Grand Canyon, Great Unconformity (IMAGE)
Caption
This view of the Grand Canyon shows the Great Unconformity, a boundary where nearly a billion years' worth of sedimentary deposits is missing from the geologic record. The boundary can be seen at roughly the middle of this image, separating the older, lumpy and angular rocks below from the younger horizontal layers above. New research suggests that the missing sediments, likely scrubbed away by glaciers during the global "snowball Earth" that ended roughly 635 million years ago, washed away to the oceans, where they lubricated subduction faults and kick-started the modern age of plate tectonics.
Credit
USGS/Alex Demas
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