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Earth History Lab

Determine the ages of rock samples using radiometric dating across four isotope systems. Measure seafloor spreading rates from magnetic reversal patterns at mid-ocean ridges. Collect data, record observations, and export complete lab reports.

Guided Experiment: Radiometric Dating of Rock Samples

Given 5 rock samples with different parent/daughter ratios and different isotope systems, can you calculate each sample's age and arrange them chronologically?

Write your hypothesis in the Lab Report panel, then click Next.

Isotope Composition

¹⁴C 50.0%¹⁴N 50.0%Parent (¹⁴C)Daughter (¹⁴N)Sample Composition

Controls

Parent Remaining50.0%
¹⁴C → ¹⁴N | Half-life: 5.73e+3 years

Results

t=−t1/2×ln⁡(N/N0)ln⁡2t = -t_{1/2} \times \frac{\ln(N/N_0)}{\ln 2}
Calculated Age
5.7 thousand years
Half-lives Elapsed
1.00
Parent
50.0%
Daughter
50.0%
Half-life
5.73e+3 yr
¹⁴C → ¹⁴N

Radioactive Decay Curve

Parent isotope Daughter isotope

Data Table

(0 rows)
#TrialIsotope SystemHalf-life (yr)Parent (%)Daughter (%)Calculated Age (yr)Actual Age (yr)Error (%)
0 / 500
0 / 500
0 / 500

Reference Guide

Radiometric Decay Formula

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.

t=−t1/2×ln⁡(N/N0)ln⁡2t = -t_{1/2} \times \frac{\ln(N / N_0)}{\ln 2}

Where NN is the current parent amount, N0N_0 is the original total (parent + daughter), and t1/2t_{1/2} is the half-life.

Isotope System Selection

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

Magnetic Reversals and Spreading

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

Rate=distance between reversalsage of reversal\text{Rate} = \frac{\text{distance between reversals}}{\text{age of reversal}}

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.

Geologic Time Scale

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)