Rock at the bottom of the Atlantic Ocean holds many secrets that could help scientists understand our planet and how life came to exist on it. That’s why a group of researchers undertook a difficult undertaking: digging a hole over three-quarters of a mile deep and extracting a record-breaking core of rock from the Earth’s mantle.
Scientists regularly extract core samples—cylindrical samples of material from deep under the Earth’s surface—to examine the composition of different layers. That data can act as a window into the planet’s past, providing information on climatic and environmental shifts, or the formation of Earth itself. Drilling in the deep sea comes with unique challenges, so researchers have often been forced to dredge rocks from the ocean floor. Analyzing the composition of those rocks can reveal valuable information, but these rocks can be altered by the pressure of the sea and by exposure to salt water.
The expedition took place between April and June 2023 in an area of the North Atlantic known as the Atlantis Massif, an underwater mountain that rises 14,000 feet (4,267 meters) from the seafloor. The site was selected because tectonic activity in the area thrusts rocks that are normally deep in the Earth’s mantle far closer to the ocean floor, making them easier to recover. That still required some deep drilling to obtain a 4,160-foot-long (1,268 meters) near-continuous core of peridotite, a type of igneous rock.
This extreme depth is far greater than any previous attempts to drill into oceanic mantle rocks. According to the study, co-authored by C. Johan Lissenberg from Cardiff University, the scientists managed to recover 71% of the drilled material, with nearly complete recovery of long sections of partially serpentinized harzburgite (that is, partially water-altered rock).
As noted in the paper, published in the journal Science, the researchers analyzed the composition of minerals within the rock and found evidence supporting a theory of how rocks born deep in the mantle rise to the surface. In that theory, pressure melts rocks that are then pressed upwards, mixing with magma in the crust before erupting on the ocean floor.
The researchers also found intrusions of a crystalline rock called gabbro, which is formed by the slow cooling of magma. They believe the gabbro plays a major role in regulating the minerals and gasses found in deep sea vents, which some scientists believe are an ideal home for the formation of primitive life. Learning more about the vents could lead to new theories on how life on Earth first began, and how it could theoretically form on other planets.
In the study, the researchers acknowledged much more analysis on what they drilled needs to be done. “The comprehensive rock record obtained during Expedition 399 provides a wealth of opportunities to make fundamental advances on our understanding of the oceanic upper mantle,” they said.
In an accompanying article, Utrecht University professor Eric Hellebrand said the “depth far exceeds those recorded in previous drilling efforts and creates opportunities to discern structural and mineralogical features of the mantle and how it interacts with the hydro- and biospheres.”
He also expressed hope that the drilling expedition could raise the bar for the study of how the Earth was formed.
“Decades of ocean floor sampling by dredging have painted a rough mineralogical picture of mantle,” he wrote. “Yet, each new drilling mission reveals surprising views of mantle and formation of the oceanic crust. More ambitious drilling projects will reveal important pieces to understand the biogeochemical effects of oceanic mantle.”
