Far beneath the tranquil turquoise waters of Bermuda lies a geological anomaly unparalleled in the world: a colossal, 12.4‑mile‑thick (20‑kilometer) layer of rock wedged between the oceanic crust and the mantle. The thickness of this layer is a record in a similar tectonic setting; its unusual composition might be the key to one of the Atlantic’s most long-standing mysteries: why the ocean floor of Bermuda remains uplifted after tens of millions of years without volcanic activity.
Lead author William Frazer, a seismologist at Carnegie Science, described the surprise saying, “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.” This “mantle raft” is less dense than surrounding rock, and its buoyancy may be keeping the island perched on an oceanic swell 1,640 feet (500 meters) higher than nearby seafloor.
Until such data are in, the prevailing hypothesis is that Bermuda’s last volcanic eruption, 31 million years ago, injected mantle-derived material into the crust, where, instead of dispersing, it solidified in place, forming a rigid buoyant block. This is a considerably different scenario than the classic hotspot model represented by, for example, Hawaii, where swells fade as tectonic plates drift away from active plumes. The Bermuda swell, on the other hand, has endured tens of millions of years with no renewed volcanism.
Seismic tomography was central to this discovery. Frazer and co-author Jeffrey Park of Yale University analyzed seismic waves from distant earthquakes recorded at a Bermuda station, tracking abrupt changes in wave velocity to image structures down to 31 miles (50 kilometers) deep. Methods such as these build upon advances in mantle imaging, where velocity anomalies and density contrasts reveal hidden heterogeneities. In the case of Bermuda, the low density of this thick layer contrasted vividly with the mantle’s background.
Geochemical clues add another layer to the story. Sarah Mazza of Smith College found that Bermuda’s lavas are unusually low in silica but rich in carbon, indicating a source deep in the mantle. Isotopic and elemental signatures point to material stored since the era of the supercontinent Pangea, between 900 and 300 million years ago. This carbon-rich mantle reservoir contrasts with the sources feeding Pacific and Indian Ocean hotspot islands and likely reflects the Atlantic’s youth and its peculiar tectonic history.
Recent research has extended the possible sources of such anomalies. Work by Cornell geoscientist Esteban Gazel proved that volcanic material can rise not only from deep mantle plumes but also from the mantle’s transition zone-a layer 250 to 400 miles down, saturated with water and carbon dioxide. In Bermuda’s case, a disturbance in this zone may have mobilized ancient subducted slabs, mixing them into ascending magma. This process could explain the island’s extreme isotopic compositions, unseen in over 50 years of oceanic lava studies.
From the point of view of materials science, subducted fragments of oceanic crust with mineral phases like bridgmanite, calcium perovskite, and high-pressure silica polymorphs may exist in the mantle raft beneath Bermuda. Some of these minerals have diagnostic seismic velocity signatures, with the velocities being as much as 7% slower than ambient mantle at some depths, thanks to phase transitions such as stishovite into CaCl₂-type silica. Such phase transitions may develop strong seismic scatterers, which can favorably assist in the detection of unusual mantle structures.
The persistence of Bermuda’s swell suggests that the injected mantle material has resisted typical subsidence forces acting on oceanic crust. Its buoyancy could be enhanced through compositional differences, for instance, enrichment in carbonates and hydrous minerals from the transition zone, which lower density and alter seismic properties. These factors are in harmony with the global view of mantle heterogeneity, wherein small-scale anomalies, sometimes only a few kilometers thick, can have outsized geodynamic effects.
Frazer’s team is now checking whether comparable thick, buoyant layers exist beneath other islands. If Bermuda proves unique, it would underline the rare convergence of geological events required to create such a structure: the legacy of Pangea’s breakup, deep mantle carbon reservoirs, and an injection of transition-zone material into the crust. As Gazel noted, With this work we can demonstrate that the Earth’s transition zone is an extreme chemical reservoir. We are now just beginning to recognize its importance in terms of global geodynamics and even volcanism.
