A closer look
You can figure out exactly when a rock was carved or exposed to the sky by measuring its sunburn from deep space. Earth is constantly bombarded by high-energy cosmic rays from exploding supernovas. When these rays smash into exposed quartz on Earth, they create rare isotopes like Beryllium-10. By counting these trapped cosmic isotopes in glacial boulders or ancient monuments, geologists can calculate the exact year that specific rock was first dragged out from underground and saw the light of day.
