Bermuda mystery solved: 12 mile-thick layer of buoyant, solid rock keeps it afloat

For millions of years, the archipelago of Bermuda has floated, an apparent anomaly in the Atlantic. 

Now, seismologists have finally solved the mystery of why this volcanic island, inactive for over 30 million years, hasn’t sunk back into the ocean. 

The answer lies not in hot magma plumes, but in a massive, never-before-seen “rock raft” deep beneath the seabed.

Beneath Bermuda’s oceanic crust lies a rock layer 20 kilometers (12.4 miles) thick.

“We identify features associated with a ∼20 km thick layer of rock below the oceanic crust that has not yet been reported,” the researchers from the Carnegie Institution for Science in the US wrote in the study paper. 

“This thick layer beneath the crust likely was emplaced when Bermuda was volcanically active 30–35 million years ago and could support the bathymetric swell,” it added. 

Interestingly, the study suggests that the Bermuda swell is not supported by a hot mantle plume or a deep thermal anomaly, as is commonly assumed for other large bathymetric swells.

Mystery of the floating island

Volcanic islands, like the famed Hawaiian chain, are typically buoyed by hot, active magma plumes pushing up the Earth’s crust.

But Bermuda’s last major eruption was roughly 30 million years ago. Geologically, it should have subsided and vanished long ago as the underlying lithosphere cooled. 

Yet, the island remains, sitting atop a bathymetric swell — a large, persistent bulge in the ocean floor.

This unique structure likely accounts for Bermuda’s continued elevation, preventing its subsidence long after the volcanic activity ceased tens of millions of years ago.

Using seismic data from a permanent station on the island, researchers analyzed recordings of large global earthquakes. 

By studying how seismic waves suddenly changed speed, they created a detailed image of the Earth’s layers up to 31 miles (50 km) beneath Bermuda.

Surprisingly, researchers found a unique, massive geological layer wedged between the oceanic crust and the rigid upper mantle.

Measuring 12 miles in thickness, this layer is unlike anything previously observed beneath an island located in the middle of a tectonic plate.

Moreover, this immense layer acts like a “buoyant raft” because it is less dense than the surrounding rigid upper mantle, thereby supporting the island’s elevation.

Buoyant structure formation

This layer is believed to be the solidified remnants of magma — or mantle rock —that was injected into the crust during Bermuda’s last volcanic burst.

When this material cooled and solidified, it formed an enormous, buoyant structure that essentially functions as a raft, holding the entire island complex aloft.

Reportedly, this 20 km thick raft raised the ocean floor by more than 1,600 feet (500 meters), providing the permanent buoyancy needed to support the archipelago for millions of years after its fiery birth.

“We associate the shallowest three layers with the volcanic edifice and a two-layered oceanic crust. The deepest layer is ∼20 km thick and likely formed during or soon after the volcanism that formed Bermuda,” the researchers concluded.

Bermuda is famous for its association with the Bermuda Triangle, a legendary area between the islands, Florida, and Puerto Rico, known for supposedly high numbers of ships and aircraft that have disappeared. 

The findings were published in the journal Geophysical Research Letters.