- The nuclei of some atoms (e.g. Uranium)
are unstable and will decay to become either
some nearby element in the periodic table or some
isotope of the same element.
207Pb -- (704 million years)
- 87Rb 87Sr -- (48.8 billion years)
- 14C 14N - (5730 years)
- The time in parenthesis is the ``half-life'' of the
specified process. The half-life represents the time required for 1/2 of the atoms in a sample to decay to the other state.
- Like popcorn, some atoms decay
right away, while others survive much longer than the average.
- There is no way to predict when a particular atom will decay. We know very precisely the probability that an atom will decay in a given time interval, but whether it really does depends on the roll of the cosmic dice.
- If you start with a pure sample of one of the above unstable
elements, after the time in parenthesis has elapsed, half of that
sample will have decayed.
- After one more half life, 3/4 of the original
sample will have decayed (i.e. half of the remaining half).
- If you are using Uranium/Lead dating, how do you determine how much lead was in the rock
originally??? A rock which is half uranium and half lead, may
be very young and just had a large amount of lead to start.
- Lead comes several stable forms -- one has 207 protons and
neutrons (the decay product of 235Uranium), and one with 204
- Uranium 235 exculsively decays to the form with 207 protons
- The ratio of 207Pb to 204Pb in naturally occurring lead in any rock (i.e. no contamination from the decay of 235U) is a constant.
- Measure the amount of 204Pb in the sample under
study and that measurement will tell you how much of 207Pb was there originally.
- Any excess 207Pb will have been produced by the decay of
- In practice, the radioactive dating
techniques used today measure the time
since a rock was last molten.
- The age of the Earth is 4.6 billion years.
according to radioactive dating techniques,
the ``ages" of rocks found on its surface are far younger.
material making up these rocks has been part of the Earth for all 4.6
- however all of these rock were
formed when molten material
on or beneath Earth's surface solidified only recently.
- The oldest rocks in the Solar System -- the ones which have not been melted since they were first formed -- are the meteorites which
fall to Earth from interplanetary space.
- The meteorites are pieces of the asteroids -- leftover fragments of
planetesimals from the era of the formation of the planets.
solidified early in the history of the Solar System and have not
- Jupiter stirred up the motions of the nearby planetesimals
and prevented them from accumulating into a planet.
- These stirred planetesimals collided at high speed and
fragmented. They were unable to stick together and
accumulate to form a large planet.
- These leftover planetesimal pieces now form the asteroid belt.
- Asteroids collide and fragment. Pieces (and sometimes
whole asteroids) can be gradually directed to the Earth via
gravitation nudges from the planets.
- When radioactive dating techniques are applied to meteorites
they are uniformly found to be 4.56 billion years old - which establishes the time of the formation of the planets.
Revised October 29, 2004