years, separated by lulls that last anywhere from about 2,000 to 8,000 years.
“This is clearly the most active of the
major Cascade volcanoes; there’s no
doubt about that,” says Clynne.
Most eruptions, he notes, are mild
affairs more akin to the one that began in
2004, when molten rock oozed into the
crater floor to heighten the lava dome
that’s rebuilding the peak.
But every once in a while, Mount St.
Helens blows itself sky high.
The 1980 eruption started with a
rumble, when a magnitude 5. 1 quake
deep inside the volcano triggered the
largest landslide ever witnessed. About
2. 5 cubic kilometers of the peak’s northern flank — a snow-dappled slope that for
weeks had been bulging outward a couple
of meters more each day — sloughed away
and raced downhill at speeds that may
have exceeded 200 kilometers per hour.
The sudden release of pressure was like
uncorking a bottle of champagne. The
blast of hot, stone-filled wind that surged
northward over the rugged, forested
terrain destroyed or damaged enough timber to build 150,000 wood-frame homes.
The rock mixed with and melted snow and
ice that had been carried from the peak,
creating debris flows called lahars with
the consistency of wet cement. As these
lahars swept down rivers and streams,
they scoured the sinuous, treelined waterways into straight-channeled, lifeless and
sediment-choked torrents.
Material carried by the largest of these
lahars, which flowed into the North Fork
Toutle River valley just north of the volcano, buried a 60-square-kilometer area
to an average depth of 45 meters and disrupted drainage patterns throughout the
watershed. Heat from the volcanic ash
in that freshly deposited blanket— a
landscape that some scientists call “the
pumice plain” — vaporized groundwater
in the underlying soil, triggering blowouts of steam that excavated craters up
to 400 meters across.
Life returns
Some researchers feared the area north of
the volcano would remain a sterile moon-
scape for decades. But scientists studying
the blast-seared zone north of the peak
found vegetation on the landslide within
a few years, says Peter Frenzen, a U.S.
Forest Service biologist at the Mount St.
Helens National Volcanic Monument.
Today about 80 percent of the landslide debris north of Mount St. Helens (upper
left) is covered with vegetation. Four years after the eruption, weedy perennials
on high-altitude ridges that were covered by snow when the blast occurred, such
as this fireweed (Epilobium angustifolium), were vanguards of recovery.
