An Introduction to Tephrochronology and the Correlation of Sedimentary Sequences Using Volcanic Ash Layers

Explosive volcanic eruptions generate plumes of hot gas and quenched molten rock that has been fragmented by the expansion of gas as the magma exits the vent. These fragments are called pyroclasts . The tephra layers are comprised of volcanic glass, crystals, and lithic material. Given that tephra is dispersed over wide areas and forms a geologically instantaneous layer, these tephra layers can be particularly useful for chronology – providing a relative chronology between sites and age if the eruption has been dated using radiometric methods. Correlating volcanic ash layers between sites and to specific eruption deposits preserved at their source volcanoes can be achieved using the composition of the volcanic glass shards. The major and trace element glass compositions remain the same for a specific eruption deposit irrespective of the distance from the vent, and they constitute the chemical fingerprint of the tephra. Volcanic deposits can be dated using many commonly employed radiometric dating methods.