At long last, ocean drillers exhume a bounty of rocks from Earth’s mantle

In 1961, geologists off the Pacific coast of Mexico embarked on a daring journey to a foreign land—the planet's interior. From a ship, they aimed to drill through the thin veneer of Earth's crust and grab a sample of the mantle, the 2900-kilometer-thick layer of dense rock that fuels volcanic eruptions and makes up most of the planet's mass. The drill only got a couple hundred meters below the seabed before the project foundered under spiraling costs. But the quest—one of geology's holy grails—remained.

This month, researchers onboard the JOIDES Resolution, the flagship of the International Ocean Discovery Program (IODP), say they have finally succeeded. Drilling below the seabed in the mid–Atlantic Ocean, they have collected a core of rock more than 1 kilometer long, consisting largely of peridotite, a kind of upper mantle rock. Although it's not clear how pristine and unaltered the samples are, it is certain the cylinders of gray-green rock present an unparalleled new record, says Susan Lang, a biogeochemist at the Woods Hole Oceanographic Institution and a co-lead of the cruise. "These are the types of rock we’ve been hoping to recover for a long time."

Researchers on land are eagerly following the ship's daily scientific logs as it continues to drill, says Jessica Warren, a mantle geochemist at the University of Delaware. "Getting down to this really fresh stuff has been a dream for decades and decades," she says. "We’re finally going to see the Wizard of Oz."

The samples can help answer a host of questions, says Johan Lissenberg, an igneous petrologist from Cardiff University onboard the ship. They can provide direct evidence for how ocean crust differs in composition from the upper mantle and better estimates of elemental abundances in the planet's primary reservoir of rock. The samples of mantle will also help researchers understand how magma melts out of the mantle and rises through the crust to drive volcanism, Lissenberg says. "This could be a whole step forward for understanding magmatism—and the global composition of the bulk Earth."

The 1961 project, called Project Mohole, was the first of a handful of unsuccessful attempts to reach the mantle. It was named after the Mohorovičić discontinuity, or "Moho," a geophysical boundary defined by a sudden spike in the speed of seismic waves where the crust, a mélange of rocks crystallized out of mantle melt and altered by water, gives way to the more homogeneous mantle. The Moho lies some 35 kilometers below thick continental crust. But it is only about 7 kilometers below ocean crust. And it is shallower still at drilling site of the JOIDES Resolution at the Mid-Atlantic Ridge, where the North American and Eurasian tectonic plates are being stretched apart, forcing the mantle upward.

Recovering a long mantle core was not the primary goal of the cruise, which is probing the Atlantis Massif, an underwater mountain, for clues to the origin of life. The massif rocks contain lots of olivine, a mineral that reacts with water in a process called serpentinization. The reactions generate hydrogen, which serves as an energy source for microbial life at the "Lost City," a nearby complex of ocean-bottom mineral chimneys deposited by gushers of superheated water.

It's long been theorized that life could have originated in such settings, which are rich in organic molecules. The cruise aimed to deepen a previously drilled 1.4-kilometer-deep hole, pushing to a depth too hot for life, where organic compounds that might have provided the raw material for the earliest life might lurk. But progress was slow.

So the ship returned to another site near Lost City, where shallow cores drilled in 2015 had found what appeared to be mantle rocks highly altered by seawater. After punching through a horizontal fault near the seabed, "the drilling just went so magically well," says Andrew McCaig, a geologist at the University of Leeds and the cruise's other chief scientist. The only hiccup came when the recovered peridotite rocks contained veins of asbestos, prompting increased safety protocols.

There's still some room for debate about whether the rocks are a true sample of the mantle, says Donna Blackman, a geophysicist at the University of California, Santa Cruz. The seismic speedup at the Moho is thought to reflect the lack of water or calcium and aluminum minerals in mantle rocks. Because the samples still show some influence of seawater, Blackman says she might classify them as deep crust. "But the petrology is interesting and special regardless," she says. And as the team continues drilling into deeper rocks, Lissenberg says, "They’re getting fresher."

Indeed, it appears the team is already sampling mantle rock that has never melted into magma, which then cools and crystallizes into different kinds of crustal rocks, says Vincent Salters, a geochemist at Florida State University. By capturing the rock at this point, he says, researchers should be able to learn how magma melts, flows, and separates—clues to the workings of volcanoes worldwide.

The rocks could also answer other basic questions, such as how much the lavas collected at midocean ridges—which are often taken as a stand-in for the mantle—differ from the mantle itself, says James Day, a geochemist at the Scripps Institution of Oceanography. The abundance of radioactive elements in the rocks could improve estimates of how much heat the mantle produces as a whole, driving the deep convective motions that are the engine of plate tectonics. And their physical strength can inform studies of how earthquakes fracture and propagate in the upper mantle. The cores could also help clarify how well the mantle is mixed, reincorporating ingredients from the continental crust that is drawn back into Earth's interior at deep ocean trenches. "There's so much more to this than understanding a little piece of ocean floor," Day says.

Research on the rocks has already begun in labs onboard the JOIDES Resolution, and eventually the cores will be available at IODP repositories for all. But all the excitement over the rock samples also comes with some bittersweetness: The expedition may be one of the last for the ship. In March, the National Science Foundation (NSF) announced that, because of cost increases and a lack of a deal with its international collaborators, it will end its operating contract for the ship in September 2024.

The ship is in great condition and could continue until 2028, says Anthony Koppers, an associate vice president at Oregon State University and a leader in the IODP community. There's still a slim possibility that the U.S. Congress will fund an extension, he says. But NSF has no plan yet to develop a successor ship. And the other two big contributors to IODP, Europe and Japan, are moving on. This month, they announced the creation of IODP³, a new global drilling program that will make heavy use of Japan's drill ship, the D/V Chikyū, which in the past has operated mostly in waters near Japan.

This was Lang's first cruise on the JOIDES Resolution, and she was astonished at how well outfitted its labs were and how knowledgeable its technical staff is. The success they’re having testifies to their decades of experience probing beneath the ocean floor, she says. "It's so unfortunate that something like this is going to be lost."