PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
  PUBLICATION_DATE    = 2009-11-15
  NOTE                = "Geologic setting description"
END_OBJECT            = TEXT
END

Big Obsidian Flow

Part of the Newberry Volcano complex, Big Obsidian Flow is a thick, viscous,
compositionally homogeneous rhyolite flow that erupted about 1300 years ago.
The Big Obsidian Flow is about 1.8 kilometers long and locally thicker than 20
meters. Surface deposits are glassy obsidian, showing a range of volatile
content.

Newberry volcano itself is a shield volcano built by flows dating back to
600,000 years ago, but Big Obsidian Flow is the most recent eruption.

The eruptive sequence that resulted in Big Obsidian Flow began with tephra
showers. These are up to 13 m thick near the vent, thinning but extending over
50 km downwind (thin deposits have been traced as far as Idaho). Pyroclastic
flows followed, extending from the Big Obsidian vent to Paulina Lake. The
flows entered Paulina Lake, perhaps causing secondary steam explosions and
displacing water from the lake into Paulina Creek.

The Big Obsidian Flow represents the final stage in this eruption. The lava
flow 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.


References

Laidley, R.A. and D.S. McKay (1971), Geochemical examination of obsidians from
Newberry Caldera, Jour. Contrib. Min. Petr., 30, 336-342, doi: 
10.1007/BF00404728.

Peterson, N.V. and E.A. Groh (1969), The ages of some holocene volcanic
eruptions in the Newberry volcano area, Oregon, Ore Bin, 31, 73-92.

Sherrod, Mastin, Scott, and Schilling (1997), Volcano Hazards at Newberry
Volcano, Oregon: U.S. Geological Survey Open-File Report 97-513