A new study presents a new way for tectonic plates – massive plates of rock that jostle to position themselves in the earth’s crust and upper mantle – to bend and sink.
It’s a bit of planetary Pilates that could solve the long-standing mystery of “subduction,” the process by which tectonic plates plunge deep inside the Earth.
Understanding the dynamics of plate tectonics has long been a top priority for Earth scientists. The rise and fall of tectonic plates has ramifications for the movement of continents, the formation of mountains and volcanoes, and the continuation of the geological carbon cycle.
“These plates are plates of cold, solid rock 100 kilometers thick which, in principle, should not be able to warp and sink at all – and yet they do, “said David Bercovici, Professor Frederick William Beinecke of Earth and Planetary Sciences at Yale Faculty of Arts and Sciences and Co-Director of the Yale Center for Natural Carbon Capture, which is part of Yale Planetary Solutions Project.
Writing in the journal Nature, Bercovici and a team of scientists present a new model showing what happens when tectonic plates slide deeper inside the Earth. Taras Gerya, professor of geophysics at ETH Zurich, is the lead author of the study; Thorsten Becker, professor at the University of Texas, is a co-author.
The study was prompted in part by previous work by Bercovici – including a study earlier this year with former Yale researcher Elvira Mulyukova, who is now an assistant professor at Northwestern University – on how grains of rock are weathered in the lithosphere or upper mantle, Earth’s uppermost layer of rock.
In the previous study, Bercovici and Mulyukova argued that the lower part of a tectonic plate can weaken due to “grain damage”, compression and mixing of mineral grains over millions of years that reduce grains to smaller and smaller sizes, softening the rock.
In the new model, a tectonic plate bends due to a repeated two-step process. The first 20 kilometers of the tectonic plate break slightly, creating faults. The damage to the grain then weakens the lower 80 kilometers of the plate just below the fault.
“The whole plate becomes weak enough to bend and sink, ”Bercovici said. “But the surprise is that the ‘bad grain damage’ feedback occurs in a repetitive sequence. As the plate bends and sinks, this causes a sausage link pattern that can be seen in the outer region of the subduction zone.
The model closely matched observations made in Japan of a tectonic plate in which large cracks in its upper section lean downward and show signs of weaker material below.
The research was supported by the National Science Foundation of the United States, the Swiss National Science Foundation and ETH Zurich.