In the Grand Canyon, there is a line in the rock where time seems to have gone missing. Ancient crystalline rocks mark its bottom, while sandstones laid down as life started to diversify during the Cambrian explosion mark the top. “It’s called the Great Unconformity, and it’s a gap in the geologic record,” says Nicolas Flament, a geoscientist at the University of Wollongong, Australia. “This gap can be anywhere between 200 million years, which is not that impressive, to more than a billion years.”
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.
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.
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.
