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Geologists have been puzzled for more than a century by something they call “The Great Unconformity”, where enormous slabs of time are missing from the geological record. Now, a new paper explains this was a product of astonishing glacial erosion during the period known as “Snowball Earth,” when almost the entire planet was covered in ice.
An unconformity is a break in the sedimentary record, such as when the age of rocks jumps sharply because one set of rocks eroded away before being covered by those substantially younger. One example was noticed in 1869 in the Grand Canyon, and subsequent research found it replicated around the world in rocks of similar age, earning the name The Great Unconformity. The authors of the new study calculated that due to glacial erosion a global average of 3-5 kilometers (2-3 miles) of the rocks were stripped away, hence "missing" from the record.
Although the Great Unconformity isn't seen everywhere on Earth, and the span of missing time varies where it does, Dr Brenhin Keller of the Berkeley Geochronology Center writes in Proceedings of the National Academy of Sciences its scale is truly enormous. Before 500 million years ago, we have approximately 0.2 cubic kilometers (0.05 cubic miles) of preserved sedimentary rock for each year of the Earth’s existence. Afterward this jumps to 1 km3 (0.2 miles3), rather than the gradual increase we would model as we get closer to today, and Keller and colleagues calculate an astonishing 1 billion cubic kilometers (200 million cubic miles) of pre-Cambrian material is missing beyond what would be expected.
Either, the authors argue, sedimentation increased dramatically at the start of the Phanerozoic era, or there was much greater erosion beforehand. They present evidence for the latter, showing crystals from the relevant era have isotopes of hafnium and oxygen consistent with being eroded from old rock and deposited at low temperatures. A phenomenal spike in erosion rates would also explain why we know of many asteroid impact craters less than 700 million years old, but only two older than that.
Between 717 and 580 million years ago the Earth went through a series of dramatic glaciations that make the recent ice ages look like minor chills. Even at the equator, the planet was covered with ice that was often piled higher than the tallest skyscraper.
The authors propose these mighty glaciers scoured the sedimentary rocks of previous eras and washed them out to sea, producing the Great Unconformity. They argue the rate of erosion required is quite consistent with that seen in modern Greenland.
If the theory is right, it not only explains one of geology’s most enduring mysteries, but supports the developing theory that the appearance of the first animals so soon after Snowball Earth was no coincidence. Instead, the nutrient pulse delivered by so much erosion created the conditions for complex life forms to survive.
