GMEG - Geology and Geophysics
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Tephrochronology is the study of volcanic ash deposits, combining petrology, geochemistry, and isotopic dating methods. The Tephrochronology Project is part of the U.S. Geological Survey (USGS) Geology & Geophysics (G & G). The Tephrochronology Laboratory (Tephra Lab) is situated in the McKelvey Building (Building 15), in Menlo Park, California.
The mission of the Tephrochronology Project is to research, develop, and provide chronostratigraphic information based on studies of volcanic ash layers and tuffs (tephra layers) to various USGS programmatic investigations. The Project also collaborates on a wide range of studies with numerous external agencies (such as universities, and local, state, and other federal agencies).
The Tephrochronology Project provides age and stratigraphic information based on studies of volcanic ash layers and tuffs (tephra layers) to various earth science investigations primarily in the western U.S. The project provides stratigraphic correlation and age control to:
|Diagram showing the origin of tephra deposits from a volcanic eruption.
(Image source: USGS Cascades Volcano Observatory website)
|Example of a tephra layer exposed on Mount Rainier, WA.
(Image source: Western Minerals and Environmental Research Science Center website)
Tephra layers are collected at critical locations where age control is required. Then, back in the Tephrochronology Laboratory, the samples are processed and physical characteristics (such as glass shard morphology and mineralogy) of the tephra components are described, and the presence and characteristics of other components (lithic fragments, cements, microfossils*, etc.) are noted. Next, the volcanic glass is separated from the rest of the tephra and analyzed by one or more chemical techniques to determine a compositional "fingerprint." Finally, computer matching software compare this fingerprint with our digital database of approximately 5,900 previously analyzed samples to identify the best matches, and a pool of candidates is generated for correlative samples. The analyzed tephra samples are evaluated in terms of petrographic, stratigraphic, and chronologic criteria to identify the best matches.
These procedures permit correlation of tephra layers at critical sites to other localities where the same tephra layers are present, and often to sites where the ages of the layers have been previously determined by one or more numerical dating methods. New tephra layers are analyzed and dated at those sites where the best materials for dating can be obtained.
The results of our tephrochronologic research are combined with other chronostratigraphic data (for example, isotopic ages, magnetostratigraphy, oxygen-isotope chronostratigraphy, and stratigraphic sequence information) to develop a four-dimensional spatial and temporal chronostratigraphic framework for Neogene sediments and rocks in the western U.S. and the Pacific margin.
(*Tephra Project scientists are also capable of providing micropaleontologic support in the forms of Cenozoic planktic and benthic foraminiferal biostratigraphy and analyses of foraminifers and pollen as proxies for shifting paleoenvironmental conditions. These capabilities permit further refinement of preliminary chronostratigraphic frameworks and reconstructions of past climate.)
In addition to our expansive digital archives of analytical (Electron Microprobe, XRF, INAA, Ion Microprobe (SHRIMP-RG), ICP-MS, etc.), descriptive, and sample locality data, and physical collections of ~7,300 raw outcrop and core samples and finished volcanic glass separates mostly from the western and central regions of the United States, the Tephrochronology Project also is a repository for tephrochronological reference materials from Alaska, New Zealand, Mexico, and Central America, and parts of Europe and Asia. We also hold in our collections Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) core samples from the northeastern Pacific Ocean, and numerous core samples from many lakes located throughout the western U.S.
Highly important collections include volcanic material collected by then Tephra Project Chief Scientist Andrei Sarna-Wojcicki just prior to, and shortly after the May 18, 1980 eruption of Mount Saint Helens in Washington. Original field notes on the nature and impact of the tephra fallout from that, and subsequent eruptions are also available for reference and study.
Another key reference set consists of 19 discrete Quaternary tephra layers in stratigraphic sequence from the Wilson Creek section of Mono Craters, CA. These tephra samples and the results from our own studies have been, and are of very great interest to many investigators working in the region on numerous studies in need of age control. Researchers requesting tephrochronologic support include from scientists from the USGS Long Valley Observatory of the Volcano Hazards Science Center, Geology of Federal Lands in the eastern Sierra Region project, SW Great Basin Project, among many others.