Back when dinosaurs were just starting to skulk, Earth had just one giant land mass, a supercontinent that scientists call Pangea. It broke up about 200 million years ago, and since then its fragments—riding on chunks of crust called tectonic plates—have been gliding, merging, and splitting their way into their present—temporary—positions. Now, geoscientists have unveiled a computer model that maps the details of that tectonic dance in 1-million-year increments—practically a frame-by-frame recap of geologic time. It shows that the plates speed up, slow down, and move around in unexpectedly short bursts of activity. It also suggests that researchers may have to rethink what drives much of that incessant motion.
“It’s a major achievement, and it’s very impressive that they can now do this analysis at this resolution,” says Thorsten Becker, a geodynamicist at the University of Southern California in Los Angeles, who was not involved in the study.
The reconstruction is the work of scientists at the EarthByte program at the University of Sydney in Australia, one of the world’s foremost research groups for plate tectonics and geodynamics, who described it in a paper published online on 12 March in Earth and Planetary Science Letters. Previous work had mapped out tectonic movement in 20-million-year increments, which were then used to analyze plate velocities. But a closer look using the latest plate reconstructions created by the EarthByte group revealed that a lot more can change in 20 million years than scientists had thought.
“It turns out that plates can change their motion (speed and direction) over geologically short periods of time, about 1 million years,” says the new study’s lead author, Sabin Zahirovic, a tectonics researcher and geodynamicist at the University of Sydney. “Which means that if you have a snapshot over 20 million years, you can easily miss an important regional or global plate reorganization.”
The researchers achieved the improved time resolution by their modeling techniques and software, performing more detailed analysis of the data used previously, and incorporating more sources of data as well. The new model shows that although plates usually creep along at an average speed of about 4 centimeters per year, some can reach much faster speeds in short sprints. For example, India, which broke off the east coast of Africa about 120 million years and is now plowing into Asia, reached speeds as high as 20 centimeters per year for a relatively brief 10 million years. A rising plume of molten rock in Earth’s mantle probably caused the speedup by “lubricating” the underside of the continent and allowing it to slide smoothly over the mantle, Zahirovic says.
The model also suggests that a major engine of continental drift—the “pull” of nearby subduction zones, where one plate plunges under another and dives into the mantle—may be less important for setting plate speeds than researchers had thought. Instead, the researchers say, the drag created by the underside of massive continents jutting out under the plate like the keel of a ship may play a bigger role by slowing plates down.
Becker says it’s notable that the model confirmed that continents have a strong anchoring effect on the plates. But he was more cautious about the group’s finding that “slab pull” from subduction isn’t as important a factor. Subduction zones bordering the plates are much harder to locate and keep track of than continents over a 200-million-year period, he says. Zahirovic acknowledges the challenge but says his team used multiple, independent sources of geologic information to “resurrect and estimate” ancient plate boundaries.
Next, Zahirovic says, he and his colleagues plan to apply what they’ve learned to try to reconstruct how plates moved before the breakup of Pangea, deep in the geologic past.
(Video credit: EarthByte)
*Correction, 23 March, 2 p.m.: This story has been corrected to reflect that EarthByte's 2012 model was the first plate reconstruction to use 1-million-year increments. The team's current paper is the first analysis of plate velocities and other measurements using those models.