Why has the seafloor around Bermuda been mysteriously high for tens of millions of years, long after its volcanoes went silent? New seismic imaging has revealed a colossal, 12.4-mile-thick (20 km) low-density rock layer beneath the island’s oceanic crust, an anomaly unlike anything else documented on Earth. This buried structure, wedged between the crust and the mantle, may be the key to understanding Bermuda’s enduring oceanic swell.
Lead author William Frazer of Carnegie Science describes the surprise: Typically, you have the bottom of the oceanic crust and then it would be expected to be the mantle. But in Bermuda, there is this other layer that is emplaced beneath the crust, within the tectonic plate that Bermuda sits on. Using recordings from distant earthquakes captured at a seismic station on Bermuda, Frazer and Yale’s Jeffrey Park applied advanced tomography to image the subsurface down to 31 miles (50 km). The data revealed a thick, buoyant layer whose seismic velocities are lower than surrounding rock, indicating reduced density.
This layer seems to have its origin linked with the last volcanic episode in Bermuda, which was 31 million years ago. During this eruption cycle, mantle material may have been injected upward into the crust, cooling into a rigid “raft” that continues to prop the seafloor about 1,640 feet (500 m) higher than surrounding Atlantic crust. Unlike in Hawaii, where swells fade as tectonic plates drift away from active hotspots, the uplift of Bermuda has been sustained without renewed volcanism-a process that defies conventional models for the evolution of oceanic islands.
Geochemical fingerprints reinforce this interpretation of a distinctive mantle source. Research on Bermuda’s lavas by geologist Sarah Mazza illustrates that they have low silica contents and are enriched in carbon, consistent with an origin from the deep mantle. Isotopic measurements, including zinc isotope variations, suggest that this carbon-rich material was emplaced hundreds of millions of years ago during the assembly of the supercontinent Pangaea. When Pangaea subsequently rifted apart, the Atlantic Ocean opened over that ancient mantle domain, making Bermuda different from islands in older oceans like the Pacific.
The Bermuda swell has remained high, consistent with predictions from models of dynamic uplift, rather than from lithospheric reheating. In these models, buoyant mantle anomalies beneath the lithosphere support elevation until plate motion removes the crust. However, there is little indication that Bermuda rises are subsiding, which suggests that the buoyant layer is unmoving and traveling with the plate. This situation is again similar to the predictions for anomalous asthenosphere that reaches up through the thermal boundary layer. It contrasts with hotspot chains that typically display age-progressive subsidence, reflecting Bermuda’s tectonic uniqueness.
Here, seismic tomography again plays a vital role. Three-dimensional imaging over recent decades has unraveled the mantle’s complexity, ranging from huge low-velocity superplumes to intricate boundary layers at the core-mantle interface. Techniques like Reverse Time Migration Full Waveform Inversion can resolve sharp contrasts in seismic properties, which capture smooth background variation and discontinuous jumps.
For Bermuda, the sensitivity of the method to changes in wave speed enabled detection of the unusual density and composition of the buried layer, affording a glimpu to global tectonic reorganizations: geological modeling indicates that uplift coincides with the collapse of stagnant slabs at the 660 km mantle discontinuity during the closure of the Tethys Ocean. Such slab avalanches require compensating upwellings, potentially feeding regions like Bermuda with deep compositionally distinct mantle material. This interplay between slab descent and localized upwelling is a hallmark of the Wilson Cycle, in which supercontinents assemble and break apart along preconditioned lithospheric zones.
The find raises more general questions: might other islands hide similar buried layers, remnants of ancient mantle injections? Frazer is now taking the search worldwide, using the same seismic methods to scan beneath other oceanic swells. If Bermuda is not unique, these structures may form a previously unknown class of mantle-crust interactions, rewriting the history of volcanic island creation, survival, and extinction. For now, Bermuda’s mystery lies not in the myths of the Bermuda Triangle but in the deep-time record locked within its foundation: a silent geological raft, born of ancient volcanism and supercontinental tectonics, still holding the island high above the Atlantic floor.
