A tephra layer is a deposit of volcanic ash, pumice, and other pyroclastic materials that settles out of the air following an explosive volcanic eruption. These layers are significant in stratigraphy and geochronology for dating geological and archaeological events, correlating deposits across regions, and studying past volcanic activity.
Reference: Thorarinsson, S. (1981). “Tephra Studies and Tephrochronology: A Historical Review with Special Reference to Iceland.” Journal of Quaternary Science, 20(1), 3-21.