Far out in the Atlantic, Bermuda rises from the ocean as if it is defying the rules of geology. Its volcanoes went silent more than 30 million years ago, yet the island still perches atop a broad swell of seafloor, elevated by about 500 meters above its surroundings. For decades, scientists puzzled over why this uplift persisted without the heat of an active mantle plume. Now, seismic imaging has revealed the answer: a colossal, low‑density rock layer deep beneath the island, unlike anything seen elsewhere on Earth.
The finding emerged from an ingenious use of global earthquake data. A single seismic station on Bermuda has been quietly recording the vibrations from hundreds of large quakes around the world. As these seismic waves passed through Earth’s interior, they changed speed and direction at boundaries between different rock types. By stacking and filtering the faint echoes from 396 earthquakes, researchers built a vertical profile of the subsurface down to about 50 kilometres. What they found startled them beneath the normal oceanic crust lay an unprecedented 20‑kilometer‑thick layer</b of rock, sandwiched within the tectonic plate between crust and upper mantle. In most ocean islands, the crust rests directly on rigid upper mantle. Here, however, the seismic signals revealed an “underplated” layer solidified magma injected during Bermuda’s last volcanic eruptions 30–35 million years ago. This magma never reached the surface; instead, it pooled beneath the crust, cooling into a massive sheet of rock slightly less dense than the mantle around it. That density difference, on the order of 1.5%, is enough to buoy the seafloor upward by roughly the same height as the Bermuda swell. “Typically, you have the bottom of the oceanic crust and then it would be expected to be the mantle,” said William Frazer of Carnegie Science. “But in Bermuda, there is this other layer that is emplaced beneath the crust, within the tectonic plate that Bermuda sits on.”500 meters
The implications reach beyond Bermuda. Similar low‑density underplating has been proposed for other volcanic islands, but nowhere else has such extreme thickness been measured. In French Polynesia, for example, underplated layers are thinner and less buoyant. The Bermuda layer’s size and composition suggest a unique mantle history, possibly tied to the breakup of the supercontinent Pangea. Geologist Sarah Mazza notes that Bermuda’s lavas are rich in carbon and low in silica, chemical signatures pointing to recycled deep‑mantle material dating back hundreds of millions of years. This volatile‑rich mantle may have been emplaced when the Atlantic Ocean opened, creating a patch unlike anything beneath older oceans.
This is in line with broader research into the possible mechanisms of intraplate volcanism, especially in regions that are far from plate boundaries. The finding also points at possibilities other than hot plumes as triggering causes of volcanism. Ancient subduction can be an agent in mantle heterogeneities, and these heterogeneities can cause intraplate volcanism. The studies of eastern Australia and Zealandia indicate that the mantle transition zone could store oceanic crust, which may provide buoyant, volatile-rich domains. Such domains can be mobilized by tectonic processes and thus result in unusual patterns of volcanism and uplift without making age-progressive chains. The thick underplate of Bermuda may be the frozen remnant of such a process, its buoyancy maintained for tens of millions of years.
Technically, it required new seismic tomography methods to image such a structure. From P‑ and S‑wave travel times, researchers can locate velocity variations with depth; lower speeds reflect rock that is of lower density or partially molten. In the Bermuda profile, normal crustal velocities were recorded above, then velocity abruptly decreased in the underplated layer, reflecting its buoyant composition. Gravity measurements confirmed the model: the island has a negative gravity anomaly-the one that would result if low‑density rock were buried beneath it.
This raft‑like layer also provides an explanation for why the heat flow in Bermuda is normal and not elevated, as it would have been with an active plume. Without ongoing melt supply, a structure geologically cold, its composition alone is enough to keep the island “afloat.” This contradicts the long-held belief that oceanic swells need to be thermally supported. Instead, Bermuda serves as proof that compositional buoyancy can maintain topography long after cessation of volcanism.
Frazer and co‑author Jeffrey Park now hope to look for similar features beneath other islands. If they exist, these submerged life rafts could rewrite theories about how the Earth’s oceanic crust evolves and why some islands refuse to sink back into the sea.
