A granite outcrop at the base of Pikes Peak in Colorado reveals a geological feature called the "Great Unconformity," in which young rocks sit on top of much older stone. (Credit: Christine Siddoway)
A team of geologists led by CU听Boulder is digging into what may be Earth鈥檚 most famous case of geologic amnesia.
Researchers have spotted that phenomenon, called the 鈥淕reat Unconformity,鈥� at locations around North America, including in the Grand Canyon and at the base of Pikes Peak in Colorado. There lie sites of missing time, where relatively young rocks dating back about 550 million years sit right on top of much more ancient stone鈥攊n some cases more than 3 billion years old.听
In other words, a huge chunk of geologic history has vanished from in between.听
鈥淩esearchers have long seen this as a fundamental boundary in geologic history,鈥� said Rebecca Flowers, an associate professor in the Department of Geological Sciences.
Top: Rebecca Flowers stands near exposed rock on Pikes Peak; bottom:听A hiker walks past one site of the Great Unconformity near the town of Manitou Springs, Colorado. (Credits: Christine Siddoway)
For a study , she and her colleagues drew on a technique known as 鈥渢hermochronology鈥� to take a fresh look at that fundamental boundary. They found that the Great Unconformity might not be the result of a single, catastrophic event in the planet鈥檚 past like many scientists thought. Instead, a series of smaller calamities may have triggered many different unconformities around the world.听
The results could help scientists better understand the flourishing of complex life that occurred not long after that tumult settled down, about 540 million years ago in an era called the 鈥淐ambrian Explosion.鈥�
鈥淭here is a lot of the geological record that is missing,鈥� Flowers said. 鈥淏ut just because it鈥檚 missing doesn鈥檛 mean that this history is simple.鈥�
Pikes Peak
To study that less-than-simple history, Flowers and her colleagues turned to Pikes Peak. In a granite outcrop near the mountain town of Manitou Springs, geologists can find one of the clearest cases of the Great Unconformity.听
Follow the strata down, and you will see young rocks鈥攍ess than 510 million years old鈥攁nd older 鈥渂asement鈥� rocks鈥攄ating back about 1 billion years. But you won鈥檛 find anything in between.
Geologists know that something must have happened in the past to erase all that history, Flowers said. What that was and when exactly it happened, however, are still a mystery.
鈥淥nly recently have we had the ability to reach far enough back in time to start filling in that gap,鈥� she said.
Rocks, Flowers said, carry a kind of memory. By probing the particular atoms that have been locked up inside geologic samples, savvy scientists can create a heat-based history of those rocks鈥攅ssentially, how hot or cold the sample was at various points in its lifetime.
Using that method, the researchers discovered that the Pikes Peak basement rocks were brought to the surface of the planet about 700 million years ago. For Flowers鈥� team, that finding was key. 听听
When all that rock rose to the surface, she explained, it would have suddenly been at the mercy of wind, snow and other extremes. And those elements could have led to erosion鈥攁 lot of erosion鈥攅ssentially wiping the geologic history of the region clean. Imagine shaking an Etch-a-Sketch but on a monumental level.听
鈥淓arth is an active place,鈥� Flowers said. 鈥淭here used to be a lot more rocks sitting on top of Mount Everest, for example. But they鈥檝e been eroded away and transported elsewhere by streams.鈥�
Blame Rodinia
But what lifted those rocks up in the first place? Flowers and her colleagues think it has something to do with Rodinia. That鈥檚 the name of a massive supercontinent鈥攖hink Pangea, only much older鈥攖hat formed at Earth鈥檚 surface roughly 1 billion years ago.听
鈥淎t the edges of Rodinia, where you have continents colliding, you鈥檇 see these mountain belts like the Himalayas begin to form,鈥� Flowers said. 鈥淭hat could have caused large amounts of erosion.鈥�
The researchers also realized something else: The Great Unconformity might not have been so great in the first place. As Rodinia crashed together then pulled apart over hundreds of millions of years, all that geologic activity may have caused many separate cases of memory loss around the world鈥攏ot just one.听
鈥淲e鈥檙e left with a feature that looks similar across the world when, in fact, there may have been multiple great unconformities, plural,鈥� Flowers said. 鈥淲e 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.鈥�
It鈥檚 something to ponder the next time you go for a hike on Pikes Peak.听
Coauthors on the new study include Rachel Havranek, a graduate student in Geological Sciences at 兔子先生传媒文化作品; Francis Macdonald of the University of California, Santa Barbara; and Christine Siddoway of The Colorado College.
