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Tephrochronology
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. 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, in at least one bog in eastern North America, in ocean floor sediments, and more than 5,000 km from it's source in Greenland ice. 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 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 employed. 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 SiO2, TiO2, Al2O3, FeO, MnO, MgO, Na2O, K2O, P2O5, and Cl. Additional components (e.g. BaO, Cr2O3, SrO, V2O3, ZrO2, and F) may also be quantified when needed. For quality control purposes, secondary standard glasses including Lipari obsidian and BHVO-2g are routinely analyzed with the unknowns in the same analytical session. 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.
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