(CN) — As billions of years have passed since its formation, Earth's rocks tell the tale of its history, except for "geologic amnesia" sites around North America scientists claim to have figured out in a study released Monday.
In sites like the Grand Canyon and the base of Pikes Peak, rocks dating back about 550 million years sit atop more ancient rock layers, some dating back more than 3 billion years, leading geologists to wonder what happened in the years in between.
In a study published in the Proceedings of the National Academy of Sciences, geologists led by Rebecca Flowers from the University of Colorado Boulder aimed to discover the reason behind the phenomenon scientists call the Great Unconformity.
"Researchers have long seen this as a fundamental boundary in geologic history," Flowers said.
Flowers and her team used a technique called thermochronology, whereby scientists can determine the temperature of rocks at certain points in history. They discovered that the Great Unconformity might not be the result of a single cataclysmic event, but rather a series of smaller events that affected Earth's surface.
Using the results, the scientists said they could better ascertain how life came to flourish after major geologic events subsided roughly 540 million years ago in a period known as the Cambrian Explosion.
"There is a lot of the geological record that is missing," Flowers said. "But just because it's missing doesn't mean that this history is simple."
The team examined granite at Pikes Peak in Colorado near the town of Manitou Springs.
"Follow the strata down, and you will see young rocks — less than 510 million years old — and older 'basement' rocks dating back about 1 billion years. But you won't find anything in between," the research team noted in a statement.
Flowers said the mystery of the Great Unconformity was difficult to solve for years.
"Only recently have we had the ability to reach far enough back in time to start filling in that gap," she said.
Using thermochronology, the geologists were able to determine that the older basement rocks were brought up to the surface around 700 million years ago, exposing them to wind and weather that made them vulnerable to erosion and wiping out the geological history.
Flowers compared it to wiping out an Etch-a-Sketch, but on a dramatically larger level.
"Earth is an active place," she said. "There used to be a lot more rocks sitting on top of Mount Everest, for example. But they've been eroded away and transported elsewhere by streams."
The research team theorize that Rodinia, the 1 billion-year-old ancient massive supercontinent that pre-dates Pangea, could be responsible for the Great Unconformity.
"At the edges of Rodinia, where you have continents colliding, you'd see these mountain belts like the Himalayas begin to form," Flowers said. "That could have caused large amounts of erosion."
The geologists said they also theorize that the Great Unconformity could have been instead a series of smaller geologic activities.
"We're left with a feature that looks similar across the world when, in fact, there may have been multiple great unconformities, plural," Flowers said. "We may need to change our language if we want to think about the Great Unconformity as being more complicated, forming at different times in different locations and for different reasons."
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