USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Small Explosions Interrupt 3-year Quiescence at Mount St. Helens,
Washington
--
Bobbie Myers, 1992,
Small Explosions Interrupt 3-year Quiescence at Mount St. Helens,
Washington: IN:
Earthquakes and Volcanoes, v.23, n.2, p.58-73
(Observations and data reported in this article are from work by
the author and others at the Cascades Volcano Observatory and the
University of Washington Geophysics Program.)
Introduction
On December 11, 1989, geologists working in the crater at Mount St.
Helens discovered two thin layers of ash separated by fresh snow -
clear evidence that at least two small explosions had occurred
recently. The explosions were neither seen nor heard, but on
December 7 scientists suspected that a small ash-producing
explosion had occurred when seismometers near the volcano recorded
a long explosion-like signal and tilt and displacement meters
showed minor deformation of the dome. There were no other large
seismic signals to account for the second ash layer, which was most
likely associated with one of several smaller signals in early
December. The December ash-producing explosions were the first
eruptive activity at Mount St. Helens since October 1986.
There have been at least five more ash-producing explosions since
December 1989, all without recognized seismic or other geophysical
precursors. The ash from these explosions appears to be pulverized
pieces of dacite dome without glass shards, which suggests that no
new magmatic material was ejected. Several of the explosions were
accompanied by snow and rock avalanches, pyroclastic density flows,
ballistic showers and debris flows.
These ash-producing explosions are part of a series of at least
28 explosion-like seismic events that began on August 24, 1989.
Seismic signals from these events resemble those associated with
confirmed ash-producing explosions in April-May 1986. Yet not all
of the 1989-1991 events produced ash plumes. Observations during
four events clearly showed that neither a steam nor ash plume was
generated. There is little information about the other events
because they occurred when the mountain was not visible, nor was
there physical evidence of ashfall or surface changes when
scientists visited the crater days to weeks later. Considerable
deformation of the north side of the dome occurred during the
series of explosion-like seismic events. Sections of the dome
slumped northward and two new vents were formed. However,
monitoring the changes associated with individual events was often
impossible because several key EDM (electronic distance meter)
targets and tiltmeters were destroyed by the series of events.
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03/26/03, Lyn Topinka