School βΊ Fossils βΊ Ages 15β16
Fossils and deep time
Modes of preservation and the biases of the fossil record; relative dating with superposition and index fossils; and absolute dating with radioactive half-lives. With a coin model of radioactive decay and age calculations.
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A fossil is evidence of life preserved in a geologic context. Most fossils are old (the BGS gives typically more than 10,000 years), although there is no fixed age.[5][3]
Preservation needs rapid burial, which prevents decomposition, weathering and scavenging, and favours durable hard parts. The chance that any one organism becomes a fossil is very small, and the record is biased: about 99% of fossils found are of marine animals, while insects, birds and upland organisms are rare.[4][1]
Modes of preservation include permineralisation (minerals from groundwater fill the spaces in cells; the most common), replacement (organic material is entirely replaced and the remains literally turned to stone), moulds and casts, compressions, and preserved remains in amber or ice, the rarest form.[6][3][4]
Relative dating orders events. In undisturbed layers the bottom layer was deposited first and is oldest (the principle of superposition, formally proposed by James Hutton). Because fossils record the progressive development of life, William Smith used them as markers to recognise a layer wherever it was exposed; geologists use fossils to identify rocks of the same age worldwide.[7]
Index fossils are forms of life that existed only during limited periods of geologic time, so they are guides to the age of the rock that contains them. Ammonites are excellent guide fossils.[8][3]
Absolute dating uses radioactive decay: a parent isotope decays into a daughter product. The half-life is the time for half the parent atoms in a sample to decay. Uranium-238 decays to lead-206 with a half-life of 4.5 billion years; potassium-40 decays to argon-40 with a half-life of 1.25 billion years.[9]
After n half-lives the fraction of parent left is (1/2)βΏ. In general t = (1/Ξ») ln(1 + D/P), where D and P are the numbers of daughter and parent atoms and Ξ» is the decay constant. The idea is simple, but measuring tiny amounts of isotopes precisely is difficult laboratory work.[9]
Try it: Coin model of radioactive decay
You need
- 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.
What to do
- Put 100 coins in the tray: each coin is an undecayed 'parent atom'. One shake = one time step.
- Shake the tray and tip the coins out. Remove every coin showing heads: these have 'decayed' into daughter atoms. Count the coins left.
- Repeat with the remaining coins until fewer than 3 are left, recording the count after each shake.
- Plot parent coins left (y) against number of shakes (x). Add the ideal curve 100 Γ (1/2)βΏ.
- From your graph, find the 'half-life' (shakes to fall from 100 to 50, and from 50 to 25). Is it constant?
- Pool the class results and compare the scatter of a single group with the class mean.
What happens, and why?
Each coin has the same chance of 'decaying' at every shake, so about half are left after each step, whatever the starting number: that is what a constant half-life means. Real isotopes decay spontaneously in the same way, which lets slowly decaying isotopes act as geologic clocks. Small samples scatter more than large ones.[9]
Play: Dating and preservation
Does each idea belong to relative dating, radiometric (absolute) dating, or fossil preservation?
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Quiz
Other ages: Ages 6β7 Β· Ages 10β11