The oldest intact rocks found on Earth date back to approximately which age?

• A. 3.2 billion years ago
• B. 3.8 billion years ago
• C. 4.0 billion years ago
• D. 4.4 billion years ago

Answer: (C)

Oldest rocks show when Earth’s crust first solidified and remained as a coherent rock body. Radiometric dating pins the age of the oldest confirmed intact rocks at about four billion years. The earliest well-preserved rock is the Acasta Gneiss from Canada, dated around 4.0 billion years ago. Zircon grains in Jack Hills, Australia, reach about 4.4 billion years, but those are tiny mineral grains within sedimentary rocks, not a single intact rock core, so they aren’t counted as the oldest rocks themselves. Other ancient rocks, like those from Isua, Greenland, are younger (about 3.7–3.8 billion years). So four billion years is the best approximate age for the oldest intact rocks.