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More than a billion years of geological history is missing from parts of the Grand Canyon and, some geologists believe, much of the rest of the world. A new investigation of this mysterious gap in time, which has puzzled geologists for 152 years, attributes it to the break-up of the supercontinent Rodinia, 700 million years ago.
The rocks of northern Arizona have been deposited over the last two billion years. The Colorado River has exposed these layers as it carved its way downward, and therefore back in time. However, for much of the Canyon's length a huge swathe is missing, with rocks from one era piled directly on top of those laid down much earlier, a feature known as an “unconformity”.
Unconformities are common, but the one the Grand Canyon exposes is so immense it was named the “Great Unconformity” after being found in 1869, and has perplexed geologists ever since. New evidence presented in Geology offers an explanation using differences along the Canyon's length.
"The Great Unconformity is one of the first well-documented geologic features in North America,” the appropriately named Barra Peak, a student at the University of Colorado, Boulder, said in a statement. “But until recently, we didn't have a lot of constraints on when or how it occurred.”
The mystery has deepened with the discovery of other unconformities elsewhere in the world from the same era. Some geologists believe these are connected, attributing them to the enormous glaciation known as Snowball Earth scraping underlying rocks bare. Others think it's nothing more than random events, some of which inevitably match in time.
Rodinia included most of the world's continents, but separation began around 800 million years ago and subsequently accelerated. Peak argues Rodinia's great divorce produced a series of violent faulting events in the region around what is now the Canyon. Rocks were dragged upwards and exposed to the elements, eventually being washed to the ocean.
With so many geologists having studied the unconformity without reaching a conclusion, Peak needed to do something different. She and her co-authors considered the fact that the pressure of overlying layers heats rocks up, inducing chemical reactions that create a record of past temperatures, a process known as thermochronology.
By measuring helium diffusion from ancient rocks, the authors concluded the eastern and western parts of the Canyon have quite different temperature histories. Some 700 million years ago, as Rodinia was breaking up, the western basement rock was brought to the surface where it weathered, but the eastern half remained buried 12-15 kilometers (8-10 miles) deep.
Just how much time is missing in the unconformity varies, but near Lake Mead rocks 520 million years old sit directly on top of stone 1.4 - 1.8 billion years old. “There's more than a billion years that's gone,” Peak said. “It's also a billion years during an interesting part of Earth's history where the planet is transitioning from an older setting to the modern Earth we know today.”
Peak and colleagues are now broadening their focus to other North American sites where the Great Unconformity can be seen. Although the geological opportunity the Grand Canyon provides is hard to match, other places can indicate if what Peak observed was part of a widespread phenomenon.
