Earth’s tectonic plates are always on the move and likely have been for much of our planet’s history. Rewinding their paths, however, poses problems reaching past the most recent supercontinent, Pangaea, which broke apart around 200 million years ago.
To understand where continents once resided on the globe, scientists must choose a frame of reference. In a relative plate motion reference frame, one plate stays still—say, Africa—and everything else moves relative to that fixed piece of Earth. In contrast, absolute plate motion models let all the plates move within a reference system of the deeper Earth—holding hot spots or other deep mantle features constant, for instance. To complicate matters, both tectonic plates and the mantle move relative to Earth’s spin axis.
By developing a new model based on letting tectonic plates and the mantle move together, a team of scientists explored how deep mantle structures might have responded to plate motions for the past billion years. The team, led by Dietmar Muller, a geophysicist at the University of Sydney, began with the time period when Pangaea’s predecessor—Rodinia—existed.
Tectonics Rules! (Maybe)
Three tectonics-based “rules” govern Muller’s model. First, he and his colleagues minimized how rapidly Earth’s outer shell—the lithosphere—rotates relative to the mantle, a phenomenon called net lithospheric rotation. Muller explained that the high viscosity of the mantle limits the overlying lithosphere’s wholesale movement.
Second, Muller’s team restricted the mobility of subduction zones through the billion-year time span on the basis of how ocean trenches behave today. When an old, dense oceanic plate subducts, the hinge—where the plate bends—typically rolls back slowly, away from the overriding plate. Hinges rarely recede rapidly, nor do trenches often travel the other way toward the overriding plate, said Muller.
Third, because continents have a thick keel that sticks into the viscous mantle, Muller gave continents speed limits.
However, these rules may not apply to earlier episodes of Earth’s history. Elvira Mulyukova, a geodynamicist at Northwestern University, pointed out that the low-viscosity layer between the lithosphere and the rest of the mantle—the asthenosphere—is relatively weak. Therefore, the need to minimize net lithospheric rotation is an assumption, not a rule, she said.
Extrapolating today’s slab rollback speeds to a billion years ago is another assumption, said Zheng-Xiang Li, a geologist at Curtin University. Rollback may not have behaved as it does today, which means the speeds may have varied in the past.
In Muller’s preferred model, which starts 1,000 million years ago, the ocean surrounding the supercontinent Rodinia contained scattered subduction zones. Until about 600 million years ago, the deep mantle structure reflected these distributed subduction zones. Ridgelike networks latticed around the core-mantle boundary, rising and falling like mountain chains made of dead tectonic plates.
Between about 600 million and 500 million years ago, a band of subduction zones helped distribute the continents around the circumference of the globe, with oceans on either side of the loop of landmasses. This resulted in distinct plume-spawning lower mantle structures beneath both oceans. However, the ever-moving continents eventually ended their encirclement and dispersed. Subducting slabs sliced through one of the lower mantle structures, which disintegrated between 500 million and 400 million years ago. The other upwelling shifted, settling below the paleo–Pacific Ocean.
From 400 million to 200 million years ago, the Pacific-centered mantle structure took up its position, unbothered by subducting plates. However, no counterpart existed in the deep mantle on the opposite side of the globe, which instead housed a graveyard of subducted slabs thanks to the closure of ancient oceans as Pangaea coalesced around 320 million years ago.
Eventually, Pangaea splintered, but the mantle structure underlying the Pacific persisted. After Pangaea’s parts dispersed, the African mantle upwelling—possibly responsible for modern rifts and volcanoes—took shape.
Muller’s model matches most other plate motion models for the past 200 million years of Earth history, in part because the recent rock record is much easier to reconstruct. In particular, today’s ocean floor, striped with symmetric magnetic signatures that increase in age away from spreading centers, goes back 200 million years, letting Earth scientists more readily reverse the paths of continents, said Mulyukova.
Beyond 200 million years ago, some plate motion models, when coupled to the mantle, yield unreasonable behavior like strange lateral mantle currents, explained Muller. “If we want to have a geodynamically reasonable model that obeys physics principles,” he said, “we need to apply these rules.”
Knowing whether the model’s output is reasonable, said Muller, “is not that easy.” As a check, he and his colleagues compared the model’s past plate locations with known locations of volcanic eruptions, indicated by kimberlites and large igneous provinces, which originate from plumes deep within the mantle and have erupted throughout Earth’s history. “We can simply compare where [deeply sourced volcanic regions] line up with the places where our model would predict a good likelihood of mantle plumes coming up,” he explained.
Muller’s model does not use paleomagnetic data as an external check in the same way as the deeply sourced volcanic information, in part because paleomagnetic data cannot constrain changes in longitude. This constraint means that if a tectonic plate traveled due east or west, that movement wouldn’t be recorded in the paleomagnetic signal, said Muller.
Moreover, paleomagnetic data reflect motions of the plates relative both to the mantle (plate tectonics) and to the wholesale rotation of the solid Earth—the crust and mantle together—relative to the spin axis (true polar wander), said Muller.
True polar wander, said Li, is a huge geodynamic process that can be modeled only indirectly in current models but, ultimately, must be incorporated to accurately use paleomagnetism data. When future geodynamic models can incorporate Earth’s rotation and centrifugal force, then they could use paleomagnetic data as a check for the results, Li explained. “We have to really get over this hurdle to build Earth’s spinning—the centrifugal force—into the geodynamic process,” he said.
“Paleomagnetism Is the King”
“Geodynamicists have worked hard, from a modeling perspective, to understand how the Earth should behave,” said Muller, by using physical constraints like mantle viscosity.
However, “within the current uncertainties of rheology [flow behavior] of the mantle…you can make models in which the mantle is almost stagnant or moving faster than plates,” said Douwe van Hinsbergen, a geologist at Utrecht University. “We do not know from observations how the mantle below our feet convects.”
As slabs subduct, their mineralogy and structure at the atomic level change, which can make a slab stiffer or weaker, said Mulyukova, “and therefore more or less capable to…shovel things around along the core-mantle boundary.” Understanding these transitions from laboratory work and incorporating them into geodynamic models, she said, is important.
Though temperature, and by extension density, is the main reason subducting plates sink, phase transitions, which occur as the physical properties of minerals morph into forms stable at higher pressures and temperatures, also contribute to the phenomenon. Phase transitions will change trench motions (Muller’s second rule) by changing the pulling forces (like density and viscosity) that tug subducting slabs into the mantle, said Mingming Li, a geodynamicist at Arizona State University.
Ultimately, using how the mantle might work and how plates should move to explain geological data “is the wrong way around,” said van Hinsbergen. Instead, he argued that geological data—like paleomagnetism—should be used to explain mantle and plate behavior.
“My biased opinion is paleomagnetism is the king,” said Li, who was otherwise very positive about the study. “There’s no other handle to test the linkage between mantle structure and reconstructed absolute paleogeography.”
“We are bound by a lot of limitations,” he continued, “but we are fortunate to live in a very exciting time. I call it a second plate tectonic revolution.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer