Scientists are testing whether slow movements deep within Earth’s mantle helped create the Great Unconformity, a major gap in the geological record. Earlier explanations have focused on glaciation, changing sea levels, and the cracking or stretching of Earth’s crust, but these ideas do not fully explain why the rifts mainly occurred along continental edges. New simulations examine whether mantle convection could have pushed some regions upward and pulled others downward over the past billion years.
Other explanations included cracking, pulling, and stretching of Earth’s crust, as well as lower global sea levels. However, the rifts mainly occurred at the edges of the continents, while sea-level changes would have affected all continents in a similar way.
“There’s no question that there was massive glaciation. There’s no question that there was sea level change over time. The question is: Was that enough to piece together the puzzle of the Great Unconformity?” Flament says.
Testing mantle-driven changes
A fourth idea proposed that the mantle itself was moving the continents up and down through a process called dynamic topography.
“It had been proposed, but it had not been quantitatively tested,” Flament said. His team set out to examine the idea with computer simulations.
A lava-lamp planet
From roughly 100 to 200 kilometers below Earth’s surface down to nearly 3,000 kilometers below it, the mantle slowly convects.
“It’s like a lava lamp, where something comes up and something comes down,” Flament explains. “Cold rocks sink, hot rocks rise.”
Although the mantle is solid, its rocks can move slowly between Earth’s surface and its depths over periods of about 200 million years. That movement exerts forces on the surface. When cold slabs of ocean floor sink into the mantle at subduction zones, they pull the ground downward. When hot mantle rises, it pushes the ground upward.
The scale of this vertical movement remains debated. “It’s a few hundred meters short, but we still don’t know what exactly it is,” Flament said.
Simulating a billion years of mantle convection
Dhungana and Flament ran six simulations of mantle convection covering the past billion years. The simulations were based on reconstructions of how tectonic plates moved during that time.
The researchers grouped regions where the ground rose or sank in similar ways. They then used those patterns to identify where unconformities might have formed and where they might have persisted to the present day.
Flament cautioned, however, that the starting configuration used in the simulations remains uncertain.
Source: arstechnica.com