SPEED DATING! TEPHROCHRONOLOGY AS A GEOLOGIC DATING TOOL – TIME TESTED AND PROVEN!

Tephrochronology is the study of volcanic ash deposits, combining petrology, geochemistry, stratigraphic correlation, & isotopic dating methods. Volcanic glass in tephra derived from a given source & point in time has a unique composition & chemical fingerprint that can be used to determine its eruptive source, age, & areal distribution. In turn, these data permit development of local & regional, high-resolution temporal & spatial stratigraphic frameworks. Accurate & precise chronostratigraphic frameworks are critical for mapping, correlation of marine, lacustrine, fluvialtile, & terrestrial sections, environmental reconstructions, determining the periodicity of natural hazards such as earthquakes, & other geologic investigations.

The USGS Tephrochronology Project collaborates with numerous government & academic partners by providing tephra analyses & data interpretation through the use of our extensive databases, which contain sample data collected over more than four decades. Our western U.S. collection alone consists of ~7,700 samples. We typically characterize volcanic glass shards using petrography & using electron microprobe analysis (EMA) to measure the abundances of major & minor elemental oxides such as SiO₂, Al₂O₃, Fe₂O₃, CaO, Na₂O, K₂O, MgO, MnO, TiO₂. A correlation matrix is then used to compare EMA results to all previous analyses to identify the best matches & generate chemical correlatives. Geologic context is critical in all tephrochronologic evaluations. Thus, our tephrochronologic correlations are also based on the stratigraphy & paleontology of study sites to refine a framework from 3-D to 4-D. Geochemical correlations to sites where numerical ages for strata are available permit further chronostratigraphic refinement & a more accurate geologic reconstruction of a region.

Glass shards in tephra from multiple eruptions of one volcano can have highly similar compositions. The limitations of EMA can result in indeterminate tephrochronologic interpretations. For example, the glass compositions of the 2.06 Ma Huckleberry Ridge & the 0.62 Ma Lava Creek B ash beds are often indistinguishable. In these cases, data using trace element methods such as INAA, ICP-MS, etc., or from another relative dating technique, e.g., magnetostratigraphy are used to make definitive correlations.