The tale of the oldest rock found in the United States reads a little bit like one of those silly bar jokes. You know the kind, where four geologists, one of them from Wyoming, walk into a bar and start chatting about who has the oldest rock.
In this case it wasn’t a bar, it was Zoom, and there were no beers.
“It was just this sort of idle (question), ‘Which one of us studies areas that have the oldest rock?’” Frost said. “It was almost like a chat, not quite over beer, but that kind of thing.”
Frost’s candidate was rocky crust in Wyoming with bits of the mineral zircon that she has dated back to 3.82 billion years.
“But the way we interpreted that rock, the result was that the rock itself was a bit younger,” Frost said. “So, it was more like 3.45 billion years old. It had just picked up some older grains (of zircon).”
Two of the other geologists, Pat Bickford and Bob Stern, put forward their candidates, which they felt made a stronger case than Frost’s for oldest rock in the U.S.
“Then Paul Mueller said, ‘Well I haven’t published it yet, but I think I have the oldest rock, and it’s in the Upper Peninsula of Michigan,’” Frost recalled. “So that’s how it all started.”
The idle chit-chat soon turned into real science, as the geologists realized they were onto something that needed further examination.
That led to a paper published in the Geological Society of America in April, examining the oldest of old rocks in Wyoming, Minnesota and Michigan, as well as identifying a new record holder in Michigan, Watersmeet gneiss rock that the researchers say is 3.62 billion years old.
The old record holder was Morton gneiss rock in Minnesota. While that was once thought to date back to 3.82 billion years old, newer, more accurate methods have determined those rocks are more like 3.52 billion years old.
This outcropping of rock has been dated as 3.45 million years old. (Courtesy Carol Frost) 
This rock contains 3.82 billion-year-old zircons, with a nighthawk sitting on top. (Courtesy Carol Frost)
How To Ask A Rock How Old It Is
Asking a woman her age has long been a don’t-ask situation. Rocks, however, can be asked about their age without seeming impolite, though it’s still a very complicated answer.
“We have to use natural radioactivity,” Frost said. “Most rocks have small traces of uranium in them. Not enough that it causes any safety concern, but the uranium decays very slowly over time to isotopes of lead.”
To determine the age of ancient metamorphic rocks like Morton and Watersmeet gneisses, mineral grains that have naturally occurring uranium in them are separated out, and the ratio of parent radioactive atoms to lead isotopes is examined.
“Because the decay is at a constant rate, you can calculate the age,” Frost said. “So that’s what we do. And there are different ways of doing the analysis.”
One complication though, is that just because a rock has some mineral grains at a certain age doesn’t mean the rock itself is necessarily that old.
“Maybe the rock crystalized from a magma that picked up some older grains from the rocks it moved through,” Frost explained. “So that mineral grain is older than the rock. That’s just a taste of the kind of complications that arise.”
In the case of Frost’s Wyoming rock, that’s the mechanism she and her colleagues concluded had occurred. The zircon particles in her rock crust were picked up by magma from some older rock it had traveled through.
“So, my rock — I keep calling it my rock — but it has a cluster of these minerals that are 3.82 billion years old,” Frost said. “Which is older than (Mueller’s) rock.”
But the rock itself wasn’t as old as the grains it had picked up. That dated to more like 3.45 billion years, Frost said, making it younger than Mueller’s 3.62 billion years.
Wyoming Still Has A Chance
The presence of several 3.82-billion-year-old zircons, though, does suggest that Wyoming may have older rocks yet to be found, Frost said. That has her holding out hope that Wyoming will one day steal the title of oldest rock still in place not just from Michigan, but maybe even from the world record holder in Canada. That rock is called Acasta gneiss, and Frost said it’s not quite 4 billion years old.
“Wyoming, the Minnesota River Valley area, and the Upper Peninsula of Michigan all have rocks of about the same age, originally, and, probably, were stuck together at one time,” Frost said. “And then all the plate tectonic activity that happened over the last two and one-half billion years, they’ve gotten fragmented from each other.”
Wyoming, she believes, can make a stronger case right now than Michigan for having sections of older rock crust that have just not yet been found.
“We only found one grain that old in the Michigan rock,” Frost said. “But mine had a cluster of eight to nine zircons all that age.”
The richer source of zircons indicates more likelihood of eventually finding older rock, Frost said.
Regardless, the Wyoming-Minnesota-Michigan block of rock crust are together the oldest continental crust known in the United States to date.
Wyoming geologist Carol Frost and three of her colleagues have identified a new oldest rock in America. Found in Michigan, researchers estimate it’s more than 3.5 billion years old. Here, she's working in the Tetons. (Courtesy Carol Frost) 
Wyoming geologist Carol Frost and three of her colleagues have identified a new oldest rock in America. Found in Michigan, researchers estimate it’s more than 3.5 billion years old. (Courtesy Carol Frost)
Where To See Wyoming’s Oldest Rocks
Some of Wyoming’s oldest rocks can be seen in the low and rounded granite hills that lie just to the north of Jeffrey City, on the road that leads from Muddy Gap to Rawlins. It’s a rather desolate area, where deer and antelope play among picturesque hills, but it’s something of a geologist’s heaven.
The rocks in that area were first lifted and exposed to wind and weather by something called the Laramide Orogeny in geologist lingo. That mountain-building event occurred between 75 million years ago, according to a United States Geological Survey paper on the geology of Rocky Mountain National Park. The Laramide Orogeny is responsible for the mountains across Wyoming, starting with the Black Hills area on the eastern edge of the state, including Devils Tower. It’s also responsible for the Rocky Mountains, Beartooth and Absaroka Mountains, Wind River, Uinta Mountains, and numerous other mountain ranges in Wyoming.
“It’s the mountain building event named for the town of Laramie, which brought up all the mountain ranges in Wyoming,” Frost said. “So, if you want to look at old rocks, you’ve got to go look in the mountains.”
Frost, who is retired, said it won’t be her that ultimately finds the nation’s oldest rocks, but she predicts it will be someone in Wyoming.
That’s because Wyoming has some of the best geology in the world, thanks to not just the Laramide Orogeny that lifted the earth’s crust upward, away from rock-eating magma, but also the state’s ever-blowing wind. It has worked steadily over millions of years to expose older and older layers of rock, making them easier to see and to find.
“Geologists have a special relationship with Wyoming,” Frost said. “And many, many universities run their summer field camps in the state of Wyoming because of that.”
In fact, when Frost applied for a job in Wyoming, she faced a field of 100 other applicants, all vying for the same job. Winning the job was like winning the geologic lottery.
“We’re closing in on minerals that are almost as old as the Earth itself, which tells you when the Earth formed, cooled and crystallized, and made minerals that are still being made today when granite magma cools,” she said.
That’s exciting stuff, Frost added, because as time goes by, there’s less and less of earth’s oldest crust left to find. Wyoming has a lion’s share of what’s left, just waiting for the right geologist to come along and discover it on some lonely mountain ridge, where the wind never ceases to blow.
Renée Jean can be reached at renee@cowboystatedaily.com.
