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Radioactive Dating• Measuring the relative amounts of radioactive elements and their

isotopes in a rock reveals the time since solidification.

• Radioactivity is prevalent in everyday life

– Stars forged virtually all of the atomic nuclei in the Earth.

– Exploding stars produce vast quantities of radioactive debris.

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Radioactive Dating• The Earth formed from this debris and incorporated a small portion

of radioactive material.

– The heat generated by the decay of this material still warms the Earth's interior to temperatures high enough for volcanic activity.

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Martian Volcanoes

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Radioactive Dating Solidification of a rock freezes in an inventory of radioactive

materials (and their non-radioactive isotopes) that then begins to decay. The radioactive dating clock begins ticking at this point.

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Radioactive Decay A radioactive atomic nucleus undergoing decay changes its

elemental and/or isotopic identity by the emission of a elementary particle.

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Radioactive Decay The emitted particle leaves the nucleus with tremendous energy

making it readily possible to detect the decay in a cloud chamber (below) or a Geiger counter.

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Half Life Over time, then, a radioactive sample becomes less radioactive

as unstable atoms decay to stable forms.

The decay is a random event for any atom resulting in ½ of the remaining radioactive atoms decaying in a fixed time interval.

Each radioactive isotope* has its own characteristic “half-life” 235U → 207Pb (704 million years)

87Rb → 87Sr (48.8 billion years)

14C → 14N (5730 years)

½ will remain after one half life

¼ will remain after two half lives

1/8 will remain after three half lives.

* The number of protons in a nucleus establishes the elemental identity (8 protons = oxygen), however the nucleus can carry different numbers of neutrons (16O, 17O, 18O)

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A Very Simple Example

Somebody hands you a brick of Uranium/Lead that they promise was pure Uranium some time ago.

Measuring the relative quantities of Uranium and Lead will reveal the age of the “rock”.

100% Uranium

50% Lead 50% Uranium50% Lead

25% Uranium

75% Lead

Brand New – Age 0 years

Two half lives - Age 1.4 billion years

One half life -Age 710 million years(pictured after refining to separate the components)

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Life is Complex Rocks do not come labeled “I was once pure Uranium”

In fact, the radioactive elements are tiny fractions of the whole sample.

Somehow one must determine how much of the radioactive element was there on “Day 0”.

235U decays to become 207Pb exclusively Lead with 82 protons and 125 neutrons.

Nature also makes 204Pb Without contamination from the decay of 235U the ratio of the

number of atoms of 204Pb to 207Pb is a constant.

Measure the amount of 204Pb in a sample and that tells you how much of the 207Pb was there on “Day 0” Any excess has come from the decay of 235U!

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The Age of the Earth and Solar System Simply find the oldest rocks and you have established the age of

the Earth?

That won't work because virtually every rock on the Earth's surface is geologically “young” The first rocks that solidified on Earth have long since been

destroyed – ironically because of the radioactivity heating the Earth's interior!

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The Oldest Rocks in the Solar System The asteroids are left-overs from the epoch of Solar System

formation.

They solidified at that time and have not re-melted since.

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The Oldest Rocks in the Solar System Meteorites are fragments of asteroids that have made their way

to Earth.

They, with a few very interesting exceptions, have radioactive dating ages of an absolutely uniform 4.56 billion years.

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Radioactive Dating – The Bottom Line The Solar System, Earth included, formed 4.6 billion years ago

as revealed by the uniform radioactive dating ages of meteorites.

The oldest rocks ever found on the Earth itself are closer to 4.3 billion years old, and most rocks on Earth are less than 1 billion years old. The Earth continually resurfaces and remelts its surface resetting

the radioactive dating clock.