Massive ancient structure beneath Bermuda reshapes Earth’s geological map

Bermuda’s position above the Atlantic seafloor has long been considered a geological puzzle. The island sits on a dome that stands roughly 500 m (1 640 feet) higher than the surrounding crust and spans nearly 500 km (310 miles).

Unlike Hawaii or Iceland, Bermuda has no ongoing volcanism, no chain of progressively older volcanic islands, and no evidence of a deep thermal plume beneath it.

The study led by William D. Frazer and Jeffrey Park of Yale University used high-frequency seismic imaging to reveal what holds this elevated structure in place. Their analysis identified four major subsurface layers beneath the island. The deepest is a 20 km (12 miles) thick zone located just below the fossil oceanic Moho, interpreted as an underplated layer formed during Bermuda’s ancient volcanic activity.

This discovery provides the first direct evidence that Bermuda’s uplift can be supported by compositionally light material within the lithosphere itself. The layer acts as a buoyant foundation that maintains the island’s elevation through long-term isostatic balance rather than thermal support from below.

Modeling of Bermuda’s topography shows that this layer is approximately 50 kg/m³ (about 3.1 pounds/m³) lighter than the mantle it replaced. Though the contrast is small, it is sufficient to maintain 400–600 m (1 300–2 000 feet) of elevation for tens of millions of years.

Imaging the hidden structure beneath Bermuda

To image the subsurface, the researchers used a method known as multiple-taper correlation receiver function analysis. This technique measures how seismic waves from distant earthquakes change as they cross boundaries between different rock layers. The data set included 396 high-quality earthquake recordings from the permanent Bermuda borehole seismic station.

The results reveal four distinct layers: a volcanic edifice about 3.3 km (2 miles) thick, oceanic crust roughly 7 km (4 miles) thick, a fossil Moho at a depth of 10.8 km (6.7 miles), and a newly identified underplated layer extending to 32 km (20 miles).

Within this deep layer, seismic waves travel at about 7.3 km/s (4.5 miles/s), suggesting a mafic composition that is denser than crustal rock but slightly lighter than the surrounding mantle. This difference produces positive buoyancy that supports Bermuda’s broad swell.

The structure has remained stable since volcanic activity ceased about 30–35 million years ago. It now behaves as a solid, buoyant mass integrated into the island’s lithosphere, maintaining its elevation through static balance rather than dynamic convection.

Rethinking the source of ocean island swells

Most oceanic swells are explained by mantle plume theory. In that model, a hot column of material rises from near the core–mantle boundary, melts the overlying lithosphere, and creates a volcanic chain. The process also forms a broad topographic swell as the plume’s heat expands the crust from below.

Bermuda defies this explanation. The last volcanic activity ended more than 30 million years ago, yet the island’s elevation has not decreased. Global seismic tomography shows no plume beneath the area, and heat-flow data confirm that the region’s mantle temperature is normal.

The new findings suggest that Bermuda’s buoyancy is the result of lithospheric alteration rather than ongoing mantle flow. During the island’s formation, magmas may have pooled at the base of the crust instead of erupting, cooling to form thick intrusions that lightened the local lithosphere.

This mechanism could apply to other intraplate islands as well. If large volumes of buoyant rock accumulate at the crust–mantle boundary, they can produce lasting elevation even after volcanism stops. Such processes would allow the lithosphere itself to store the memory of ancient upwelling events.

An exceptional underplated layer in the global context

Underplating has been identified beneath many intraplate volcanoes, but Bermuda’s layer is unusually thick. At Hawaii, the underplated zone reaches 5–10 km (3–6 miles). At the Marquesas and the Azores, it measures around 10 km (6 miles). Bermuda’s 20 km (12-mile) layer is roughly twice as thick as any of these examples.

Despite its thickness, the density contrast is modest. In the Marquesas, underplated material can be 200 to 300 kilograms per cubic meter (12 to 19 pounds per cubic foot) lighter than the mantle. Bermuda’s contrast, only 50 kilograms per cubic meter (3.1 pounds per cubic foot), is small but compensated for by the large volume of buoyant material.

Residual gravity and topography data from the Bermuda Rise reinforce this interpretation. The island shows higher topography but lower gravity, a combination that signals the presence of low-density rock below the crust. The underplated layer likely extends laterally up to 100 km (62 miles) from the island’s center, tapering outward into the surrounding seafloor.

The layer’s composition remains uncertain, but possible explanations include large mafic plutons that never erupted, mantle material depleted of dense minerals, or rock chemically modified by volatile-rich fluids. Each would yield a slightly less dense composition capable of long-term buoyancy.

Implications for the evolution of oceanic lithosphere

The Bermuda study reveals that the lithosphere can retain internal buoyant structures long after volcanic activity ceases. Rather than being a passive shell moving over active plumes, the oceanic lithosphere can be reshaped by processes that alter its composition and density from within.

This insight helps explain why Bermuda maintains normal heat flow and stable elevation while lacking a deep mantle plume. The island’s underplate provides structural, not thermal, support. The finding unites several previously puzzling observations into a single, self-consistent model.

If similar processes occur elsewhere, many oceanic swells previously attributed to hot upwellings could instead result from compositional changes within the lithosphere. Such a shift in understanding would redefine how scientists interpret the formation and persistence of intraplate volcanoes across Earth’s oceans.

Frazer and Park’s work demonstrates that even when surface volcanism fades, the deep record of those events can persist for millions of years, locked into the lithosphere as a hidden source of buoyancy.

References:

¹ Thick Underplating and Buoyancy of the Bermuda Swell – William D. Frazer et al. – Geophysical Research Letters – November 28, 2025 – https://doi.org/10.1029/2025GL118279 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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