Tephrochronology is the study of volcanic ash deposits.
Volcanic ash layers often have unique chemical and physical characteristics that can be used for correlation.
The basic science behind this method is that calcareous shell material incorporates the two strontium isotopes in the same ratio that occurs in seawater at the time the organism was alive.
At different times in Earth's history, the relative abundance of these two isotopes in seawater gradually changed through time (such as during the Permian, the Late Cretaceous, and parts of the Tertiary).
In addition to the chemical and physical characteristics of volcanic ash, select igneous minerals in the ash can be used for absolute dating (discussed below).