From 24 August 1989 until 18 June 1991, Mount St. Helens produced at least 28 shallow, explosion-like seismic events with signatures similar to those produced by gas explosions on the dome during the mid 1980s. At least six were accompanied by violent emission of non-juvenile tephra, ejection of blocks of rock nearly 1 km from the vent, and avalanching of debris off the north side of the dome. All six confirmed emissions and most (although not all) other seismic events took place hours to days after storms. The short delay between storms and emissions suggests that the events that follow storms originate at very shallow depth, probably within the dome itself. Although the exact causal mechanism is not known, it is speculated that slope instability or accelerated growth of cooling fractures following storms may have released gas trapped within or at the base of the dome. -from Author
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Explosive tephra emissions of Mount St. Helens, 1989-1991: the violent escape of magmatic gas following storms?