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Formation of the Pumice Plain - May 18, 1980
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Beginning at about 9:00 A.M., (May 18, 1980 eruption of Mount St.
Helens), and increasing in intensity until about noon, pyroclastic flows
swept down the north face of Mount St. Helens and filled low spots between the
freshly deposted hummocks of the debris-avalanche deposit to depths greater
than 100 feet. These flows, dominantly fresh pumice, were driven by volcanic
gases and gravity. ...
The flat, light-tan surface formed by the overlapping sheets, tongues, and lobes
of the pyroclastic deposits is called the Pumice Plain.
-- Exceprt from:
Pringle, 1993, Roadside Geology of Mount St. Helens National Volcanic Monument
and Vicinity: Washington Department of Natural Resources, Division of Geology
and Earth Resources Information Circular 88
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Mount St. Helens
Harrys Ridge (north)
Link to: USFS Mount St. Helens National Volcanic Monument![[medium size]](https://webarchive.library.unt.edu/eot2008/20090109105548/http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/Images/MSH80_pyroclastic_flows_august7_on_july22_09-30-80_med.jpg){kind=link}
![[large size]](https://webarchive.library.unt.edu/eot2008/20090109105548/http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/Images/MSH80_pyroclastic_flows_august7_on_july22_09-30-80.jpg){kind=link}