Geologists have long thought that mid-ocean ridges are relatively passive participants in plate tectonics. But a new study shows that there might be more activity going on beneath the equatorial Mid-Atlantic Ridge.
The study, published in Nature, suggests that beneath the ridge, upwelling from a thin mantle transition zone (MTZ) might be driving seafloor spreading.
“It was assumed that these gravitational forces, which are pulling down, are contributing to the spreads at the ridges,” said Matthew Agius, lead author of the new study and a researcher at Roma Tre University in Rome. This conventional view explains that gravity pulls subducting plates away from the ridge, a process that is accommodated by passive mantle upwelling at the ridge itself.
In 2015, Agius learned of an experiment led by Catherine Rychert and Nicholas Harmon, associate professors in geophysics at the University of Southampton, and University of Oxford professor Michael Kendall. The original goal, however, wasn’t to figure out the drivers of spread at mid-ocean ridges. Agius, a postdoctoral fellow at the University at Southampton during this experiment, and these colleagues intended to use ocean bottom seismometers to take some of the first seismic recordings at the Mid-Atlantic Ridge and learn about lithosphere formation beneath it.
In 2016, their research cruise set out for Cape Verde and traveled from there to deploy 39 seismometer stations around the Mid-Atlantic Ridge across an area 1,000 kilometers wide. A year later, the team came back to collect the instruments and look at their data.
At the outset, the team hoped to find clues about the origins of the lithosphere. “But the quality of the data was so rich—very high quality seismic data—that it gave us the ability to zoom in deeper,” said Agius. Using P-to-S receiver functions on the seismic data beneath the stations, the team could image the MTZ, the boundary between the lower and the upper mantle, between 410 and 660 kilometers deep.
“You can only do those measurements where you have stations, so the oceans are largely unsampled,” said Christine Houser, an assistant professor and geophysics researcher at the Tokyo Institute of Technology’s Earth-Life Science Institute not involved with this study.
When they zoomed in, the researchers saw that the MTZ in the western part of their study area was thinner than expected—the 410-kilometer discontinuity was depressed, and the 660-kilometer discontinuity was uplifted. They also noticed that beneath the ridge, shear waves were slower than underneath older Atlantic seafloor, implying a hotter MTZ. These characteristics are typically found at hot spots, not ridges.
“For the first time, we have evidence of higher temperatures in the mantle transition zone [at the Mid-Atlantic Ridge],” said Agius. From that, the researchers inferred that material in the lower mantle is rising to the upper mantle. Instead of gravity, upwelling could be driving seafloor spreading.
This experiment is the first time scientists have obtained seismic data directly from the ridge, as opposed to data from land stations, which provide a hazier view of Earth’s inner mechanics at the ridge. “It introduces new evidence for the whole study of plate tectonics,” said Agius.
“This finding in itself, that there could be regions in our mantle where there’s vertical material transport that are not…[sites of] active upwelling and downwelling like slabs and plumes, is intriguing,” said Elvira Mulyukova, an associate research scientist who studies geodynamics at Yale University who was not involved in the research.
Houser, like the Southampton team, uses seismic data to map Earth’s mantle. She said the data from this new study align with her own models so far.
But Mulyukova wants stronger evidence and measurements of more geophysical properties at the ridge. The authors interpreted their observations as evidence of vertical material transfer in the mantle, but there are other possibilities. Agius and his colleagues agree that studying other properties in this area would give a more holistic view.
If proven to be true, this team’s findings could change the understanding of major aspects of Earth’s history. “This would have an implication for the thermal history of the planet, the geochemical history of the planet [and] the geodynamo.”
—Jackie Rocheleau (@JackieRocheleau), Science Writer