Could an ancient “geological raft” be holding Bermuda afloat? New seismic imaging has revealed a colossal, 12.4‑mile‑thick (20‑kilometer) layer of rock buried beneath the island’s oceanic crust-a feature unlike anything seen elsewhere on Earth. Instead of the expected transition from crust to mantle, researchers found an extra, buoyant layer embedded within the tectonic plate itself, and its presence may explain why Bermuda’s seafloor remains elevated despite 31 million years without volcanic activity.
Led by William Frazer of Carnegie Science and Jeffrey Park of Yale University, the work utilized recordings from a seismic station on Bermuda to capture the vibrations of powerful earthquakes around the globe. Analyzing how seismic waves changed their speeds and directions while passing beneath the island allowed the team to image structures down to about 31 miles, or 50 kilometers, deep. The anomalous layer stood out not only for its thickness but also for having lower density compared to the mantle rock that surrounded it-a property that would help keep the crust above it buoyant.
This find puts a whole new spin on the Bermuda Rise, a broad oceanic swell that elevates the seafloor about 1,640 feet, or 500 meters, above its neighbors. The uplift of hotspot‑driven swells, like Hawaii’s, decays as tectonic plates move away from the heat source. But Bermuda’s swell has endured tens of millions of years. Frazer’s team theorizes that mantle material was injected into the crust in the last big volcanic pulse, cooling into a thick, buoyant “raft” that still supports the region today.
Geochemical evidence supports a mantle source unlike that of typical hotspot islands. Sarah Mazza of Smith College found that Bermuda’s volcanic rocks are unusually low in silica and rich in carbon, pointing to a deep mantle origin. Isotopic signatures in zinc suggest that this carbon was delivered hundreds of millions of years ago, during the assembly of the supercontinent Pangea, when slabs of oceanic crust were pushed deep into the mantle. This material is likely to reside in the mantle’s transition zone-a layer between 250 and 400 miles (400-640 km) deep, rich in water and carbon dioxide, known to influence melting behavior.
Investigations of the transition zone have found it may function as a long‑term chemical reservoir. Investigations on other mantle characteristics, like Large Low Seismic Velocity Provinces, have indicated that ancient and compositionally different regions may exist for hundreds of millions of years, possibly sourcing mantle plumes at their periphery. However, in Bermuda’s case, eruption style and chemistry indicate yet another mechanism: a perturbation in the transition zone forced deep, volatile‑rich material upward, bypassing the classic plume process entirely.
The resultant magma was unusually rich in carbon dioxide and water, reducing both its density and melting point. When it reached the ocean floor approximately 30 million years ago, it constructed the volcanic structure that is now Bermuda. As the pulse of magma grew weaker, the mantle material injected into the crust remained in place, a thick buoyant layer now imaged by seismic waves.
From a geophysical perspective, the discovery emphasizes how important the use of advanced seismic tomography is in determining sub‑crustal anomalies. Techniques similar to those applied along the Mid‑Atlantic Ridge – where Vp/Vs ratios and velocity gradients reveal either magmatic or tectonic crust formation – may point to structures hidden far from active plate boundaries. In the case of Bermuda, the low density and seismic velocity profile of this anomalous layer clearly differentiates it from normal mantle peridotite and is in line with a composition modified by partial melting and volatile enrichment.
Persistence of the Bermuda Rise into the present also suggests unusual mantle dynamics. Whereas many slow‑spreading ridges are underlain by a focused, three‑dimensional upwelling, the processes at work here may involve instead a more uniform, two‑dimensional mantle flow modified by ancient subduction inputs. The stability of uplift suggests that this buoyant layer is mechanically integrated into the plate and moves with it, rather than dissipating as a transient thermal swell.
Frazer’s team is now surveying other ocean islands to see if similar “fossil” magmatic rafts exist elsewhere. If found, they could represent a previously unrecognized class of intraplate volcanic features ones born not from steady mantle plumes, but from rare, deep‑mantle disturbances that leave lasting imprints in the lithosphere.
