Augustine Volcano, located in Alaska’s Cook Inlet, is one of the region’s most active volcanoes. Although recent eruptions, like the one in 2006, have been relatively small, deposits from the past 2,000 years reveal five much larger explosive eruptions. This study focuses on three of those major events—Tephras B, M, and C—which occurred between 390 and 1,100 years ago.

By analyzing the chemistry and grain sizes of volcanic ash and pumice (tephra), we can learn how eruption styles and magma sources have changed over time. The tephra from these older, powerful eruptions contains a higher proportion of high-silica magma than what was erupted in 2006. In particular, Tephras M and C show more “banded pumice,” which forms when different types of magma mix quickly during an eruption—possibly making the eruption more explosive.

Chemical analysis also shows that the magma in these older eruptions was influenced by the crust beneath the volcano, likely a remnant of an ancient volcanic arc. The largest eruption, Tephra M, includes a wide variety of magma compositions, possibly marking a transition in magma sources. This research helps us better understand what drives explosive volcanic activity at Augustine.