The United States’ oldest known rock has existed for at least 3.6 billion years

A weathered sign in the Minnesota River Valley proudly proclaims: “World’s Oldest Rock.” Erected in 1975, it marks a 3.8-billion-year-old gneiss — or so scientists thought.

Turns out, it’s not the world’s oldest rock (Since 2019, that title has been held by an estimated 4-billion-year-old Canadian Acasta Gneiss). An analysis of minerals in the Minnesota gneiss and gneisses from across the country indicate that it’s probably not even the oldest in the United States, geologist Carol Frost and colleagues report in the March-April GSA Today. The age proclaimed on the sign may be overstated by at least 300 million years, the team argues. Instead, the old sign should be uprooted, revised to “America’s Oldest Rock” and hammered into Michigan’s Watersmeet Gneiss, which the researchers estimate is at least 3.6 billion years old.

But the lighthearted debate points to a deeper problem: If we can’t date rocks accurately, we risk misreading major geologic events that shaped the planet — and that will shape its future.

Like many geology debates, this one started as “a beer question,” says coauthor Bob Stern, of the University of Texas at Dallas. Extracurricular curiosity led him and Ph.D. student Clinton Crowley to geologists, including Frost, who specialize in dating ancient rocks.

The premise seemed simple. It wasn’t.

When geologists date rocks, they’re really dating minerals. “A rock can be composed of minerals that formed at different ages,” says Frost, of the University of Wyoming in Laramie. It’s like trying to date a building by analyzing its bricks, which aren’t necessarily the same age. For Frost, it’s almost metaphysical: “So, what is the age of the rock? I mean, what does the question really mean?”

The mineral zircon is a fan favorite, but its durability — able to withstand weathering, heat and pressure — means it often outlasts its host rock. After crystallizing in magma, zircons can be swept into sediments or crushed by tectonic forces, processes that form new rocks but may (or may not) distort the crystal’s age. As a result, zircons are helpful but imperfect record keepers.

The team sampled gneisses from the primordial American heartland in Minnesota, Wyoming and Michigan, the rocks’ banded striations and deformed grains hinting at a tumultuous history. “They’re messy,” says Jeffrey Vervoort, a geologist at Washington State University in Pullman who was not involved in the study. “You can say that of all of them.”

When studying gneiss samples, researchers zap their zircons with lasers and ion beams to measure the radioactive decay of uranium to lead and calculate each rock’s age.

The disputed champion, Minnesota’s Morton Gneiss, contains zircons dating to 2.6 billion, 3.3 billion and 3.5 billion years ago. “There may have been two rocks of different ages that became mixed at a third youngest time,” Frost says. The sign boasting 3.8 billion? Probably outdated.

Most zircons in Wyoming’s Sacawee Gneiss date to 3.4 billion years ago, though nine rogue grains date to 3.8 billion years ago.

Then there’s Michigan’s Watersmeet Gneiss. Its zircon ages span a wild 3.8 billion to 1.3 billion years old, with evidence of a violent past: volcanic intrusion (which sounds as rude as it is), metamorphism and tectonic upheaval. The team settled on a minimum age of 3.6 billion years, handing Watersmeet the title of “America’s Oldest Rock” — for now.

The search for America’s oldest rock isn’t just an exercise in trivia — it raises fundamental questions about how we reconstruct Earth’s history. Without precise ages, scientists can’t pinpoint when life began, mountains grew or climates shifted.

Vervoort isn’t worried the study will upend geologic history, explaining that younger rocks are usually much easier to date. Early Earth is another matter. “When I give talks on the early Earth, I always finish with the Salvador Dali painting with the drooping clocks in a barren landscape,” he says. “It’s complex.”

Geologist Mark Harrison of UCLA applauds the researchers’ efforts but emphasizes that their results only reflect rocks available at Earth’s surface. “They may have documented [the United States’] oldest known rock,” he says, “but the challenge to young readers will be to discover an even older rock.”

Frost agrees. The 3.8-billion-year-old zircons in the Michigan and Wyoming gneisses hint at older rocks that were either recycled in Earth’s mantle or remain buried in the crust. “I would love to find them,” she says.

So, should the sign in Minnesota come down? Probably. But it might be wise to leave a little room for updates.