Hallelujah, it’s rainin’ chert!This layer is one of the most common rock types found in

Hallelujah, it’s rainin’ chert!This layer is one of the most common rock types found in sediments from the ancient ocean; chert. Chert is made of fine-grained silica, and in rocks from the Precambrian, layers of it are found all over the place.They formed over hundreds of millions of years, on different continents, in different settings, and even mixed with other common rock types like banded iron formations and carbon-rich sediments possibly left over from early organisms.Despite these cherts being so common in the Precambrian, Earth doesn’t commonly make them today. That could make some sense because of evolution; some organisms have figured out how to make shells out of silica, which removes it from ocean waters. But…that leaves the question about how the ancient ocean made layers like this.There are some ideas. Some scientists have proposed that chert could form as groundwater flowed through sediments after burial; any silica dissolved in the water could precipitate new minerals like chert. Others suggested chert might at the bottom of the ocean as tiny grains pile up and water was squeezed out.Based on photos like the one you’re looking at, a group of Stanford scientists led by Elizabeth Stefurak have found strong evidence for a different mechanism. They looked at cherts from all over the world, formed hundreds of millions of years apart and they kept finding a strange texture. See how there are “grains” in this chert layer? None of the known ways to form chert can produce that texture.They found granules in rocks formed from both deep and shallow waters, so however the Earth was making these; the planet transported them all over. Although they can be compacted, they formed as nearly spherical grains. The scientists hypothesized that maybe they were commonly formed in shallow waters and carried farther out to sea by waves or submarine landslides. Imagine a landslide millimeter-sized balls of chert pouring down a slope or even raining down on the floor of the ocean; that’s very different from anything happening today, but something like that happened billions of years ago to make this rock.The geologic record is full of interesting changes we are still trying to understand. You’re looking at a rock found all over, over hundreds of millions of years, and yet because the oceans today are so different, it takes detailed study to understand the story of how those rocks formed.-JBBImage credit: Elizabeth StefurakOriginal paper: http://geology.geoscienceworld.org/content/early/2014/02/07/G35187.1.abstract -- source link

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