle sources related to ancient Pangea processes.
Scientists Discover Unprecedented 20 km‑Thick Rock Layer Beneath Bermuda — A Frozen Mantle 'Raft' That May Buoy The Seafloor
Scientists aren't sure how or why the giant layer of rock formed, but it may relate to volcanic activity that ceased in the region around 31 million years ago. . | Credit: mtcurado/Getty Images
Scientists have identified an extraordinary, roughly 12.4‑mile (20‑kilometre) thick layer of relatively low‑density rock beneath the oceanic crust under Bermuda — a feature not previously observed in similar tectonic settings anywhere on Earth.
What The Study Found
"Typically, you have the bottom of the oceanic crust and then it would be expected to be the mantle,"said study lead author William Frazer, a seismologist at Carnegie Science in Washington, D.C.
"But in Bermuda, there is this other layer that is emplaced beneath the crust, within the tectonic plate that Bermuda sits on."
The layer extends to about 31 miles (50 km) below the surface and is unusually thick (about 20 km) compared with comparable crustal or upper‑mantle structures elsewhere. The authors suggest this mass could be frozen mantle material emplaced during the island’s last volcanic episode roughly 31 million years ago. That buried material could act like a dense "raft," raising the surrounding seafloor by roughly 1,640 feet (500 metres) and helping to explain Bermuda's persistent oceanic swell.
How The Team Reached This Conclusion
William Frazer and coauthor Jeffrey Park (Yale University) used recordings from a seismic station on Bermuda of distant, large earthquakes to image the subsurface down to about 31 miles (50 km). By mapping abrupt changes (discontinuities) in seismic waves passing beneath the island, they revealed the unusually thick and relatively low‑density layer beneath the crust. Their results were published Nov. 28 in Geophysical Research Letters.
Independent Evidence And Broader Context
Sarah Mazza, a geologist at Smith College who was not involved in the study, noted that Bermuda's lavas are low in silica — a chemical signal consistent with a carbon‑rich mantle source. In a separate September paper in Geology, Mazza analyzed zinc isotopes in Bermuda samples and concluded that the carbon signature likely originates deep in the mantle and may have been emplaced during assembly of the supercontinent Pangea between roughly 900 million and 300 million years ago.
This chemistry differs from many Pacific and Indian Ocean hotspot islands and may relate to the Atlantic’s relative youth as an ocean formed when Pangea broke apart. The combination of unusual chemistry and this newly detected structural layer helps explain why Bermuda’s oceanic swell has not subsided despite millions of years without surface volcanism.
Why It Matters
Understanding this extreme, previously unrecognized subsurface feature helps geoscientists distinguish between more routine Earth processes and rarer, exceptional phenomena. Frazer is now examining seismic data from other islands to determine whether comparable layers exist elsewhere or whether Bermuda is truly unique.
Note: Popular myths about the Bermuda Triangle are unrelated to this scientific discovery; the new finding concerns deep geology beneath the island and not surface accidents or disappearances.
