About 4.567 billion years ago, a quivering bead of magma 93 million miles from the Sun cooled down until it grew a skin of rock. Eventually, it would be named Earth. We don’t know a lot about what the planet was like back then, because that primordial crust is almost entirely recycled–eroded away, pushed back down into the molten depths of the planet, or smashed to bits by the huge impacts that blasted Earth for its first few hundred million years.

Geologists have wandered the planet to find scraps of the infant Earth. One mineral that is particularly precious to them is known as a zircon. Tiny zircon crystals can withstand billions of years of abuse–getting ripped out of their original rock, incorporated into new rocks, heated up, and squeezed at tremendous pressures–and yet still retain their original chemistry. Zircons have the added attraction of holding onto radioactive isotopes such as uranium. Over billions of years, the uranium decays at a steady rate into lead. By measuring the atoms of uranium and lead in a zircon, scientists can get a tight estimate of the zircon’s age. Continue reading “Searching For the Oldest Pieces of Earth” →