Volcanic Clouds from the 1999 Eruption of Shishaldin Volcano: Comparison of Satellite and Seismic Observations

The main ash-producing phase of the 1999 eruption of Shishaldin Volcano, Alaska, occurred on April 19 and ejected ash as high as 16 km asl. High levels of tremor persisted for more than 18 hours, but ash production probably lasted less than 4 hours. The total erupted volume of the April 19 event was estimated to be at least 2.0 x 107 m3 DRE. Upper level winds transported mainly sulfur dioxide to the north above 10 km altitude, and mainly ash to the south below that level. The relationship between seismicity and ash cloud production changed following the April 19 event. Episodes of strong amplitude tremor from April 21 to 24 were associated with strong strombolian activity, but very little ash was produced that could be detected in satellite images. On several occasions in late May, extensive ash plumes were detected in AVHRR and GOES images, but seismic levels during these times of ash production remained at low-levels. Satellite data suggest that the ash produced during the May events contained more fine particles than that produced by the April 19 event.