But pre-1980 St. Helens had only small glaciers, so its 1980 cataclysmic eruption produced a massive Plinian ash column, a lateral blast, and pyroclastic flows that dominated the event, while lahars, though large, were secondary and stayed mostly in remote valleys.
Mount Rainier, by contrast, sits under the largest glacial ice volume of any mountain in the contiguous U.S., 1 cubic mile!. Rainier has about five times more ice than all other Cascade volcanoes combined. Even a modest eruption (or a non-eruptive flank collapse like the 5,600-year-old Osceola Mudflow) can melt or dislodge huge volumes of ice and snow almost instantly.
Mt. Rainier's greatest known disasters—such as the 5,600-year-old Osceola Mudflow and the 500-year-old Electron Mudflow—were gigantic lahars triggered by flank collapses or modest eruptions, not huge ash plumes. The Osceola event alone sent a lahar more than The Osceola Mudflow from Mount Rainier traveled about 60 miles to reach Puget Sound and buried more than 190 square miles of land under mud that was locally up to 100 feet deep.
These were triggered by flank collapse and hydrothermal weakening, not necessarily by a 1980-style explosion.
Because of this ice cap and steep, debris-filled valleys draining toward densely populated suburbs of Seattle–Tacoma, scientists expect Rainier’s next major hazard to be fast-moving lahars that bury towns around Puget Sound under tens of feed of mud in under an hour, even if the eruption itself is relatively small or produces little ash.
That’s why you hear “lahars” for Rainier and “explosion” for St. Helens in 1980.
When it goes, next week, next year or next century, it’s going to kill many hundreds of thousands with little warning.
Even if there is substantial geological warnings, people will not heed them.................
I had to look up about every third word, but a fascinating description. Many thanks for taking the time ...
Mt. St. Helens’ 1980 initial detonation/eruption was more like a 10 megaton lateral shaped charge aimed north by northwest followed by a more normal vertical ash eruption.