Simulic
English

Fossils and dating – rock-strata dig, index fossils and radiometric dating

BiologyEvolutionAges 17–18

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Excavate two rock sequences layer by layer, record which fossils appear and disappear, and use the law of superposition and index fossils to order the layers, correlate two sites and spot an unconformity. Then date volcanic ash and organic remains on carbon-14, potassium–argon and uranium–lead decay curves to bracket the ages of fossil layers, and see how fossils form, why the record has gaps, and the geological time scale with the big five mass extinctions.

Lesson: Fossil record: relative dating, radiometric dating and the geological time scale

What it shows

Fossils are evidence for evolution, but only once we know how old they are. In this virtual dig, students excavate two composite rock sequences, record the fossils in each layer and build a range chart showing species appearing and disappearing. The law of superposition puts the layers in order; index fossils such as graptolites and ammonites link layers at the two sites and reveal an unconformity. Radiometric dating of volcanic ash (K–Ar, U–Pb) and of organic remains (carbon-14) then brackets the absolute ages. Further views show how fossils form, why the record has gaps, and the geological time scale with five mass extinctions.

How to use

Choose a Rock sequence, then press Dig at A or Dig at B, or tap a layer, to excavate from the top down; the table and range chart fill in. Tick Link matching layers to correlate the sites. In Radiometric dating, pick a Sample and a Method and press Measure, repeating to average the results. How fossils form has a Play button and condition boxes; on the Geological time scale, tap an event.

Parameters you can change

  • View Rock-strata dig, Radiometric dating, How fossils form, Geological time scale
  • Rock sequence Paleozoic (marine), Mesozoic – Paleogene, Quaternary (lake and cave)
  • Link matching layers at the two sites
  • Show layer ages (answer)
  • Dating method Carbon-14, Potassium–argon, Uranium–lead
  • Students calculate (hide age)
  • How the fossil forms Mineralised bone, Mould and cast, Amber, Trace fossil (footprints)
  • Time span of the time scale Last 541 million years, Whole history of the Earth

Questions to explore

  1. Why is a lower rock layer usually older than the layer above it, and when might this not be true?
  2. Why can carbon-14 not be used to date a dinosaur bone?
  3. How do index fossils show that some layers are missing at Site B?

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