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FIELD STATION · EARTH MATERIALS

Geologic Time & Fossils Lab

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CHOOSE YOUR INVESTIGATION LEVEL

How do we know how old rocks and fossils are?

Both levels use the same model: a cliff whose clues put its events in order, radioactive clocks that give ages in years, and fossil ranges that place a layer on the ICS geologic time scale. AP adds the equations, uncertainties, fractional half-lives and period durations.

About this model

Relative dating (① Outcrop). Each cliff comes from Codex’s generator: two to four sedimentary beds, a dike D, a fault F, an erosion surface U and a cap layer Z, with clues for superposition, inclusions, cross-cutting and truncation. The model assumes the beds are intact, began horizontal and are not overturned. Any history that obeys every clue is accepted; where no clue links two events (D and F in every cliff) their order is undetermined. The cliff picture is a teaching block, not to scale: thicknesses, positions, the fault slip and the erosion depth are drawing choices.

Radiometric dating (② Decay clock). An ideal closed sample with a known starting parent amount: N = N₀ · 2−t/T, so t = T · log₂(N₀/N). Half-lives (NUBASE2020): carbon-14 5,700 ± 30 yr (the site-wide value), potassium-40 1.248 ± 0.003 Gyr, uranium-235 704 ± 1 Myr, uranium-238 4.463 ± 0.003 Gyr. The model does not include contamination, daughters present at the start, closure temperatures, reservoir effects or calibration: a carbon-14 age here is a physical age, not a calendar date. Potassium-40 ages use the parent fraction only (89.28 % of K-40 decays give Ca-40, 10.72 % give Ar-40), never a K-Ar daughter ratio. The 400 nuclei are a magnified cartoon; the count drawn is the model’s fraction × 400. Carbon-14 samples are limited to 50,000 years; long-lived clocks to 4.5 billion years. A uranium or potassium age dates when a mineral crystallised, not when nearby sediment or fossils were laid down.

Fossils and time (③). Species A, B and C are hypothetical teaching species with made-up ranges, not real animals; fossils are assumed to be in place (not reworked). The shared window is the overlap of the ranges: oldest = the youngest of the older limits, youngest = the oldest of the younger limits. Period boundaries are the nominal ages on the ICS International Chronostratigraphic Chart, version 2026/06; real boundaries are defined at reference sections (GSSPs) and carry uncertainties this model does not show. An age exactly on a boundary is assigned to the younger period. Period colours follow the ICS/CGMW chart convention (approximate). The Precambrian bar uses eon boundaries from the same chart and Earth’s age of 4.54 billion years (USGS) for scale only.

Randomness. Every cliff, sample and site pair comes from a seed (shown under Advanced) so it can be repeated exactly. The tests draw their questions with a seed per attempt.

ICS International Chronostratigraphic Chart · IAEA AMDC: NUBASE2020 · USGS: relative time scale · USGS: radiometric time scale · USGS: fossils

What am I looking at?

How the lab works. Pick Explore or one lab at the top. Each lab has five tabs: Pre-lab, Bench, Data & graph, Claim · Evidence · Reasoning, and Test. On the Bench, the coach below the scene lists your steps; they tick themselves as you work. Press What do I do now? when you’re stuck.

① Outcrop. A block of rock cut open like a cliff. Lettered beds are sedimentary layers (each rock type has its own pattern). The dark sheet is a dike of igneous rock; the dashed line is a fault (the half-arrows show which way each side slipped); the wavy brown line with pebbles is an erosion surface. Numbered amber rings are clues: tap one to magnify it. Build a history with the event cards and press Test my history to watch the cliff form in your order.

② Decay clock. Left: the sample and a magnified view of 400 of its nuclei. Coloured spheres are parent atoms; grey cubes are daughters (for potassium-40, small blue triangles are argon-40). Right: the fraction of parent left against time, in half-lives with the years underneath. The amber dashed line is the measured amount in your sample.

③ Fossils & time. Two rock columns (Site 1 and Site 2) with fossils of hypothetical teaching species. The range chart shows when each species lived (older is lower). The amber band is the window you set with ▲ and ▼. The coloured strip is the ICS geologic time scale; zoom it with Z / X.

Keys: Space play / pause · → step · R reset · Z / X zoom.