DECIPHERING THE EARLY ERUPTIVE HISTORY OF MOUNT ST. HELENS USING MICROANALYTICAL GEOCHEMISTRY

Tephra deposits provide key information for documenting volcano eruptive histories, and to understand volcanic processes and hazards. Widespread tephra layers from major eruptions are used around the world for correlation and dating in geology and archaeology. Applications include earth surface processes, past environmental/climate changes, and human history.

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

We are working with samples of set C tephras (approximately 35 to 50 ka) collected in 2008 from key proximal reference sites described by Mullineaux (1996 USGS PP1563) including Road 25, Road 81, Road 90, and Muddy River Quarry as well as several reference samples collected by Mullineaux himself. We also have medial to distal samples of set C and older tephras from the 220,000 year sediment record at Carp Lake, Washington (Kuehn, 2014 AMQUA Meeting), from Cape Horn Creek and Green Mountain, Washington and from other locations in eastern Washington, eastern Oregon, and southern Idaho.

We are using glass geochemistry obtained via the electron microprobe laboratory at Concord University to distinguish and correlate different eruptions across the various locations. The data obtained on proximal samples to date distinguishes samples in a way that disagrees with previously published identifications of eruptive units. With further geochemical analyses we expect to clarify the number and sequence of set C and older eruptions of Mount St. Helens.