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