Geologists have long believed that mid-ocean ridges are relatively passive participants in plate tectonics. But a new study shows there may be more activity below the Mid-Atlantic Equatorial Ridge.
The study, published in Nature, suggests that below the ridge, the upwelling of a Thin Mantle Transition Zone (MTZ) could cause the seabed to sprawl.
“It was assumed that these gravitational forces, which pull downwards, contribute to the deviations at the peaks,” said Matthew Agius, lead author of the new study and researcher at Roma Tre University in Rome. This conventional view explains that gravity pulls the subduction plates away from the ridge, a process that is accommodated by the passive mantle rising at the ridge itself.
In 2015, Agius learned of an experiment carried out by Catherine rychert and Nicolas Harmon, associate professors in geophysics at the University of Southampton and professor at the University of Oxford Michael kendall. The initial objective, however, was not to determine the propagation factors at the mid-ocean ridges. Agius, a postdoctoral fellow at the University of Southampton during this experiment, and his colleagues intended to use ocean floor seismometers to take some of the early seismic recordings on the Mid-Atlantic Ridge and learn more about the formation of the lithosphere below.
In 2016, their research cruise visited Cape Verde and traveled from there to deploy 39 seismometric stations around the Mid-Atlantic Ridge over an area 1,000 kilometers wide. A year later, the team returned to collect the instruments and examine their data.
Initially, the team hoped to find clues to the origins of the lithosphere. “But the quality of the data was so rich – very high quality seismic data – that it allowed us to zoom in deeper,” said Agius. Using P-at-S receiver functions on the seismic data under the stations, the team was able to image the MTZ, the limit between the lower mantle and the upper mantle, between 410 and 660 kilometers deep.
“You can only do these measurements where you have stations, so the oceans are largely unsampled,” said Christine houser, assistant professor and researcher in geophysics at the Earth-Life Science Institute of the Tokyo Institute of Technology, not involved in this study.
When they zoomed in, the researchers found that the MTZ in the western part of their study area was thinner than expected: the 410-kilometer discontinuity was diminished, and the 660-kilometer discontinuity was noted. They also noticed that below the ridge the shear waves were slower than below the old Atlantic seabed, implying a warmer MTZ. These features are usually found at hot spots, not peaks.
“For the first time, we have evidence of higher temperatures in the mantle transition zone [at the Mid-Atlantic Ridge]”Agius said. From this, the researchers deduced that matter from the lower mantle rises towards the upper mantle. Instead of gravity, upwelling could cause the seabed to sprawl out.
This experiment is the first time that scientists have obtained seismic data directly from the ridge, as opposed to data from earth stations, which provides a more blurred view of the Earth’s internal mechanics at the ridge level. “This introduces new evidence for the entire study of plate tectonics,” Agius said.
“This discovery in itself, that there could be regions in our mantle where there is a vertical transport of materials that are not …[sites of] active upwelling and downwelling like slabs and plumes are intriguing, ”said Elvira Moulyukova, an associate researcher who studies geodynamics at Yale University and was not involved in the research.
Houser, like the Southampton team, uses seismic data to map the Earth’s mantle. She said the data in this new study so far matches her own models.
But Mulyukova wants stronger evidence and measurements of more geophysical properties on the ridge. The authors interpreted their observations as evidence of vertical matter transfer in the mantle, but there are other possibilities. Agius and his colleagues agree that studying other properties in this area would provide a more holistic view.
If true, this team’s findings could change our 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] geodynamo.
—Jackie Rocheleau (@JackieRocheleau), science writer