BETTER UNDERSTANDING THE EARLY ERUPTIVE HISTORY OF MT. ST. HELENS THROUGH IMPROVED MICROANALYSIS OF TEPHRA GLASSES

Widespread layers of volcanic ash (tephra) are used around the world for correlation and dating in geology and archaeology. Applications include Earth surface processes, past environmental and climate changes, volcanic hazards, and human history. Typically, ash layers are chemically fingerprinted and identified by analyzing glass shards on an electron microprobe. This approach, over time, has the potential to reduce chronological uncertainties in a variety of scientific disciplines.

Mt. St. Helens has a long history with many major eruptions and is a key source of volcanic ash. Because eruptions that were closely-spaced in time have very similar volcanic glass compositions, distinguishing ash from different eruptions can be challenging. Careful analytical work and relatively high precision data are needed.

At Concord, we have developed analytical routines that are up to the task. We have applied them to some of the earliest tephras from Mt. St. Helens. Analyses of glass shards from the "Set C" layers using the electron microprobe show small but reproducible differences in the abundances of several major and minor element oxides. In addition, the Cs and Cy layers, once thought to be possibly from the same eruption, are clearly different. For example, the Cs, Cy, and Cw layers, contain 0.9, 1.0, and 1.2 wt% FeO and 4.1, 4.5, and 4.3 wt% Na₂O, respectively. Additionally, Cs differs from both Cy and Cw by higher TiO₂ and MgO (0.05 and 0.15 wt%, respectively) while Cw contains 0.15 wt% lower K₂O than Cs and Cy. This electron microprobe approach can now be applied to other Mt. St. Helens ash samples from around the region. With further analyses we also hope to better understand the history of the volcano.

This research was conducted in Concord University’s Electron Microprobe Laboratory, a regional teaching and research facility supported by the WV Research Trust Fund and WV EPSCoR.