Earth History Lab

Radioactive parent isotopes decay into stable daughter isotopes at a rate defined by the half-life. The age of a sample is calculated from the ratio of parent to daughter atoms.

Where $N$ is the current parent amount, $N_0$ is the original total (parent + daughter), and $t_{1/2}$ is the half-life.

Choose an isotope system whose half-life matches the expected age range of your sample.

• ¹⁴C → ¹⁴N (t½ = 5,730 yr) - organic material up to ~50,000 years
• ⁴⁰K → ⁴⁰Ar (t½ = 1.25 Byr) - volcanic rocks, hundreds of millions of years
• ²³⁸U → ²⁰⁶Pb (t½ = 4.47 Byr) - oldest rocks and meteorites
• ⁸⁷Rb → ⁸⁷Sr (t½ = 48.8 Byr) - very ancient igneous and metamorphic rocks

Earth's magnetic field periodically reverses polarity. New oceanic crust at mid-ocean ridges records these reversals as symmetric magnetic stripes.

By measuring the distance from the ridge to a reversal of known age, you can calculate how fast the plates are moving apart. Typical rates range from 2 to 16 cm/year.

Radiometric dating provides the absolute ages that anchor the geologic time scale. Key boundaries include the following.

• Earth's formation - 4.54 billion years ago (meteorite dating with U-Pb)
• Oldest rocks - 4.03 Byr (Acasta Gneiss, Rb-Sr and U-Pb)
• Cambrian Explosion - 541 Myr (K-Ar dating of volcanic ash layers)
• Dinosaur extinction - 66 Myr (K-Ar dating of impact layer)
• Last Ice Age peak - 20,000 yr (¹⁴C dating of organic material)