1. Historical Records and Genealogies. (6,000 yrs)
2. Cooling of hot iron spheres (>75,000 yrs)
3. Rates of deposition of sediments. (1 million to 1 billion years)
4. Ocean Salinity. (90 million years)
5. Actual measurements of cooling of the earth (20-100 million years; Lord Kelvin, 1866.)

All of these attempts were refuted (in early 1900's) by discovery of radioactivity, rocks contain natural 'clocks' (isotopes) in them which tick away (decay) at a constant rate. Isotopes are atoms of the same element with different number of neutrons in the nuclei. Radioactive isotopes are unstable and decay (spontaneously transform) into a different stable element. Different elements decay at different rates. The time it takes for one-half of the parent atoms to decay to stable daughter elements is called the half-life

Parent/daughter elements half-life

*Note that Carbon dating is restricted to organic matter and also to very young events because of its very short half-life.

Radioactive isotopes are essentially natural clocks which begin ticking when the mineral they are in first forms.

To date a rock, we need to know:

1. Parent/daughter ratio (measured in a mass spectrometer)
2. half-life (known from lab measurements)

You may recall that geologists classify rocks according to how they form and that using these criteria, there are only three rock types:

Igneous = a rock which forms by minerals crystallizing out from a melt.

Sedimentary = a rock which forms from the breakdown of other rocks.

Metamorphic = a rock which has undergone a change in temperature and pressure.