News Earth The United States’ oldest known rock has existed for at least 3.6 billion years A rock formation in Michigan has beat out other contenders for the title — for now An analysis of minerals in the Watersmeet Gneiss (shown) suggest the metamorphic rock dates to about 3.6 billion years ago, potentially making it the oldest known rock in the United States. Paul Brandes By Evan Howell 2 hours ago 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. Sign up for our newsletter We summarize the week's scientific breakthroughs every Thursday.