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Absolute Age Dating - Keynote pdf

Age of the Islands

Geologists must consider unthinkable expanses of time to understand geological processes, some of which take hundreds of millions of years to complete.
Geologists now believe the Earth and the solar system is 4.6 billion years old.

Two types of age dating are used in geology:

  1. relative age dating
  2. absolute age dating

Relative age dating uses geological principles to give relative ages.
In other worlds, which rock is older and which is younger.
Geologists place rocks and geological events in chronological order.

Absolute age dating yields the absolute age of rocks or geological events:
For example, the Waianae shield volcano stopped erupting three million years ago.
Absolute age generally determined radiometric dating techniques.

Absolute Age Dating

To date rocks absolutely, geologists need a geological clock.
A geological clock must

  1. occur at a constant rate
  2. accumulate its own record

A primary geological clock is radioactive decay.


Isotopes - atoms whose nuclei have the same number of protons, but different numbers of neutrons.
For example, all atoms of carbon have 6 protons.
The common carbon isotopes are C-12, C-13, and C-14.

Most isotopes are stable; however, some isotopes are unstable
Unstable isotopes undergo spontaneous decay - the nucleus breaks apart.
Radioactivity - the natural spontaneous decay of unstable nuclei.

Radioactive atoms decay to a more stable atom.
The nucleus of a radioactive atom spontaneously disintegrates, forming an atom of a different element and emitting radiation, a form of energy.


Rb-87 is a radioactive isotope of rubidium.
Rb-87 is unstable and decays to Strontium 87, Sr-87.

Rb-87 is the parent;
Sr-87 the daughter.

Rb-87 decays by emitting an electron.
One of the neutrons in Ru-87 releases an electron and is converted to a proton.
With the addition of a proton, the Ru-87 atom is converted to a Sr-87 atom.

The rate of decay is constant over time.
We assume only parent atoms were present in the minerals when they formed, then we count the number of parent and daughter atoms.
Using this ratio with the half-life of the radioactive isotope, the time since the formation of the mineral can be calculated.
Half-life - time required for 50% of the radioactive atoms to decay.

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