Earth is roughly 4.54 billion years old. In that time, it has seen continents form and disappear, ice caps expand and retreat, and life evolve from single-celled organisms into blue whales.
But how do we know Earth's age? We start by looking inside it.
"When you're an Earth scientist who looks at a rock, it's not just a rock; it's like that rock has a story that you can try to decipher," said Becky Flowers, a geologist at the University of Colorado Boulder.
When minerals form out of magma or lava, they often contain traces of radioactive material, such as uranium. Over time, those radioactive elements decay, meaning they spew radiation, eventually transforming them into new, more stable elements that remain trapped inside the mineral.
