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Introduction
Three Sisters is one of
three active volcanic centers that lie close to rapidly growing communities
and resort areas in Central Oregon. The
major composite volcanoes of this area are clustered near the center of the
region and include South Sister, Middle Sister, and Broken Top. Additionally,
hundreds of mafic volcanoes are scattered throughout the Three Sisters area.
These range from small cinder cones to large shield volcanoes like North
Sister and Belknap Crater. 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.
Scientists at the Cascade Volcano
Observatory created a GIS data set which depicts proximal and distal lahar
hazard zones as well as a regional lava flow hazard zone for Three Sisters OFR 99-437 Scott and others, 1999) The various distal
lahar zones were constructed from LaharZ software using 20, 100, and 500
million cubic meter input flow volumes. Additionally, scientists used the
depositional history of past events in the Three Sisters Region as well as
experience and judgment derived from the study of volcanoes to help construct
the regional hazard zone.
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