Here’s something I’d never heard of: The Great Unconformity. The Grand Canyon is like a timeline of earth’s history, with each layer going back farther in time as you descend its walls, all the way back to rock that formed 1.8 billion years ago at the bottom.
The only problem is that a billion years worth of rock is missing. The timeline jumps from the Tonto group of 700 million years ago to the Vishnu basement layer 1.6 billion years ago. Geologist John Wesley Powell was first to discover this way back in 1869, but still no one really knows what happened to all that rock.
There are several competing theories, apparently, and they all likely played some role. For example, rock layers were weathered away by ice during the snowball earth period.
Geologists at the University of Colorado, though, published work this week providing thermochronological evidence for what seems to be the primary cause.
During the breakup of the first Pangea supercontinent, the western end of the Grand Canyon region was lifted so high that the basement layer came to the surface, but it was still miles underground at the eastern end.
The whole North American plate seems to have tipped like a saucer as Rodina split apart, and millions of years of rock slid into the sea.
basement rock
exposed at the surface
the great divorce