Today, Earth’s surface is a mosaic of shifting tectonic plates that give us moving continents and mountain ranges and may even have made life possible. However, it hasn’t always been this way. In the beginning, the continents as we know them didn’t exist, and stagnant oceanic crust encompassed Earth, forming a “lid” like Venus has today. Now, researchers have offered a possible explanation for how the lid cracked: A mantle plume—a ribbon of hot rock originating deep in the earth—may have created the first plate boundary, setting plate tectonics in motion.
Specifically, the researchers hypothesize that mantle plumes can give birth to subduction zones, where one piece of Earth’s rigid outer layer—or lithosphere—rides over another, pushing it into the mantle. Today, the pull of sinking slabs at subduction zones provides much of the driving force behind plate tectonics. Scientists have previously suggested numerous ways that new subduction zones might develop. However, most explanations suffer from a fatal flaw: they occur only on a planet that already has active plate tectonics. For instance, subduction can start in areas of weak lithosphere, but today, most lithospheric weakness occurs in places where tectonic forces have already stretched or deformed the crust. To solve this chicken-and-egg dilemma, researchers have identified several non-tectonic processes that could induce subduction, including meteorite impacts, strong convection of mantle material, and now mantle plumes.
The idea grew partly out of recent work on a vast deposit of oceanic basalt known as the Caribbean Large Igneous Province, led by Scott Whattam of Korea University in Seoul and Robert Stern of the University of Texas, Dallas. An exceptionally hot mantle plume caused the basalt to erupt about 100 million years ago, and also appears to have triggered subduction along the Caribbean coasts of Central and South America. To explore whether the same process could have kick-started plate tectonics, Whattam and Stern partnered with Taras Gerya, a geophysicist at ETH-Zurich in Switzerland who was already modeling how plumes interact with the lithosphere. In the new study, published online today in Nature, the team describes how mantle plumes can initiate subduction in a three-dimensional model.
In the first step, magma from the plume head rises to the surface and erupts to create a hot, buoyant plateau of oceanic crust, like the Caribbean Large Igneous Province. The melt percolating up through the plateau also weakens it, eventually causing the plateau to collapse. In the process, its edges override the surrounding lithosphere, pushing the lithosphere down into a funnel-shaped depression. As the funnel deforms, the lithosphere weakens and rips, producing several independent slabs that dangle into the mantle. Finally, these cold, dense slabs begin to sink, and subduction is off and running.
To Gerya’s surprise, the model also produced a mosaic of small plates. As subduction ramped up beneath it, the overlying oceanic plateau stretched and broke, creating familiar geologic features such as spreading centers and transform faults. “All types of presently existing plate boundaries can be produced by plume-lithosphere interactions,” Gerya says. That’s important because plate tectonics not only requires subduction—it also requires multiple plates.
However, the researchers found that subduction could begin only under certain conditions. The plume had to intrude into old, cold lithosphere that would sink easily, and it had to be powerful enough to sufficiently weaken the crust above it to induce collapse. In addition, subduction needs water to lubricate the moving tectonic plates.
Without those ingredients, the model showed that the process froze, producing areas of damaged crust that could potentially form tectonic plates in the future. Such failed subduction zones appear to exist on Venus today and may well have formed on Earth in the past, Gerya says, adding that plate tectonics may have started and stopped several times before picking up momentum about 3 billion years ago.
Some researchers question whether the necessary conditions could have ever occurred earlier in Earth’s history. For one, there wouldn’t have been much old, cold lithosphere on the hot early Earth, says Kent Condie, a geochemist at New Mexico Tech in Socorro. At that time, most lithosphere would have been young and buoyant and would have resisted sinking into the mantle to start subduction, he says.
In addition, it’s unclear whether mantle plumes formed as frequently billions of years ago. And if they did, no one knows whether they could have weakened the lithosphere as much as the new model implies, says Patrice Rey, a geophysicist at the University of Sydney in Australia. Regardless of that uncertainty, however, Rey likes that the new hypothesis makes testable predictions—for instance, that large quantities of volcanic rocks should accompany the initiation of subduction. “It forces you to look at the geological record—to say, OK, is this possible? What is the prediction here? Do we have that in nature?”
Everyone agrees that’s no easy task. “Trying to understand the mantle is worse than trying to understand other galaxies,” says David Bercovici, a geophysicist at Yale University. Scientists have to look through almost 3000 kilometers of rock and more than 4 billion years of time, he says. However, they are likely to keep trying. The origin of plate tectonics is “the big holy grail for our field,” Bercovici says, and this study promises to fuel more debate.