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Between 1980 and 1986, hundreds of small ash emissions
(at Mount St. Helens) also occurred. These
small events were known as gas and ash emissions or explosions, and they
originated from cracks and small vents on the dome and sent plumes of steam and
ash 500 feet (150 meters) to 22,000 feet (6,700 meters) above the volcano. The
more vigorous events also erupted showers of hot rock fragments from the dome,
and some events generated small debris flows.
Most of these events took place without recognized precursors, which made
advanced warning of these events impossible. Verbal or written statements were
issued to all concerned agencies after the larger or more visible events.
If an event occurred during normal working hours, the ECC and
CVO updated media and public information telephone tapes and contacted the FAA,
emergency management officials, and others on the call-down list shortly after
the event. Because the seismic amplitude of these events was too small to
trigger alarms on the UW State-wide network, scientists were often unaware
(until normal working hours) of events that occurred during non-working hours.
In most cases, this was not a problem because these small events were frequent
enough that government officials and the public became familiar with them.
However, the FAA and emergency management personnel occasionally needed to
contact CVO for more information after the larger events that may have been
visible to pilots or the public. This contact was made by calling the USFS/ECC
telephone beeper number; the USFS, in turn, contacted the CVO duty scientist by
telephone or through the beeper, if necessary.
Examples of statements issued by the CVO and UW after eruptive events at Mount
St. Helens that occurred without precursors.
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Mount St. Helens Volcano Updates after Non-Precursory Events
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Type of Notice and When Issued
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Statements
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June 18, 1982 - 0930 PDT;
DAILY UPDATE
(Distributed by CVO computer "news" system)
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University of Washington-USGS reports continued low-level seismic activity.
Small gas-emission events continue to occur daily. The most recent event was at
0621 this morning, June 18. The plume rose to 12,000 feet.
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April 18, 1986 - 1200 PDT;
DAILY UPDATE
(Distributed by CVO computer "news" system)
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Since yesterday's report at 1300, there have been three more gas and ash
emission events. The first occurred at 1428 on the 17th and produced a plume to
14,000 feet (according to a local pilot). The second event occurred at about 1
a.m. today. Minor ashfall from this event was reported SE. of the mountain.
Both of these events appeared to be slightly smaller (seismically) than the
1716 event on the 16th. The third event occurred at 1116 today and was observed
by geologists working in the crater and flying in an airplane around the
mountain. The event appeared to be about the same size (seismically) at Yellow
Rock as the 1716 event on the 16th. Geologists reported that rock fragments
were thrown on and around the dome. The ash plume rose to 14,000 feet and later
drifted higher and to the southeast. The event lasted about 5 minutes
seismically, with the vigorous emission lasting just under 2 minutes.
When crew first reached the crater this morning they reported a light dusting of
tephra on the east and southeast flanks of the mountain, some tephra on the
crater floor, lots of ejected material (rock fragments) on top of the dome, and
lots of new snow in the crater. A pit dug in the snow revealed four, thin
(dispersed) ash layers from previous events.
A new update will be issued on Monday unless there is significant activity over
the weekend.
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May 6, 1986 - 1300 PDT;
INFORMATION STATEMENT
(Distributed by ECC formal call-down and CVO computer "news" system)
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More than 50 steam and ash explosions have occurred on the dome at Mount St.
Helens during the last 3 weeks. Seismicity has increased from slightly elevated
levels at the end of April to moderate level as today. Most monitoring equipment
on the dome has been damaged by the explosions and no longer operates. One
remaining tiltmeter on the dome shows gradually accelerating tilt. Other
deformation measurements have been hampered by inclement weather,
inaccessibility of the dome because of the explosions, and loss of many targets.
Hazards from explosions are most likely to be restricted to the crater and
flanks of Mount St. Helens. However, small mudflows caused by explosions in the
crater may flow into the North Fork Toutle River, and areas downwind from the
volcano may occasionally be dusted by small amounts of ash such as has
occurred recently.
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