USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Volcano Hazards in the Three Sisters Region, Oregon
--
W.E. Scott, R.M. Iverson, S.P. Schilling, and B.J. Fischer,
2001,
Volcano Hazards in the Three Sisters Region, Oregon:
U.S. Geological Survey Open-File Report 99-437
Summary
Three Sisters is one of three potentially active
volcanic centers that lie close to rapidly growing
communities and resort areas in Central Oregon.
Two types of volcanoes exist in the Three Sisters
region and each poses distinct hazards to people
and property. South Sister, Middle Sister, and
Broken Top, major composite volcanoes clustered
near the center of the region, have erupted
repeatedly over tens of thousands of years and
may erupt explosively in the future. In contrast,
mafic volcanoes, which range from small cinder
cones to large shield volcanoes like North Sister
and Belknap Crater, are typically short-lived
(weeks to centuries) and erupt less explosively
than do composite volcanoes. Hundreds of mafic
volcanoes scattered through the Three Sisters
region are part of a much longer zone along the
High Cascades of Oregon in which birth of new
marie volcanoes is possible.
This report describes the types of hazardous
events that can occur in the Three Sisters region
and the accompanying volcano-hazard-zonation
map outlines areas that could be at risk from such
events. Hazardous events include landslides from
the steep flanks of large volcanoes and floods,
which need not be triggered by eruptions, as well
as eruption-triggered events such as fallout of
tephra (volcanic ash) and lava flows. A proximal
hazard zone roughly 20 kilometers (12 miles) in
diameter surrounding the Three Sisters and
Broken Top could be affected within minutes of
the onset of an eruption or large landslide. Distal
hazard zones that follow river valleys downstream
from the Three Sisters and Broken Top could be
inundated by lahars (rapid flows of water-laden
rock and mud) generated either by melting of
snow and ice during eruptions or by large
landslides. Slow-moving lava flows could issue
from new mafic volcanoes almost anywhere
within the region. Fallout of tephra from eruption
clouds can affect areas hundreds of kilometers
(miles) downwind, so eruptions at volcanoes
elsewhere in the Cascade Range also contribute to
volcano hazards in Central Oregon.
This report is intended to aid scientists,
government officials, and citizens as they work
together to reduce the risk from volcano hazards
through public education and emergency-response
planning.
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03/03/06, Lyn Topinka