Big chunks of history (and rock) are missing in North America, study says

At rocky sites across America, including Pikes Peak and the Grand Canyon, 550 million-year-old rock sits on top of rock that has existed as long as 3 billion years. But there's nothing in between. It's as though that time period between 3 billion years ago and 550 million years ago has literally been wiped from Earth.

It's called the Great Unconformity, and some researchers believe it's the result of one massive cataclysmic event in Earth's history.

"Researchers have long seen this as a fundamental boundary in geologic history," said Rebecca Flowers, study author and associate professor in the University of Colorado at Boulder's department of geological sciences, in a statement.

That boundary represents the boundary between rocks that don't contain fossils, and those that do. About 540 million years ago, the so-called Cambrian explosion occurred. This event marks where there is a sudden wealth of a diverse range of animal fossils in Earth's rocks.

Flowers and her colleagues investigated the boundary in a study published Monday in the journal Proceedings of the National Academy of Sciences.

At Pikes Peak, they found the clear dividing line between rocks that were less than 510 million years old and rocks that reached 1 billion years old. The older rocks are what is referred to as "basement" rock.

"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. Only recently have we had the ability to reach far enough back in time to start filling in that gap."

The researchers used thermochronology to study rock samples. This allowed them to study atoms inside the samples that preserve the heat history of the rocks at Pikes Peak. Together, the heat history of the samples provided a timeline of when the rocks were hot or cold.

Thermochronology enabled the researchers to pinpoint when basement rocks were exhumed at Pikes Peak. An event caused an upheaval of rock there 700 million years ago.

Then, that rock was exposed to the elements, like wind and snow. Extremes like these can cause enough erosion to effectively erase rocks from a region. And enough events like this occurring in different regions would do away with chunks of rock -- and the history they held -- altogether.

"Earth is an active place," Flowers 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."

Flowers and her colleagues believed that smaller catastrophic events created many unconformities around the world, rather than one giant event that could cause a Great Unconformity.

Rodinia, a giant supercontinent older than Pangaea, formed Earth's surface. And the activity caused by the supercontinent would have been enough to spark erosion events, the researchers said.

"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."

Rodinia was formed 1 billion years ago and broke apart between 633 million and 750 million years ago. The violent crashing and pulling apart of landmasses would have been enough to spark geologic erasure in multiple regions.

"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."