Whose measurements first showed that some volcanic ash layers can be traced worldwide?

Icelandic geologist Sigurdur Thorarinsson was a pioneer in studying volcanic ash, or tephra, as a geological time marker. By comparing ash layers from eruptions such as Hekla and Katla, he showed that thin deposits could be traced over wide regions and used to date sediments and archaeological sites. This method, called tephrochronology, became extremely valuable in geology and climate studies. It helps scientists correlate layers from different places and reconstruct eruption histories. Thorarinsson's work made volcanic ash a powerful scientific tool rather than just a hazard.