Earth’s rocks have a missing layer spanning up to a billion years. We finally know where it went

Geologists have known since the 1870s that in places such as the Grand Canyon, sedimentary rocks about 540 million years old or younger sit directly on crystalline basement up to two billion years old, a boundary called the Great Unconformity. The missing interval can span hundreds of millions to more than a billion years and appears across North America, northern Europe and parts of northern China. Researchers Santosh Dhungana and Nicolas Flament at the University of Wollongong used the Australian supercomputer Gadi to run six mantle‑convection models extending back one billion years, constrained by plate‑motion reconstructions. The models consistently show that the ancient continental blocks Laurentia (core of North America), Baltica (northern Europe) and North China were uplifted from roughly a billion years ago to about 540 million years ago, then subsided over the past 540 million years. This up‑and‑down sequence — prolonged uplift followed by a single major subsidence — exposed crystalline rock to erosion and later buried the resulting surface under new sediment, allowing the unconformity to survive. The predicted uplift coincides with the Cryogenian Snowball Earth glaciations (about 720–635 million years ago), and the authors argue that mantle‑driven elevation created the long‑lasting exposure needed for extensive glacial erosion, which is more efficient than rivers on gentle slopes. In contrast, Australia and South America experienced multiple uplift‑subsidence cycles, which would have erased any early boundary, matching the weaker or absent Great Unconformity there. The study emphasizes the sequence of movement rather than precise uplift heights, noting the models tend to overestimate long‑wavelength elevation changes by about a factor of two. Flament said, "Sometimes what is missing can tell us more about the story of Earth than what we can see." The authors caution that plate reconstructions become less certain farther back, that results vary among the six simulations — especially for Baltica and North China — and that mantle flow must be considered alongside regional tectonics, erosion and sediment transport, not as a complete substitute. They also note the model does not directly demonstrate a causal link to the Cambrian explosion, though erosion, sedimentation and ocean chemistry are identified for further investigation.