THE 1,100 YBP ERUPTION OF TEPHRA C AT AUGUSTINE VOLCANO, ALASKA: VARIATIONS IN ERUPTIVE STYLE AND MAGMA SOURCES AT AN INTERMEDIATE ARC VOLCANO

Augustine Volcano in Alaska is an excellent natural laboratory for investigating volcanic evolution, as its Holocene eruptive activity has included large, lapilli-producing eruptions and small, ash-dominated eruptions (Waitt & Begét, 2009). This work compares the large 1,100 ybp eruption of Tephra C to the smaller, modern eruptions at Augustine including those that occurred in 1986 and 2006.

We sampled Tephra C from two proximal deposits on Augustine. The deposit was sub-sampled at both sites to characterize changes through the course of this eruption, including whole rock geochemistry, sample componentry, grain size distribution, and microlite number density.

Pumice clasts from Tephra C range from andesite to dacite, consistent with mixing of mafic and felsic magmas, and similar to tephra from the modern eruptions of Augustine. There are subtle differences, however. Tephra C pumice contains lower K₂O than pumice erupted in 1986 and 2006, and MnO is similar in high-silica andesite-dacite from all three eruptions but is lower in the lower-silica andesite in Tephra C compared to 1986 and 2006. This suggests that Tephra C may have different mafic parents than the modern eruptions. Stratigraphic sub-samples indicate at least three eruptive pulses or paroxysms in the eruption of Tephra C. This includes the visibly darker middle layer, rich in banded pumice with lower mean grain size, which we interpret as a mid-eruption mixing event, driving a lower-explosivity paroxysm. A return to higher explosivity in the later stages of the eruption, indicated by a larger mean grain size and a greater proportion of high-silica andesite, contrasts with the behavior observed in 2006, when the eruption did not return to high explosivity once the proportion of high-silica material decreased (Larsen et al., 2010).

Our results indicate that changes in explosivity during the eruption of Tephra C were accompanied by subtle changes in eruptive products. Furthermore, tephra erupted through the course of the Tephra C eruption illustrates that periods of lower explosivity and a decrease in high-silica components can be followed by a return to higher explosivity. In monitoring future eruptions at Augustine, transitions to lower-silica, “weaker” eruptive behavior should not be considered a sure sign of eruption end.