Bermuda's geological mystery solved
Bermuda has not only a mythologised "triangle mystery", but also a real scientific mystery: why the archipelago is visibly elevated above the surrounding ocean floor, as if "floating" on an elevated platform.
A new study published in Geophysical Research Letters offers a possible explanation: a massive layer of rock about 20 kilometres thick has been discovered beneath Bermuda, located between the oceanic crust and the mantle.
Bermuda is considered a geological anomaly and is markedly different from most volcanic islands. Like them, it sits on a major seafloor uplift, the so-called bathymetric swell. According to the classical model, such swells are fuelled by heat from a rising column of hot mantle matter - a mantle plume - that "inflates" the lithospheric plate.
However, in the case of Bermuda, current evidence does not indicate an active hot plume beneath the island. Moreover, volcanic activity here ceased more than 30 million years ago, and the uplift itself was expected to have "settled" long ago. The question remained: what keeps this section of the seafloor elevated?
How they looked under the island
To figure it out, the researchers used data from a seismic station permanently operating in Bermuda. They analysed records of distant earthquakes: as seismic waves pass through the subsurface, they reflect off the boundaries between rock layers and produce faint "echoes". The time of arrival of these reflections can be used to reconstruct the structure of the Earth's crust and upper mantle.
During the analyses, the scientists identified the expected elements of the section: the base of the island, the oceanic crust and the crust-mantle boundary (moho). But beyond that, they discovered an additional hidden layer that was 2-3 times thicker than similar structures on most oceanic islands.
"Subcrustal buildup" instead of a hot plume
The authors interpret the finding as a powerful underplating - "sub-crustal build-up": magma in the past penetrated beneath the oceanic crust and there solidified, forming a dense, but slightly less heavy than the surrounding mantle, "backing" layer.
It is this difference in density, the researchers believe, that creates additional buoyancy and helps hold bathymetric uplift from below - without the need for ongoing warming and active volcanism.
The team estimates that the layer may have formed during an era when Bermuda was volcanically active - about 30 to 35 million years ago. The volcanoes have since died down, but the "backing" has survived and continues to support seafloor uplift.
The authors note that these results provide a new perspective on the nature of oceanic "swells" and show that their stability can be explained not only by heat and plumes, but also by long-lived structural effects from ancient magmatic activity.