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

[Kreigh Tomaszewski]

A rock formation in Michigan has beat out other contenders for the title —
for now

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
<https://rock.geosociety.org/net/gsatoday/science/G613A/GSATG613A.pdf>,
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.

https://www.sciencenews.org/article/united-states-oldest-known-rock