Earth is Missing a Huge Part of Its Crust. Now We May Know Why.

A fifth of Earth's geologic history might have vanished because planet-wide glaciers buried the evidence. From a report: The Grand Canyon is a gigantic geological library, with rocky layers that tell much of the story of Earth's history. Curiously though, a sizeable layer representing anywhere from 250 million years to 1.2 billion years is missing. Known as the Great Unconformity, this massive temporal gap can be found not just in this famous crevasse, but in places all over the world. In one layer, you have the Cambrian period, which started roughly 540 million years ago and left behind sedimentary rocks packed with the fossils of complex, multicellular life. Directly below, you have fossil-free crystalline basement rock, which formed about a billion or more years ago.

So where did all the rock that belongs in between these time periods go? Using multiple lines of evidence, an international team of geoscientists reckons that the thief was Snowball Earth, a hypothesized time when much, if not all, of the planet was covered in ice. According to the team, at intervals within those billion or so years, up to a third of Earth's crust was sawn off by Snowball Earth's roaming glaciers and their erosive capabilities. The resulting sediment was dumped into the slush-covered oceans, where it was then sucked into the mantle by subducting tectonic plates.

Effectively, in many locations, Earth buried the evidence of about a fifth of its geological history, the team argued this week in the Proceedings of the National Academy of Sciences. The notion is elegant but provocative, and the authors themselves predict that some geoscientists will express skepticism. 'I think, though, we have extraordinary evidence to support that extraordinary claim,' says study leader C. Brenhin Keller, a postdoctoral fellow at the Berkeley Geochronology Center.

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