Tukuk-OSAtoms That Tick at a Fixed Rate
Some atomic nuclei are unstable — radioactive isotopes — and they spontaneously transform into a different, stable "daughter" atom, releasing energy as they go. The remarkable fact that makes dating possible is that you cannot predict when one individual atom will decay, but in a large sample the population decays at a fixed, unchanging rate, measured as a half-life: the time for half of the parent atoms to turn into daughter atoms. Half-lives are unaffected by temperature, pressure or chemistry, so a radioactive isotope is a clock that keeps ticking no matter what happens to the rock around it. Different isotopes tick at wildly different speeds, from fractions of a second to billions of years, which is why scientists pick the isotope to match the age they expect.
From Ratio to Age
Dating is a measurement of ratios. In a mineral that formed as a crystal, you can count how many parent atoms remain and how many daughter atoms have accumulated. Because each half-life halves the parent and adds a matching dose of daughter, the parent-to-daughter ratio tells you how many half-lives have passed since the crystal locked the atoms in. Uranium-238 decays to lead-206 with a half-life of about 4.47 billion years — ideal for the oldest rocks and, indeed, for dating meteorites to set the age of the Solar System at ~4.54 billion years. Potassium-40 to argon-40 (half-life 1.25 billion years) dates volcanic layers, and since volcanoes erupt lavas and ash that crystallise freshly, they give clean "zero the clock" moments.
Carbon-14: The Clock for Things That Were Alive
For recent, once-living material there is radiocarbon (carbon-14). Living things constantly exchange carbon with the atmosphere, so their carbon-14 matches the air. When they die, the exchange stops, and the carbon-14 already inside decays with a half-life of ~5,730 years. Measuring how much is left tells you how long ago the organism died — perfect for wood, bone, charcoal and cloth up to about 50,000 years old. Because the short half-life burns the signal out, carbon-14 cannot date dinosaurs or the oldest rocks; those need the slow uranium and potassium clocks instead.
Dating Fossils Without Touching Them
Most fossils sit in sedimentary rock — layers of grains eroded from older rocks — and the grains themselves are older than the fossil, so you cannot date the sediment directly. Instead, geologists date the igneous layers above and below (a volcanic ash bed, a sill intruding through the sequence) and bracket the fossil's age between them. They also use relative dating: deeper layers are generally older (superposition), and certain short-lived species (index fossils) mark a known slice of time wherever they turn up. Combining the bracketing with radioisotopes is how the whole geological time scale got its numbers.
Why the Clocks Can Be Trusted (and Checked)
A common objection — "what if the starting amount or the rate changed?" — is answered with cross-checks. Scientists use multiple isotope systems on the same sample; if uranium, potassium and other independent clocks agree, the date is robust. They also validate against independently counted records like tree rings (dendrochronology) and annual lake-bed layers, and they screen for signs that a rock was heated or disturbed, which can reset or smear a clock. Agreement between physics, stratigraphy and independent archives is what turns "the Earth is very old" from an assertion into a measurement — the same kind of scale that makes the planet's extremes measurable in more than one way.