In the Grand Canyon there is a line in the rock. Below it lie ancient crystalline rocks. Above it are sandstones laid down in the Cambrian, when life began to diversify. Between the two there is nothing at all. “It’s called the Great Unconformity, and it’s a gap in the geologic record,” Nicolas Flament, a geoscientist at the University of Wollongong in Australia, told Jacek Krywko of Ars Technica. “This gap can be anywhere between 200 million years, which is not that impressive, to more than a billion years.”
Every archive loses things. A chronicle can be burned, a ministry’s files can go missing, a census can be forgotten in a drawer. The Earth keeps its records in layers of stone, and here, at one sharp seam, whole ages are gone. Flament and his colleague Santosh Dhungana now name a suspect in a paper in Nature, titled “The deep Earth origin of the Great Unconformity”. The suspect is the planet’s own interior.
The usual suspects
In North America, the oldest sediments resting on the unconformity are between 635 and 482 million years old. Geologists have followed the seam across most of the continent, and through parts of northern Europe and northern China. “Something seems to have happened, in particular in North America,” Flament says.
For a long time the favourite answer was ice. Between roughly 720 and 635 million years ago glaciers reached the tropics, and some scientists believe the entire planet froze over, the scenario called Snowball Earth. Flament says ice sheets erode rock about ten times more effectively than rivers do. But a planet frozen from pole to pole should have left the same scar everywhere, and it did not. Rifting, the stretching and tearing of the crust, works mostly at the edges of continents. A global fall in sea level should have struck every continent 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.
There was a fourth idea: that the mantle itself had been raising and lowering continents, a process known as dynamic topography. “It had been proposed, but it hadn’t been quantitatively tested,” Flament says. So his team tested it.
The power that never shows itself
From about 100 to 200 kilometres beneath the surface down to nearly 3,000 kilometres, the mantle turns over slowly. “An analogy would be a lava lamp, where some stuff comes up and some stuff comes down,” Flament says. “Cold rock sinks and hot rock rises.” The mantle is solid, yet a piece of rock can travel from the surface to the deep Earth in some 200 million years. Where cold slabs of ocean floor dive down at subduction zones, the ground above is dragged lower. Where hot rock wells up, the ground is lifted. How far it moves is still argued over. “The short of it is several hundreds of meters,” Flament says, and the exact figure “is still to be worked out.”
One could see a parable here, offered only as reflection. The most lasting power is often the kind nobody sees. A continent cannot know what is lifting it. All it sees is the ice arriving.
Dhungana and Flament ran six simulations of mantle convection reaching back 1 billion years, each driven by reconstructions of how the tectonic plates moved. They then grouped regions that rose and fell in similar ways, to find where an unconformity should have formed and where it should have survived. Flament is open about the weak foundations. Magnetic traces in old rocks record how far a continent sat from the equator, but not its longitude. “It’s possible that some continents may be in the wrong hemisphere,” he admits. The ancient ocean floor, recycled into the mantle long ago, had to be invented from scratch.
Even so, the models agreed. 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 over rising mantle and was pushed upward. Five of the six models showed Laurentia rising for a long time and then sinking in the Cambrian. For northern Europe and North China, four of them did.
Ice on a raised table
The uplift on its own was too gentle to strip off enough rock, the team concluded. “The uplift could create a tilt for the whole continent, but it’s not great at creating a steep slope,” Flament says. Glaciers filled the gap, because ice can grind through rock even on low ground and gentle slopes. In the models, the land was still rising when Snowball Earth arrived. “This gentle uplift that’s continuous, and then that glacial erosion that comes on top, work together,” Flament says. “They’re not coupled. They just happen to coincide in time.” Then, in the Cambrian, the continents sank, the seas flooded in, and new sediment buried the worn surface.
This also explains why the scar is so plain in North America and so hard to find elsewhere. Compared against the sediment preserved on each continent, North America and Eurasia show a single long rise followed by a single long fall. The researchers read that as the ideal recipe. South America and Australia 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 separate case. Mountain belts hemmed it in, and its erosion and sediment were shaped by those mountains rather than by the mantle below. “Imagine a book where a huge section in the middle has been ripped out,” Dhungana said in the university’s announcement, carried by phys.org. “Our research suggests these pages weren’t lost by chance.”
Only once
If the mantle lifts and lowers continents again and again, why is there only one Great Unconformity? The team points to a combination of three things: a rise-and-fall cycle lasting more than 900 million years, a near-global glaciation that arrived at exactly the right moment, and a rock record that grows harder to read the further back one goes. Flament suspects an older candidate, linked to a glaciation 2.4 billion years ago. But rocks that old contain no fossils to help date them. “Having life in sediments makes it a lot easier to have a sense of time,” he says.
What happened next may matter as much as the gap itself. The erosion that carved the unconformity washed huge amounts of ground rock into the oceans, possibly fertilising them just before the Cambrian explosion, when calcium became plentiful in seawater. “That’s when life learns to mobilize calcium and create skeletons and shells,” Flament says. He is careful not to give the deep Earth all the credit. “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.”