Who knew that the greatest Bermudian mystery wasn’t to be found in the Triangle, but rather deep within its ocean floor? Seismologists have discovered an unprecedented geological feature – a 20-kilometer thick bed of low-density rock sandwiched between the crust and the mantle underlying Bermuda. Not only is “underplating” an unusual feature found nowhere else below an oceanic island, but it may now explain the long-standing puzzle of the Bermudian Island Group sea swell’s unwillingness to subside despite its most recent volcanic activity a whopping 31 million years ago.
Generally, underneath the oceanic crust is believed to be the mantle, which is made up of dense rock material; this constitutes most parts of the earth’s interior. However, as William Frazer, the study’s lead author at the Carnegie Science Institute, pointed out: “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 abnormally thick layer is two to three times thicker than its equivalents elsewhere, and is found to be less dense than its surrounding mantle material, with a difference in density of 1.5% or roughly 50 kilograms per cubic meter.
This finding came about due to careful observation of seismic waves radiating from remote earthquakes. Frazer and Jeffrey Park of Yale University chose to use a seismic station installed on the island of Bermuda to record where and how the waves changed speed or reflected off interfaces below the Earth’s surface. These patterns often indicate boundaries between rock layers, just like light passing from air into glass. This information has shown that there is a ‘secret’ landscape below where the Moho discontinuity should be, which might be where magma pushed its way towards the surface during the last volcanic cycle on Bermuda and cooled to solidify in place.
Generally, in the majority of volcano ranges, for example, Hawaii and the Galapagos, the oceanic swell exists because of the feeding of mantle plumes, which are hot and buoyant columns of material from deep inside the Earth. As the plates move, the crustal material moves away from the plumes, resulting in the cessation of volcanism, ultimately causing the swell to collapse. Bermuda does not follow this particular theory. There is no plume under Bermuda, neither more mantle heat flow, yet the swell persists, uplifting the seafloor topographically by as much as 500 meters. One theory accounting for this particular anomaly suggests the thickness of the underplate material provides a kind of geological raft with enough buoyancy to push the crust upwards, supporting the swell.
There’s also information to be gleaned from the chemical composition of the volcanic rocks in Bermuda. Geologist Sarah Mazza of Smith College analyzed the lava and discovered that it is low in silica and high in carbon, indicating that the lava originated from the Earth’s mantle, which contains this high carbon. Mazza’s measurement of the zinc isotopes in the sample indicates the presence of this carbon in the Earth’s mantle resulted from the accretion of the supercontinent Pangaea, between 900 and 300 million years ago. This particular reservoir of the Earth’s mantle appears to have some differences from the other oceans’ hotspots because the Atlantic Ocean is younger due to the break-up of Pangaea.
The tectonic environment adds further support for the uniqueness of the Bermuda situation. Pangea’s disruption created an Atlantic Ocean with an extension-related formation of new oceanic crust that is not amenable to subduction recycling. This process could have conserved the carbon-rich mantle layer beneath the Bermuda region and facilitated the creation of an anomalously buoyant underplate during the period of its volcanoism. In the case of intraplate volcanism on other continents, the volumes and densities are more appropriate for the formation of shallow swells that last for only a few million years.
Seismic imaging, the technique that helped determine Bermuda’s hidden shape, is a potent remote sensing technique that can ascertain the Earth’s interior. By analyzing the arrival times and amplitude of seismic waves caused by distant earthquakes, such as those thousands of kilometers away that originate from the earthquake belts of Central Asia, the shape of the density contrasts and the edges of the covered strata have been resolved with a resolution of 50 kilometers. Here, the data showed that the underplating could extend 50 to 100 kilometers from the center of the island, probably with a bowl-like shape.
Frazer and his team are currently conducting research in the surrounding islands to find if there are similar thick, dense layers of rock present, or if Bermuda is the only case in the world. “Understanding a place like Bermuda, which is an extreme location, is important to understand places that are less extreme,” Frazer explained. If this is the case, the underplating of Bermuda could revolutionize the way geologists understand the oceanic swell and the supercontinents.
