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The age of the planet, though, was important to Charles Darwin and other evolutionary theorists: The biological evidence they were collecting showed that nature needed vastly more time than previously thought to sculpt the world.A breakthrough came with the discovery of radioactivity at the beginning of the 1900s.Measuring the uranium-to-lead ratios in the oldest rocks on Earth gave scientists an estimated age of the planet of 4.6 billion years.Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.Certain isotopes are unstable and undergo a process of radioactive decay, slowly and steadily transforming, molecule by molecule, into a different isotope.This rate of decay is constant for a given isotope, and the time it takes for one-half of a particular isotope to decay is its radioactive half-life.An hourglass is a helpful analogy to explain how geologists calculate the ages of rocks.
After all, the ever-changing Earth rarely left a complete geological record.For the other radioactive “clocks,” it is assumed that by analyzing multiple samples of a rock body, or unit, today it is possible to determine how much of the daughter isotopes (lead, strontium, or neodymium) were present when the rock formed (via the so-called isochron technique, which is still based on unproven assumptions 2 and 3).Yet lava flows that have occurred in the present have been tested soon after they erupted, and they invariably contained much more argon-40 than expected.1 For example, when a sample of the lava in the Mt. Helens crater (that had been observed to form and cool in 1986) ( age yield incorrect old potassium-argon ages due to the extra argon-40 that they inherited from the erupting volcanoes, then ancient lava flows of unknown ages could likewise have inherited extra argon-40 and yield excessively old ages.Using this technique, called radiometric dating, scientists are able to "see" back in time.PART 1: Back to Basics PART 2: Problems with the Assumptions PART 3: Making Sense of the Patterns This three-part series will help you properly understand radiometric dating, the assumptions that lead to inaccurate dates, and the clues about what really happened in the past.