1,300 Years Ago: Pumice and Ash First to Erupt

The most recent eruption about 1,300 years ago began with a series of explosive events in the summit caldera that formed layers of tephra downwind. Large blocks of pumice and dense lava fragments as large as 1 meter in diameter were deposited within the caldera. At this site on the east flank of the volcano, the explosive activity resulted in pumice deposits mostly less than 3 m thick! Ash from these eruptions have been traced as far east as Idaho. See figure 3 of the Newberry report for illustration of activity.

1,300 Years Ago: Pyroclastic Flows Erupted Next

As the intensity of the explosive activity increased, pyroclastic flows swept from the vent inside Newberry caldera and rushed into Paulina Lake, probably causing secondary explosions from the lake. Just beyond the margin of these pyroclastic-flow deposits, a layer of gray ash shown here atop older brown-colored volcanic rocks is evidence of a fast-moving ash cloud that raced ahead of the pyroclastic flow. See figure 3 of the Newberry report for illustration of activity.