1
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
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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.
3
4
Martian Volcanoes
5
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.
6
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.
8
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”
U → 207Pb (704 million years)

235

87
Rb → 87Sr (48.8 billion years)

14
C → 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)
9
A Very Simple Example

Somebody hands you a brick of Uranium/Lead that they promise
was pure Uranium some time ago.
Brand New – Age 0 years
100% Uranium
50%
50%Lead
Lead
50% Uranium
One half life Age 710 million years
(pictured after refining to
separate the components)
75% Lead

25%
Uranium
Two half lives Age 1.4 billion years
Measuring the relative quantities of Uranium and Lead will reveal
the age of the “rock”.
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Life is Complex

Rocks do not come labeled “I was once pure Uranium”



Somehow one must determine how much of the radioactive
element was there on “Day 0”.
U decays to become 207Pb exclusively
235


Lead with 82 protons and 125 neutrons.
Nature also makes 204Pb


In fact, the radioactive elements are tiny fractions of the whole
sample.
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!
11
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.