In the Grand Canyon, there is a line in the rock where time seems to have simply given up. Ancient crystalline rocks sit at the bottom, sandstones from the Cambrian explosion sit at the top, and in between is a gap in the geologic record so rude it has its own name: the Great Unconformity. “This gap can be anywhere between 200 million years, which is not that impressive, to more than a billion years,” says Nicolas Flament, a geoscientist at the University of Wollongong, Australia. So, mildly impressive to extremely impressive, depending on your tolerance for missing eons.
In a recent Nature study, Flament and his colleague Santosh Dhungana argue that this gap was carved with help from deep within the planet. According to their models, slow currents of hot rock in Earth's mantle pushed entire continents up for hundreds of millions of years, letting glaciers grind them down and then let them sink, allowing fresh sediments to bury the scar. The largest hole in the rock record might be a fingerprint of processes happening deep in the planet's interior - because apparently even geology has a shadowy deep state.
The oldest sediments resting on the Great Unconformity in North America are between 635 and 482 million years old. Geologists have traced it across most of North America and parts of northern Europe and northern China. “Something seems to have happened, in particular in North America,” Flament says. Which is a polite way of saying the rest of the world got off comparatively lightly.
The leading explanation was glaciation. Between roughly 720 and 635 million years ago, glaciers reached the tropics, and some scientists think the entire planet froze over, a scenario called Snowball Earth. “Ice sheets are very good at grinding, scouring, scraping the surface,” Flament explains. He said that glaciers are about 10 times more effective than rivers at eroding rock away. But if the whole planet froze, the Great Unconformity should be found everywhere. And it's not. Awkward.
Other explanations included rifting, pulling and stretching of Earth's crust, and a global drop in sea levels. But rifting acts mostly on the edges of continents, and a sea level change should have affected all continents alike. “There's no doubt that there was extensive glaciation. There's no doubt that there have been sea-level changes throughout time. The question is, was that enough to put the Great Unconformity puzzle together?” Flament says. The answer, apparently, was no.
A fourth idea held that the mantle itself was moving continents up and down, a process called dynamic topography. “It had been proposed, but it hadn't been quantitatively tested,” Flament says. So, his team set out to do so - because someone eventually has to check the homework.
From roughly 100 to 200 kilometers beneath the surface down to nearly 3,000 kilometers, Earth's mantle slowly convects. “An analogy would be a lava lamp, where some stuff comes up and some stuff comes down,” Flament explains. “Cold rock sinks and hot rock rises.” The mantle is solid, yet a piece of rock can slowly travel from the surface to the deep Earth over some 200 million years. That motion tugs on the surface. Where cold slabs of ocean floor sink into the mantle at subduction zones, the ground gets pulled down; where hot mantle rises, it gets pushed up. The amplitude of this up and down motion is still debated, though. “The short of it is several hundreds of meters, and it's still to be worked out exactly what that [precise value] is,” Flament says. So: hundreds of meters, give or take a few hundred meters.
Dhungana and Flament ran six simulations of mantle convection going back 1 billion years, driven by reconstructions of how tectonic plates moved over that time. Then they grouped regions where the ground rose and fell in similar ways to find where the unconformity should have formed, and where it should have survived to the present. Flament, however, says the starting configuration of these simulations remains uncertain. Magnetic records in ancient rocks reveal how far continents were from the equator, but not their longitude. “It's possible that some continents may be in the wrong hemisphere,” Flament admits. The ancient ocean floor, long since recycled into the mantle, had to be invented entirely from scratch in the models. Still, the simulations produced a consistent story - which is more than most models manage when they're half-invented.
Laurentia, the ancient core of North America, had little or no subduction along its margins between about 930 and 510 million years ago, much like Africa today, so it sat above some rising mantle and was pushed up. Five of the six models showed this pattern of Laurentia seeing long uplift followed by sinking in the Cambrian. For northern Europe and North China, this pattern showed up in four. So, a solid majority, if not quite a consensus.
But the uplift alone, Flament's team concluded, was too gentle to strip away enough rock to create the Great Unconformity. “The uplift could create a tilt for the whole continent, but it's not great at creating a steep slope,” Flament says. The missing piece of the puzzle, he argues, were glaciers that ground through rock even on low ground and gentle slopes. The models show the uplift was still building when Snowball Earth hit. “This gentle uplift that's continuous, and then that glacial erosion that comes on top, work together,” Flament explains. “They're not coupled. They just happen to coincide in time.” When the continents sank in the Cambrian, seas flooded in and buried the eroded surface.
The models made by Flament's team also explain why the gap is so obvious in North America but hard to find elsewhere. Compared with records of sediment preserved on each continent, North America and Eurasia show one long rise followed by one long fall, which the researchers interpret as the ideal recipe for the Great Unconformity. South America and Australia, on the other hand, went through two cycles. “This would form potentially two unconformities, and then the Great Unconformity would be more difficult to see,” Flament says. Africa was a different case altogether - hemmed in by mountain belts at the time, its erosion and sediment were shaped by the mountains rather than by the mantle beneath. So, once again, Africa is doing its own thing.
But if the mantle goes through such up and down cycles on a regular basis, the remaining question was why we can see only one Great Unconformity, and not many. The team says the Great Unconformity is unique in the geological record because of a combination of a rise-and-fall cycle lasting over 900 million years, a near-global glaciation that happened at the right moment, and rock records that get harder to read farther back in time. In other words, the geological equivalent of a perfect storm, minus the storm.
Flament suspects there may be an older candidate, tied to a glaciation 2.4 billion years ago, but rocks that old lack the fossils that make younger sediments easier to date. “Having life in sediments makes it a lot easier to have a sense of time,” he says. His work, however, may help explain what happens right after the Great Unconformity formed. The intense erosion that carved the Great Unconformity washed rock material into the oceans, possibly fertilizing them just before the Cambrian explosion, when calcium became abundant in seawater. “That's when life learns to mobilize calcium and create skeletons and shells,” Flament says. He admits, though, that mantle dynamics likely wasn't the only process that enabled the Cambrian explosion. “I think it would be misleading to say the mantle set the pace here. But I think it was one of the forces that worked together.” A rare display of scientific restraint: the mantle was involved, but it didn't do it alone.