Earth’s missing chapter
An international team of scientists thinks it has solved one of geology’s great mysteries: What happened to a massive, missing layer of Earth’s crust? The Great Unconformity—a gap in the geological record of anywhere from 250 million years to 1.2 billion years—can be observed at the Grand Canyon, where the rocky layers offer a window into Earth’s history. One strata is made up of sedimentary rocks from the Cambrian period, which started some 540 million years ago, and below is a layer of crystalline rock that formed about 1 billion years ago. The new study suggests the missing layer or layers vanished during a hypothesized period known as Snowball Earth, when most of the planet was covered in ice, reports NationalGeographic.com. Researchers believe that roaming glaciers ground up a 3-mile-deep layer of the crust. Using a chemical analysis of ancient zircons—hardy minerals that lock in the geochemical conditions of their environment during formation—the scientists concluded the resulting sediment was dumped into the oceans and then sucked into Earth’s mantle by moving tectonic plates. “Earth does a really good job at erasing the tracks of its past,” says study co-author Bill Bottke, of the Southwest Research Institute in Boulder.