The United States’ ancient rock is at least 3.6 billion years old

Geologists recently discovered that the “World’s Oldest Rock” sign in Minnesota’s River Valley is likely incorrect in proclaiming the rock as the oldest in the world. Analysis of minerals from the rock suggests that it may not even be the oldest in the United States, with a current estimation placing it at least 300 million years younger than initially believed. The sign should be updated to read “America’s Oldest Rock” and be placed in Michigan’s Watersmeet Gneiss.

Geologists Carol Frost and her team conducted a study on ancient rocks, using minerals like zircon to determine their ages. Zircon, a durable mineral capable of withstanding various environmental pressures, is commonly used in dating rocks. However, zircon’s ability to outlast its host rocks can sometimes complicate the dating process, affecting the accuracy of the final age determinations.

The team sampled gneisses from various locations in Minnesota, Wyoming, and Michigan, with the minerals within these rocks providing evidence of a tumultuous geological history. Zircons from these rocks were analyzed to determine their ages through radioactive decay measurements. Samples from Morton Gneiss in Minnesota and Sacawee Gneiss in Wyoming presented zircon ages spanning billions of years, challenging existing assumptions about these rocks.

Michigan’s Watersmeet Gneiss emerged as a contender for America’s oldest rock, with zircon ages ranging from 3.8 billion to 1.3 billion years old, pointing to a history of volcanic activity, metamorphism, and tectonic upheaval. The team concluded that Watersmeet Gneiss has a minimum age of 3.6 billion years, making it a strong candidate for the title of “America’s Oldest Rock.”

The search for America’s oldest rock is not just a trivial pursuit but a fundamental quest to understand Earth’s history accurately. Precise dating of rocks is essential for reconstructing past geological events, such as the evolution of life, mountain formation, and climate shifts. While the study may not revolutionize geologic history, it highlights the challenges in dating ancient rocks, particularly those from Earth’s early history.

Geologist Mark Harrison of UCLA commended the researchers for their findings but noted that the results represent only rocks available at the surface. The search for even older rocks continues, with the possibility of ancient rocks buried in Earth’s crust waiting to be discovered. Frost expressed a desire to uncover these potentially older rocks, suggesting that the study leaves room for future updates and discoveries in the field of geology.

The debate surrounding the “World’s Oldest Rock” sign in Minnesota raises broader questions about the reliability of rock dating and its implications for understanding Earth’s history. While the sign may need revising, the study serves as a reminder of the complexity involved in dating ancient rocks accurately. As geologists continue to uncover new evidence and refine dating techniques, the search for Earth’s oldest rock remains an ongoing and intriguing scientific endeavor.