Newberry volcano is a broad shield volcano located in central Oregon.
It has been built by thousands of eruptions, beginning about
600,000 years ago. At least 25 vents on the flanks and summit have
been active during several eruptive episodes of the past 10,000 years.
The most recent eruption 1,300 years ago produced the Big Obsidian
Flow. Thus, the volcano's long history and recent activity indicate
that Newberry will erupt in the future.
The eruptive sequence that culminated in the Big Obsidian Flow 1,300 years ago exemplifies several aspects of a typical rhyolitic eruptive sequence at Newberry volcano. The eruptions began with tephra showers that deposited pumice lumps and dense lava blocks as large as 1 meter (3 feet) within the caldera. These tephra deposits, which are thicker than 13 meters (43 feet) near the vent, diminish in thickness and grain size downwind. For example, 50 kilometers (30 miles) downwind from the caldera near Brothers, Oregon, these tephra deposits are 25 centimeters (1 foot) thick and have average grain size of 3 millimeters (0.1 inch). Newberry tephra can be traced as a fine-grained ash deposit as far east as Idaho. As the eruption progressed, pyroclastic flows swept downslope from the Big Obsidian vent to Paulina Lake. The boat ramp at Little Crater Campground is excavated in these pyroclastic-flow deposits, as is the caldera road upslope from Paulina Lake. The flows entered Paulina Lake, perhaps causing secondary steam explosions and displacing water from the lake into Paulina Creek.
The final stage of eruption produced the Big Obsidian Flow itself,
a lava flow that moved slowly, probably advancing only a few meters
or tens of meters per day as it oozed down an inner caldera wall and
ponded on the caldera floor. The Big Obsidian Flow is about
1.8 kilometers (6,000 feet) long and locally thicker than 20 meters (65 feet).
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