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.
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.
Lava lamp planet
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.
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.