USGS/Cascades Volcano Observatory, Vancouver, Washington
Geology of Interactions of Volcanoes, Snow, and Water:
Mount St. Helens, Washington
Lahars formed by pyroclastic surge swiftly melting
snowpack on 18 May 1980
The initial explosions at Mount St. Helens on 18 May 1980
developed into a huge, hot flow (so-called "pyroclastic surge")
that generated catastrophic floods off the east and west flanks
of the volcano. Near source deposits had attributes of a hot and
dry "surge", not a cool and wet one. The tail of the surge had
passed the volcano flanks before flood (or "lahar") arrived. The
floods must have originated by swift snowmelt at the base of a
hot and dry turbulent surge. Impacting hot debris transferred
downslope momentum to the snow surface, initiating thousands of
small slushflows. The slushflows grew swiftly, uniting downslope
into great slushy debris laden floods. The floods accelerated to
more than 100 km/hr (63 mi/hr) as they swept down and off the
volcano flanks as great debris rich floods, channeled into the
heads of valleys, which they ravaged for tens of miles
downvalley.
Waitt, R.B., 1989, Swift snowmelt and floods (lahars) caused by
great pyroclastic surge at Mount St. Helens volcano, Washington,
18 May 1980: Bulletin of Volcanology, v. 52, p. 138-157.
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11/03/97, Lyn Topinka