Mantle convection may have helped create the Great Unconformity

A new study says Great Unconformity mantle convection may help explain one of the largest gaps in Earth’s rock record. The researchers’ models suggest slow movement in the mantle lifted some continental interiors, exposing them to erosion before they later sank and were covered by sediment.

The proposal matters because ice sheets, sea-level shifts and tectonic changes have all been offered as explanations for the Great Unconformity. The study, published in Nature, argues that long-lived mantle-driven uplift helped make glacial erosion effective in particular regions rather than across every continent alike.

What is the Great Unconformity?

The Great Unconformity is a boundary where relatively young sedimentary rocks lie directly on much older crystalline basement, with hundreds of millions of years of intervening rock absent. In the Grand Canyon, Cambrian-era sedimentary layers overlie ancient crystalline rock; depending on the location, the missing interval ranges from roughly 200 million years to more than 1 billion years, according to Ars Technica.

The feature has been documented across much of North America and in parts of northern Europe and North China. Its uneven distribution has made a glaciation-only explanation difficult: extensive ice erosion should have left a more uniform pattern if it were the sole cause.

A three-stage explanation

Santosh Dhungana and Nicolas Flament of the University of Wollongong modeled mantle convection over the past billion years. The mantle is solid hot rock that moves extremely slowly: hotter material rises and cooler material sinks, a process that can raise or lower the land above by hundreds of meters.

The researchers ran six simulations based on reconstructions of tectonic plate movement. Five of those simulations showed Laurentia, the ancient core of North America, undergoing a long period of uplift followed by subsidence between about 930 million and 510 million years ago, Ars Technica reported. Four simulations found a comparable pattern for northern Europe and North China.

The proposed sequence has three parts. First, rising mantle gently elevated continental interiors, allowing erosion to strip their surface. Second, ice sheets during the Snowball Earth interval, roughly 720 million to 635 million years ago, scoured the uplifted land. The authors conclude uplift on its own would have been too gradual to account for the missing rock. Finally, as the continental blocks subsided, seas advanced and new sediments buried the eroded surface, preserving the boundary.

Why the pattern differs by continent

The model offers an explanation for why the unconformity stands out most clearly in some regions. Laurentia, Baltica in northern Europe and North China experienced one extended rise followed by one extended fall in the simulations, a sequence the researchers say favors both erosion and preservation of the resulting surface.

Australia and South America, by contrast, went through two uplift-and-subsidence cycles, the authors said. That history could leave multiple, less distinct unconformities rather than one conspicuous boundary.

What the models cannot settle

The study is a model-based explanation, not proof that mantle motion alone created the Great Unconformity. Ancient magnetic evidence can establish a continent’s distance from the equator but not its longitude, and the researchers had to reconstruct ocean floor that has since been recycled into the mantle.

Glaciation, rifting, sea-level change, continental rearrangement, mountain building and volcanism remain part of the wider scientific discussion. The authors also say erosion may have changed ocean chemistry ahead of the Cambrian diversification of life, but they do not identify mantle processes as the sole driver of that event.

This story draws on original reporting from Ars Technica.