Transcript: The age of the Earth is estimated via the fundamental physical process of radioactive decay. It's possible to measure the age of things on the Earth by, say, counting tree rings, but the oldest ages are only several thousand years. Geologists can use the sedimentation record of rock and ice to estimate ages, but the Earth is a restless place having resurfaced itself every hundred million years or so through out its history. So it's hard to measure ages in more than millions of years with sedimentation. Radioactive decay of the Earth's oldest rock gives us the most reliable estimate of the Earth's age. We need a decay process whose half-life is nearly matched to the expected age of the Earth. The favorite decay processes for this purpose are the decay of potassium-40 to argon-40 with a half-life of 1.25 billion years and the decay of uranium-538 by a series of processes to lead-206 with a half-life of 4.5 billion years. Using these fundamental techniques geologists and astronomers have deduced that the age of the Earth is about 4.5 billions years.