A sample of mantle rock viewed under a microscope.Credit: Johan Lissenberg
A record-breaking expedition to drill into rocks at the bottom of the Atlantic Ocean has given scientists their best glimpse yet of what the Earth might look like underneath its crust.
Researchers extracted an almost uninterrupted 1,268-metre long sample of green-marble-like rock from a region where Earth’s mantle — the thick, interior layer that makes up more than 80% of the planet’s bulk — has pushed up through the sea floor (see ‘Deep-sea drilling’). The samples, described on 8 August in Science1, offer unprecedented insights into processes that lead to the crust’s formation.
Ocean-drilling ship that revolutionized Earth science due to retire
“We had that story in our head” about what this kind of rock should look like, but it’s completely different when “you see it there on a table”, says Natsue Abe, a petrologist at the Japan Agency for Marine-Earth Science and Technology in Yokohama.
The expedition’s achievements are a “fantastic landmark”, says Rosalind Coggon, a marine geologist at the University of Southampton, UK. “Ocean drilling provides the only access to samples of Earth’s deep interior that are key to understanding our planet’s formation and evolution.”
Geoscientists worry that it will be a long time until they can follow up with more studies, because the decade-long International Ocean Discovery Program (IODP) is coming to an end, and the United States is retiring its workhorse research ship, JOIDES Resolution.
Researchers examine rock cores retrieved during an ocean-drilling expedition.Credit: Erick Bravo, IODP JRSO (CC BY 4.0)
Oceanic crust — the type of crust found mainly underneath Earth’s seas, rather than its continents — is mostly made up of dense, volcanic rock called basalt. It is much thinner and younger than continental crust, because the rocks are recycled continually by the movements of tectonic plates.
Basalt forms when magma pushes up through undersea cracks along formations called mid-oceanic ridges. The magma itself originates from a process called partial melting in the mantle — which is largely made up of translucent-green, magnesium-rich minerals. As material in the mantle rises, the pressure over it drops, which causes some of these minerals to melt and form microscopic films of magma between rock crystals.
Usually, only magma erupts onto the sea floor. But at some sites, mantle rock also makes it to the surface, where it interacts with sea water in a reaction called serpentinization. This alters the rock’s structure — giving it a marble-like appearance — and releases various substances, including hydrogen.
Easy to drill
In May 2023, JOIDES Resolution was visiting a site where this has happened: an undersea mountain called the Atlantis Massif, located just west of the Atlantic’s mid-ocean ridge. The 143-metre-long ship is equipped with a 62-metre-tall crane for undersea drilling.
The researchers on board chose to drill at Lost City, a site on the southern side of the massif. The region is punctuated with hydrothermal vents where extremophile microorganisms feed on the hydrogen that seeps out.
“We had only planned to drill for 200 metres, because that was the deepest people had ever managed to drill in mantle rock,” says Johan Lissenberg, a petrologist at Cardiff University, UK. But the drilling was surprisingly easy and three times faster than usual, returning long, unbroken cylinders of rock called cores. “So, we just decided to keep going,” says Lissenberg. The team stopped only when the expedition was coming to its scheduled end.
The researchers have now published their initial findings. “What we report is literally what you can do on the ship. A team of 30 scientists poring over the cores 24 hours a day for two months, and logging centimetre by centimetre as it’s coming up.”
When the scientists examined the structure of the rock in detail, they observed ‘oblique features’, a telltale signature of the prevailing theory of how magma separates from the mantle to become part of the crust, says Lissenberg. The mantle rock was also interspersed with other types of rock in the cores, suggesting that the mantle–crust boundary is not as sharp as seismographic data normally suggest, says Jessica Warren, a geochemist at the University of Delaware in Newark. Together, these results “are key to how we understand the formation of tectonic plates in the oceans”, she says.
Uncertain future
The trip capped a worthy four-decade career for the JOIDES Resolution, which the US National Science Foundation (NSF) had been renting from a private company. But the NSF has announced that it can no longer afford the US$72 million per year that it costs to run the ship after it fulfilled its IODP obligations, and that the programme would be discontinued. This leaves some scientists, especially those at early career stages, uncertain about the future of the field, says Aled Evans, a marine geologist at the University of Southampton.
One remaining ‘grand challenge’ for geoscientists is to drill through the basaltic layer and across the boundary between crust and mantle — called the Mohorovičić discontinuity or ‘Moho’. This would allow them to access pristine mantle rock that hasn’t reacted with seawater. “We haven’t drilled into the real mantle yet,” says Abe. The unexpectedly smooth drilling at Lost City bodes well for those future attempts, which could be carried out by Japan’s research ship Chikyū, she adds. “Mantle rocks are the most common part of our entire planet,” says Evans. “Sampling them would tell us something fundamental about what our planet is made of.”
