What does a steam-and-ash plume look like?

These images were taken on June 16, 1982.  Similar steam-and-ash explosions took place during 1989-1992. The duration of the event, as indicated by the seismic record, was just over twenty minutes.   Click on the thumbnailed image to see a full size picture.  Gas-and ash explosions are recorded by seismograph stations operating on and near the volcano. In general, seismic recordings of gas-and-ash explosions at Mount Saint Helens are characterized by emergent, low-frequency signals of extended duration, often with pulsating amplitude changes.  Relative signal amplitudes are similar among stations on the flanks of the mountain but are much higher at seismograph stations within the crater.  The photo sequence was taken over about 12 minutes, the final photo was probably taken near the end of the seismic event. The plume has drifted in the wind, and the coarser ash can be seen dropping out of the plume.  Some emissions barely clear the crater rim, others reach altitudes as high as 30,000 feet and pose a hazard to aircraft. Dust clouds from rockfalls in the crater (especially during the dry summer months), thunderheads, and condensation plumes are sometimes mistaken for gas-and-ash plumes.