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.The apatites taken from volcanic rocks yield crystallization ages.
Two exceptions are fission track and luminescence dating, which make use of changes that happen to materials surrounding some isotopes in rocks.
This radiometric method is often used to date crystals and glasses in volcanic rocks that have cooled quickly, such as volcanic ash.
Disadvantages include few tracks in samples less than 100,000 years old, and annealing of tracks in glass at ambient surface temperatures.
Quaternary materials dated by fission tracks include volcanic ash, archaeological materials, tektites and impact glass, and natural clinker.
During this time, they were exposed to sunlight, which wipes out any previously stored energy from radiation. 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).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 method that shines laser light is called optically stimulated luminescence (OSL).