Tephra beds are ideal stratigraphic markers because they are deposited instantaneously; they may be dated by of horizons in volcanic regions by identification of different layers of ash (tephra).
Tephra layers (beds) are ideal stratigraphic markers because they are deposited instantaneously.
Potassium, which is present in most rocks and minerals, has a single , K 40.
This decays by two different processes into Calcium 40 and Argon 40.
Dates produced by using this technique have been checked by .
The technique is best used on material more than 100,000 years old -- such as the dating of layers associated with the earliest remains of hominids, notably in the Olduvai Gorge.
The basis for this technique is that a uranium isotope, U 238, as well as decaying to a stable lead isotope, also undergoes spontaneous fission. Fission is accompanied by an energy release which sends the resulting two nuclei into the surrounding material, the tracks causing damage to the crystal lattice.
These tracks can be counted under a microscope after the polished surface of the sample has been etched with acid.
Also, the chemical content of tephra (volcanic ash) is unique for each eruption.
If artifacts lie below tephra known to have come from a certain eruption, the artifacts predate the eruption.
The concentration of uranium can be determined by the induced fission of U 235 by neutron irradiation of the sample.