Most absolute dates for rocks are obtained by radiometric methods methods, which are based on the radioactive decay of certain chemical elements. Most radiometric methods directly measure the isotopes and their decay products in rock samples. Two exceptions are fission track and luminescence dating, which make use of changes that happen to materials surrounding some isotopes in rocks.
Fission track dating
This radiometric method is often used to date crystals and glasses in volcanic rocks that have cooled quickly, such as volcanic ash. The commonest minerals dated using this method are zircon and apatite.
Zircon is common in volcanic ash, and its crystals contain very small amounts of the uranium-238 isotope. As the uranium decays, subatomic particles split away – this process is called fission. These particles leave tiny tracks in the crystal structure of the zircon, which geologists count using a powerful microscope. The more tracks there are, the longer the uranium has been decaying for. High temperatures remove the tracks from the crystal, so when the ash leaves the hot volcano, its fission track ‘clock’ is at zero. Tracks start building up after the crystals have cooled and settled in a layer on the ground or at the bottom of the sea.
This measures how long certain minerals have been exposed to natural radiation. Many sediments contain crystals of the minerals quartz and feldspar, as well as very small amounts of radioactive elements. As these elements decay, radiation is trapped in the quartz and feldspar crystals. The longer the crystals are in the sediment, the more radiation is trapped.
Before the crystals were buried in the sediment, they were carried along by rivers or wind. During this time, they were exposed to sunlight, which wipes out any previously stored energy from radiation. This sets the geological ‘clock’ to zero.
Geologists collect samples of crystals from sediment, being careful to keep them away from sunlight. In a laboratory, heat or light is shone on the crystals, and this causes the stored radiation to be given off as light (in a process called luminescence). The longer the crystal had been buried, the brighter the luminescence.
When heat is shone on the sample, the dating method is called thermoluminescence. The method that shines laser light is called optically stimulated luminescence (OSL).