What is Tephrochronology?

Tephrochronology is the use of volcanic ash and pumice (tephra) as a tool for dating and correlation. Tephrochronology is employed globally with numerous interdisciplinary applications including: environmental and climate change, archaeology, Earth surface processes, ecology, animal and plant evolution, earthquake hazards & neotectonics, volcanic hazards, and even medicine. In recent years, tephrochronology has been a fast-growing discipline, in part because it is considered one of the few techniques with the potential to significantly reduce chronological uncertainties in archaeological and environmental research. Tephrochronology has also expanded to include trace concentrations of volcanic ash invisibly hidden in sediments. Such cryptotephra deposits extend the range of tephra correlation to sites thousands of kilometers away from volcanic sources, even enabling correlations across oceans and between continents.

Because volcanic ash is rapidly and widely dispersed during large, explosive eruptions, tephrochronology provides a unique capability to tie together records of, for example, environmental change over long distances and connect land, lake, sea, and glacial ice records with a temporal resolution that is largely unmatched by other dating techniques. For example, ash from the large eruption which formed Crater Lake in Oregon has been identified at hundreds of locations in western North America, at a few sites in eastern North America, in ocean floor sediments, and more than 5,000 km from it’s source in Greenland ice. Similarly, White River ash from Alaska has been found as far away as Europe.

The electron microprobe is the primary analytical tool for tephrochronology. It is most commonly used to analyze the glass fraction of tephra for major and minor element abundances and thereby provide a chemical fingerprint which often allows ash from different eruptions to be uniquely identified. Mineral abundances, mineral compositions, particle size and shape, layer thickness, trace-element abundances in the glass, stratigraphic relations, and dating are also used. Tephra correlations are most robust when multiple lines of evidence are combined.

The CU Tephra Lab

The tephra lab uses the ARL-SEMQ microprobe as the primary tool for chemical fingerprinting. Samples are typically mounted using low-viscosity epoxy in a 2.54 cm / 1 inch diameter acrylic disk, polished, carbon coated, and then placed in the microprobe for analysis. Samples are routinely analyzed for eleven major and minor elements/oxides: SiO2, TiO2, Al2O3, FeO, MnO, MgO, Na2O, K2O, P2O5, BaO, and Cl.  A separate analytical procedure is also available which includes twenty major, minor, and trace elements/oxides: SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O, P2O5, Cl, BaO, CoO, Cr2O3, NiO, Rb2O, SO3, SrO, VO2, and ZrO2. For quality control purposes, four reference glasses (Lipari obsidian ID3506, BHVO-2g, NKT-1g, and orthoclase glass) are routinely analyzed with tephra samples in the same analytical session. To enhance accuracy and precision, several advanced techniques are also employed, including a time-dependent intensity correction for sodium, mean atomic number backgrounds, combined EDS+WDS acquisition, blank corrections, and spectral interference corrections.

The resulting data may then be used to identify tephra samples by comparison with a large database containing analyzes from thousands of tephra samples, mostly from North America. The laboratory also has a large reference collection including, for example, proximal samples of most major tephra-producing eruptions of Mt. St. Helens (WA) and Newberry Volcano (OR) and samples from key distal reference locations like Summer Lake (OR). In cases where there are several potential matches with very similar chemical fingerprints, the unknown sample and reference samples may be analyzed together in the same session on the microprobe for confirmation.

Tephra identification is available as a service to external clients. Please contact us for further information.

Analytical Accuracy and Precision

Based on analysis of reference glasses with a range of compositions, the accuracy, precision, and long-term reproducibility of the CU tephra lab’s data is excellent. For some elements, the current analytical precision exceeds that of most tephra labs which submitted data to the 2011 INTAV interlaboratory comparison (Kuehn et al., 2011). The CU tephra lab is also one of only a few which routinely reports data for both P and Ba. Several examples are shown below. A complete reporting of CU tephra data compared to the INTAV submissions is also available. For tabulated results from a number of international reference glasses, see the Quality Control page of this website.