What We Know About Earth's Great Unconformity Rock Gap

A gap of as much as 1.2 billion years in the rock record — called the Great Unconformity — may trace back to slow-moving currents inside Earth's mantle, according to a study published in Nature and reported by Ars Technica on Nov. 6, 2026.

What is the Great Unconformity?

The Great Unconformity marks a boundary visible in the Grand Canyon and elsewhere where ancient crystalline rock sits directly beneath sandstone laid down during the Cambrian explosion, Nicolas Flament, a geoscientist at the University of Wollongong, told Ars Technica. The missing interval ranges from roughly 200 million years to more than a billion years, Flament said. The oldest sediments resting atop the unconformity in North America date between 635 and 482 million years old, according to the Ars Technica report, and geologists have traced the feature across most of North America and parts of northern Europe and northern China.

What does the new study say caused it?

Flament and co-author Santosh Dhungana modeled how mantle convection — hot rock rising, cold rock sinking over a region from about 100 to 200 kilometers deep down to nearly 3,000 kilometers — can push continents upward for hundreds of millions of years, per the study described by Ars Technica. That uplift exposed rock to glacial erosion; later, as the mantle currents shifted, the land sank and new sediment buried the eroded surface. Flament called the mantle "a lava lamp, where some stuff comes up and some stuff comes down." The mechanism, known as dynamic topography, had been proposed before but, Flament said, "it hadn't been quantitatively tested" until this analysis.

"Something seems to have happened, in particular in North America." — Nicolas Flament, University of Wollongong

Why isn't glaciation alone a sufficient explanation?

The leading prior explanation pointed to Snowball Earth, the period roughly 720 to 635 million years ago when glaciers reached the tropics, according to Ars Technica. Flament noted glaciers erode roughly ten times more effectively than rivers. But a planet-wide freeze should leave the unconformity everywhere, and it does not appear globally, Flament said.

What other theories were ruled out?

Researchers also considered crustal rifting and global sea-level drops. Rifting mainly affects continental edges rather than interiors, and a sea-level shift should strike every continent the same way, per the Ars Technica account. Flament summarized the puzzle: "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?"

Timeline of the Great Unconformity story

• 720–635 million years ago: Glaciers reach tropical latitudes during the Snowball Earth interval.
• 635–482 million years ago: Oldest sediments resting on the North American unconformity are deposited.
• Nov. 6, 2026: Flament and Dhungana's mantle-convection model is published in Nature and covered by Ars Technica.

What to watch next

• Whether the mantle-convection model holds up when applied to the unconformity's traces in northern Europe and northern China.
• Follow-up studies testing dynamic topography against additional glacial and sea-level data sets.
• Peer response to the Nature paper from geoscientists who favored rifting or sea-level explanations.