No two volcanoes are the same, but they all form in the same handful of ways. All, it seems, except for the ancient volcano forming the foundations of the island of Bermuda.
After examining rocks from deep under the island, scientists discovered that this quiet volcano formed in a way that is, so far, completely unique. The work, reported this week in the journal Nature, not only solves a long-standing mystery about this beautiful isle in the Atlantic, it also describes a whole new way to make a volcano. (Explore the volcanoes that make up the Pacific Ring of Fire.)
To crack the case, the team examined a 2,600-foot-long pillar of rock that is the only core sample taken from Bermuda. Drilled from near an airport back in 1972, the core had been kept in storage at Dalhousie University in Nova Scotia and was gathering dust. While the team suspected something unusual must be going on, a comprehensive geochemical assessment of the rock took them completely by surprise.
“After 50 years of people doing geochemical research on oceanic lavas, no one has found the signature we’ve found in Bermuda,” says study coauthor Esteban Gazel, a geochemist at Cornell University. “Sometimes, by luck, you just find something new and different.”
With a new model for volcano-making in hand, Bermuda may not be alone: Other volcanoes could exist in the Atlantic Ocean that formed by the same or similar processes, says study coauthor Sarah Mazza, a geochemist at the University of Münster. “We just haven’t found them yet,” she says.
And Aurélie Germa, a volcanologist and geochemist at the University of South Florida, says that the team’s model for building Bermuda “will probably help solve some geochemical inconsistencies” at other volcanoes in similar tectonic settings “that nobody could have really explained before.”
Tectonic lead balloons
Until now, the known methods for building volcanoes included plumes rising within the hot and plasticky mantle, two tectonic plates diverging from each other at a mid-ocean ridge, or one plate diving beneath another in a subduction zone. These processes can all create magma patches in Earth’s crust that can then give rise to eruptive peaks at the surface.
The volcanic underbelly of Bermuda has traditionally been explained by a mantle plume, which is how the Hawaiian archipelago formed: a stationary plume created numerous volcanic islands that grew, erupted then died out as the tectonic plate above it kept moving.
Geophysical research does suggest there is an upward-moving warm structure beneath Bermuda. But the evidence for it being a Hawaii-like plume isn’t exactly strong. There is no Hawaiian-like island chain to speak of, and if there was a plume, there should also be volcanism at a spot far southeast of Bermuda where it is predicted to be today, and there isn’t.
That’s where the rock core comes into play, which chronicled the volcanic underworld hiding beneath Bermuda’s pink beaches. For one, it has unusual proportions of different lead compounds, formed by the decay of two types of uranium. This could only be explained if the source within the mantle was geologically young, but the mantle under Bermuda should be extremely old.
The Atlantic Ocean, though, is a special place: It only exists because of the breakup of the supercontinent Pangaea hundreds of millions of years ago, Mazza says. Either during the formation or destruction of this supercontinent, slithers of tectonic plates were shoved down into the mantle below what would one day be the Atlantic Ocean. If tapped into, these doomed slabs could be the source of young material the team was looking for.
The madcap mantle
These slabs were likely stored in the mantle’s so-called transition zone, a physically unusual region between 250 and 400 miles underground. Diamonds that sneak up from the mantle’s transition zone reveal that it contains multiple oceans’ worth of water, as well as plenty of compounds like carbon dioxide.
Sure enough, the core sample contained signs of these tell-tale geological fingerprints. The water content of minerals like pyroxene, known to be incredibly dry, were oddly wet in Bermuda’s core samples. Silica, a major chemical compound in volcanic rocks, also wasn’t as common as it should be, indicating that it was pushed out by a high proportion of carbon dioxide.
With the source of Bermuda’s building blocks in mind, the team then used computer models to work out what happened next.
Around 30 million years from the present, a disturbance in the area caused the mantle at the transition’s deeper boundary to flow upward. As it did so, it borrowed some material from these stagnating slabs and took the mixture along for the ride.
As the environment around the mixture drastically changed, melting took place, creating a molten soup. That soup made its way straight up to the crust, where it created a strange type of magma that erupted onto the seafloor and built Bermuda.
Back to the future
Volcanologists have long speculated on the source of the materials used to build volcanoes in the middle of tectonic plates. This study finds compelling evidence for a reservoir of these materials in the middle of the mantle for the first time, and in doing so, it describes a new way to make a volcano.
The study is a “real geo-detective success,” says Janine Krippner, a volcanologist at the Smithsonian Institution’s Global Volcanism Program who praises the team’s multi-pronged approach.
Val Finlayson, an isotope geochemist at the University of Maryland, says that the research is a “pretty thorough exploration of the origins of Bermuda.” It was possible a regular plume could explain the situation, since deep-rooted mantle plumes can eat up recycled tectonic material as they ascend, potentially creating some of the geochemical signatures seen by the team. However, the team’s “comprehensive, multidimensional approach” makes their new story all the more convincing, she says.
And while the team is confident in their theory, they also aren’t unequivocal: “At the end of the day, this is an interpretation” of what made Bermuda, Gazel says, and additional work is required to corroborate it.
Rebecca Williams, a volcanologist at the University of Hull, adds that this remarkable story only came about thanks to a sample collected in the 1970s, when the scientific knowledge needed to reveal its secrets simply didn’t exist.
“Who knows what future discoveries may lie in our rock collections,” she says.