The Earth’s outer layer is split into slabs, called tectonic plates. As the plates slide across the Earth’s surface, their constant, often violent interactions with one another create volcanoes, earthquakes, rifts and mountain ranges. But the Earth may not always have been shaped this way.
When geologists recently modeled how mineral combinations reacted at particular temperatures and pressures, they found that some of our planet’s oldest rocks — and the first continents — may not have needed plate tectonics to form. In their report, published in the journal Nature, they suggest that for a long period of Earth’s early days, the planet was covered by one solid plate, like an eggshell.
Scientists still aren’t sure how we might have gotten from an early solid crust to our present-day tectonic plates. “That's the $64,000 question,” says Michael Brown, a geologist at the University of Maryland and one of the report’s co-authors. But Brown and his colleagues have a few ideas.
Brown explains that during the Archean Eon — spanning 4 billion years ago to 2.5 billion years ago — the Earth’s upper mantle was much hotter than it is today, and our planet was highly volcanic.
“You have to imagine a world where there are many more volcanoes distributed across the globe than we see at present,” he says, adding that today, we tend to think of volcanic activity just in specific regions, like Hawaii or Iceland.
“But this world would have had the possibility for volcanism anywhere across the surface,” he explains. “It would have had the possibility of volcanism being generated by upwellings in the upper mantle, as well as these larger, what we call 'plumes' that are generated at the core-mantle boundary, that are these big tubes of hot magma that come up through 2,900 kilometers of Earth’s mantle.”
Those magma tubes are the main culprits in one theory about how we got from a solid shell to fractured tectonic plates. According to Brown, they could have produced so much magmatism at the Earth’s surface that they pushed the outer shell down, and it began to sink under its own weight.
“This would generate a local — on the scale of something like the Pacific Ocean, perhaps — but a local area of subduction,” he says. “And it may be that with enough of these plumes in the late part of the Archean, that we can generate a globally linked network of plate boundaries.”
Another alternative, he says, has to do with just wear and tear. The shell could have weakened in certain linear zones, which, in turn, linked up over time. “So, we have a network of weaknesses in this outer shell that eventually are able to move apart as magmatism forces them apart and some move together. And again, we eventually generate a globally linked network of plate boundaries.”
Either way, we end up with a network of tectonic plates, like the one we have today. But the controversial aspect of the geologists’ report is the suggestion that tectonic plates formed sometime long after Earth did, after a period with a single, solid outer shell. As Brown explains, a competing theory holds that tectonic plates essentially formed with the Earth, 4.5 billion years ago.
“In other words, this outer shell that would have formed first of all, as the magma ocean crystallized after the moon-forming impact, would have very soon developed into a network of spreading, moving-apart, converging, moving-together and moving-sideways plate boundaries that would have enabled that shell to, if you like, crack into a number of plates,” Brown says.
“And one of the arguments in favor of that is a particular fingerprint in the chemistry of igneous rocks. And I think one of the things our paper does is demonstrate that this chemical fingerprinting is not a unique signature of a particular tectonic environment. And we have to be more careful how we identify things when we go way back in time.”
At present, Brown says there’s a likely scientific consensus that plate tectonics began around 3 billion years ago. “I don't mean by that, [that] there’s 98 percent of us [who] believe that but probably more than half of the geologists interested in this topic would subscribe to that view at the moment, and that may change with time,” he says.
But like everything in science, Brown says it will be a community effort to understand more about Earth’s early shell — and not one without disagreement.
“This is the way we work,” he adds. “We try to prove ourselves wrong. We can't prove ourselves right.”