Hot rock flowing deep within Earth’s mantle pushed continents upward for hundreds of millions of years, according to a recent Nature study that offers a new explanation for the Grand Canyon’s mysterious Great Unconformity.
Nicolas Flament, a geoscientist at the University of Wollongong, Australia, and his colleague Santosh Dhungana built computer models showing that mantle currents lifted landmasses so glaciers could grind them down before sinking them beneath fresh sediment layers.
In the Grand Canyon, this geological gap sits between ancient crystalline rocks at the base and sandstones from the Cambrian explosion at the top. The missing time spans anywhere from 200 million years to over a billion years, arstechnica.com reported.
Geologists have tracked the feature across most of North America, parts of northern Europe, and northern China. The oldest sediments resting on the boundary in North America date between 635 and 482 million years old.
The leading explanation for the erosion has long been glaciation. Between 720 and 635 million years ago, ice sheets reached the tropics during a period some scientists call Snowball Earth. Flament notes that glaciers are roughly 10 times more effective than rivers at scouring rock away, but if the entire planet froze, the gap should appear everywhere uniformly, which it does not.
Other hypotheses included crustal stretching known as rifting and global sea-level drops. However, rifting acts primarily on continental margins, while sea-level changes should affect all landmasses equally.
Flament’s team tested a fourth idea called dynamic topography, where mantle movements push the surface up and down. Earth’s solid mantle slowly convects from 100 to nearly 3,000 kilometers beneath the surface like a lava lamp. Cold rock sinks at subduction zones while hot rock rises, pulling the surface down or pushing it up by several hundred meters.
Dhungana and Flament ran six mantle convection simulations spanning the last billion years, driven by tectonic plate reconstructions. They grouped regions with similar ground motion histories to locate where the unconformity should form and survive. Flament points out that the starting configuration of these simulations remains uncertain because ancient magnetic records reveal latitude but not longitude.
“It’s called the Great Unconformity, and it’s a gap in the geologic record,” Flament said. He added that the missing interval can range “between 200 million years, which is not that impressive, to more than a billion years.”
Flament summarized the core question regarding past theories by stating, “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?”
