eruptions of 1980 at Mount St. Helens: The First 100 Days," by Bruce C Foxworthy and Mary
Hill (1982). You can obtain this book (many color photos) and many other cool maps and
publications from the Map Sales office at the USGS office Menlo Park (345 Middlefield Rd.).
Mount St. Helens: The First 100 Days
On 20 March (59 days before the major eruption, or E-59), the seismology lab at the University
of Washington records the first of thousands of M3 to M5 earthquakes centered beneath the
volcano. On 27 March (E-52), eruptions of ash begin, and a few days later cracks and craters
form and begin to grow on the summit. By early April, it seems likely that magma, not just ash,
may ultimately be erupted from this volcano (evidence: harmonic tremor, gas composition).
Bulging, no eruptive action
12 April (E-36): Eruptive activity decreases and then ceases, but a surface bulge starts growing
on the north flank of the volcano. By 4 May (E-14), the bulge is growing upward at 5 feet/day.
The threat of landslides caused by collapse of the bulge constitutes the main volcanic hazard that
keeps surveillance funded and entry restricted.
Eruptive action resumes
7 May (E-9): Ash eruption resumes, but still no magma. High levels of sulfur dioxide (SO2)
suggestive of magma are recorded on 14 May (E-4), but no harmonic tremor is recorded from 8
May until the main eruption. Thus, the exact timing of the main eruption came as somewhat of a
surprise. Data from the morning of the 18th (available by noon) show that magma probably had
risen to within 300 m of the surface just before the landslide and eruption.
Main eruption
Triggered by a M5.1 earthquake at 8:32 am, the north flank of the mountain proceeds to collapse
in a mega-double-landslide. Within seconds, eruptive ash clouds are visible in photographs, so
the "uncapped soda bottle" analogy still applies here. The landslide was overtaken by a laterally-
directed blast of ash (driven by decompressed gas) within the first minute. The ash blast went up
and over ridges, sandblasting everything in its path and killing Johnston, Martin, and Blackburn.
A tongue of the landslide followed, scaling the 1400'-high ridge on which Johnston had been
stationed.
Impact of mudflows
70% of the ice mass on mountain was removed, most of it melted and mixed with ash, trees, etc.
to form monster mudflows. The biggest took out several bridges, and almost took out the bridge
on I-5. Debris and mud clogged the Columbia River, which stranded many ocean freighters for
up to 4 weeks, and the intake filters of Longview, which had to invoke emergency water use.
Later explosive eruptions
25 May, 12 June; the latter nailed Portland, which was still removing ash and imposing
emergency speed limits 10 days later.