A father-daughter duo from the Cowboy State didn’t know it at the time, but the large rock they found is a massive discovery that’s 2.5 billion years old and a unique find for Wyoming rock hounds.
The large rock found by Laramie resident Patrick Corcoran and his daughter Cora is called a banded iron formation. It dates back to an important time in Earth’s history, originating more than 2.5 billion years ago in the ancient oceans that covered Wyoming.
“I’ve never seen one this big in my time in Wyoming,” said Dr. Laura Vietti, the geology museum and collections manager at the University of Wyoming, where the Corcorans donated their find. “It is very helpful to have established supporters of the museum, like Patrick and Cora, because they were able to find this rare specimen and bring it to us.”
The large rock certainly stands out, with a reddish color and relatively smooth and shiny, with clearly visible bands formed over millions of years of sedimentary compression. For those who don’t immediately know, it can be mistaken for a giant piece of petrified wood.
The rusting of the oceans
Banded iron formations are unique sedimentary rocks formed more than 2 billion years ago. The formation gets its name from the distinctly colored layers that are strikingly visible in the boulder found by the Corcorans.
“It is composed of alternating layers of a sparkling metal, usually hematite, and a red flint that alternates over and over again,” Vietti said. “And that’s why it’s called a banded iron formation.”
The alternating bands of hematite and flint describe an important chapter in Earth’s history. Banded iron formations are the earliest evidence of oxygen in the oceans and provide the basis for multicellular life on Earth.
“Before multicellular life emerged on Earth, the atmosphere was very oxygen-depleted and the oceans had no dissolved oxygen,” she said. “When photosynthetic cyanobacteria began to evolve and pump oxygen into the ocean and atmosphere, they essentially rusted the oceans.”
Because iron does not dissolve in water, any rust floating in the ocean settled on the seafloor, forming thin but widespread, banded iron formations. The reddish color of the UW specimen comes from the 2-billion-year-old rust caused by cyanobacteria that oxygenate the oceans and atmosphere and make our planet habitable today.
Along boulders
Banded iron formations are found on every continent on Earth, including Wyoming. But that doesn’t mean they’re easy to find and often aren’t located in the Cowboy State.
“We have the right age of rocks and the right depositional environments to form banded iron formations,” Vietti said. “But they’re not easy to get to, and there aren’t many of them.”
Most of Wyoming’s banded iron formations are located at the highest points of mountain ranges and are inaccessible to all but the most tenacious climbers. And there is little interest in reaching these deposits as banded iron formations are not an economically viable resource.
The Corcorans found their 2-billion-year-old boulder in the flat expanse of a private ranch with nary a mountain in sight. Vietti classified their find as a washed-out cobblestone making a wet descent from a mountain exposure.
“Washed out cobblestones were captured from the mountains by streams or glaciers,” she said, “and tumbled down the mountains and were deposited on the plains of Wyoming. You will find them as you walk across the prairie. most of our tire iron comes from.”
Tumbling water and ice rounded and smoothed the boulder’s jagged edges, revealing its dramatically colored layers. It’s a perfect piece for the UW Geological Museum, and a perfect spot has already been found for it.
Bigger is better (outside)
The UW Geological Museum has collected several specimens of banded iron formations in Wyoming. Two small pieces will be on display in “Wyoming’s Oceans,” a new exhibit at the museum that showcases Wyoming’s watery past while highlighting the collections and research of the university’s past and current biologists, geologists and paleontologists.
The banded iron boulder of the Corcorans is important because of its size. Vietti said this is the largest specimen from Wyoming she has seen in her career.
“Most of the banded iron formation boulders you find in Wyoming are the size of a thimble and a grapefruit,” she said. “This particular specimen is very important because it is so large and shows the unique banded iron layers very well.”
This example of Wyoming tire iron is slightly smaller than a mini refrigerator and much heavier. Because the design of “Wyoming’s Oceans” has already been completed, the Corcoran’s banded iron boulder is too big and too late for the company to be included in the new exhibit.
Vietti foresaw an academic future for the takeover.
“It will be an exhibition specimen, but mainly outdoors for now,” she said. “We will put a sign or label on it for the general public. And it will be used as a teaching specimen for our Intro to Geology labs (at UW.).”
Patrick and Cora Corcoran are happy that their big boulder has a promising future ahead of it, regardless of location. It has survived two billion years of Wyoming’s geological history, so it should have no problem withstanding the natural forces of a college campus.
“It’s terribly heavy and terribly big,” Vietti said. “We don’t really have room for it in the museum right now. We might put it inside in the future, but I think it will be fine outside right now. And we’re very happy to have it there. ”
Andreas Rossi can be reached at arossi@cowboystatedaily.com.