There are many open questions about how our planet formed 4.55 billion years ago: When did plate tectonics start? When did the Earth's mantle begin to vigorously circulate in a process called convection? What was Earth like early in its lifetime? Because no rock records from the earliest years of the Earth remain, researchers turn to minerals called zircons, which are resilient against physical and chemical alteration over time and therefore preserve a precise chemical record about the moments i...