Teacher sheet · Ages 15–16 · about 45–50 minutes
Fossils and deep time
Lesson page: echoesof.science/school/fossils/ages-15-16/ · Pupil worksheet: echoesof.science/school/fossils/ages-15-16/print/
Learning aims
- Describe modes of fossil preservation and the biases of the fossil record.
- Use superposition and index fossils for relative dating.
- Use half-lives to calculate absolute ages.
- Model radioactive decay and discuss random scatter.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why is a jellyfish fossil front-page news, but a shell fossil is not? |
| 10 min | Read the text; worked example of a half-life age calculation. |
| 15 min | Coin model and graph. |
| 5 min | Play Dating and preservation. |
| 10 min | Quiz; discuss what each dating method can and cannot tell us. |
Materials for the experiment
- 100 coins (or 100 cubes with one marked face) per group
- A tray or box with a lid
- Graph paper or a spreadsheet
Safety: Keep coins in the tray and pick up any dropped ones so nobody slips. Wash hands after handling coins.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Radiometric dating is done on the fossil bone itself. | Long-lived isotopes such as uranium-238 and potassium-40 are used to date rock and mineral specimens; fossils can then be placed in order using superposition. |
| After two half-lives nothing is left. | After each half-life half of what remains decays: 1/2, then 1/4, then 1/8 … |
| The fossil record is a complete picture of past life. | It is incomplete and biased towards hard parts and marine environments. |
Worksheet answers
- Read the shakes needed to halve the number (about 1); the process is random, so small samples scatter around (1/2)ⁿ, and pooled class data lie closer to it.
- The ammonites are older than 150 million years, because by superposition the lower layer was deposited before the ash layer above it.
- 12.5% = (1/2)³, so three half-lives: 3 × 704 = 2,112 million years, about 2.1 billion years.
- Four layers labelled oldest at the bottom; the same index fossils drawn in matching layers of a second column, with lines joining layers of the same age.
Quiz answers
- A mineral has 25% of its original potassium-40 left. The half-life is 1.25 billion years. How old is it? 2.5 billion years. 25% = (1/2)², so two half-lives: 2 × 1.25 = 2.5 billion years.[9]
- A mineral contains equal numbers of lead-206 and uranium-238 atoms. About how old is it? 4.5 billion years. D/P = 1 means one half-life has passed: 4.5 billion years.[9]
- In undisturbed rock layers, which layer is oldest? The bottom layer. The bottom layer was deposited first.[7]
- What makes a good index fossil? It existed only during a limited period of geologic time. A short time range pins down the age of the rock.[8]
- Which is the rarest form of fossil preservation? Preserved remains, such as an insect in amber. The BGS calls preserved remains the rarest form.[3]
- Why are insects and birds rare in the fossil record? Their remains are fragile and rarely buried quickly in sediment. The record is biased towards hard parts buried in water-laid sediment.[4]
Going further
Rearrange t = (1/λ) ln(1 + D/P) using λ = ln 2 ÷ half-life, and check that D/P = 1 gives one half-life and D/P = 3 gives two.
Sources used in this lesson
- [1] How are fossils formed? (Natural History Museum, London): reliability High
- [3] Fossils (British Geological Survey (BGS)): reliability High
- [4] How Fossils Form (US National Park Service (NPS)): reliability High
- [5] What is a Fossil? (US National Park Service (NPS)): reliability High
- [6] How do fossils form? (Australian Museum): reliability High
- [7] Geologic Time: Relative Time Scale (US Geological Survey (USGS)): reliability High
- [8] Geologic Time: Index Fossils (US Geological Survey (USGS)): reliability High
- [9] Geologic age: using radioactive decay to determine geologic age (US Geological Survey (USGS)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/fossils/sources/
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