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Mt.
St. Helens National Volcanic Monument
Mount St. Helens: The
Volcano
The Mount St. Helens
Volcano has shown continuous intermittent
volcanic activity since the 1980 eruption, including
minor eruptions of steam and ash, and
occasional pyroclastic flows. More than a dozen extrusions of lava have built
a mound-shaped lava dome in the new crater. Goat Rocks on the northern flank is one of the
older lava domes. Experts monitoring the volcano believe there will be adequate
warning of the next eruption.
The most recent seismic
activity occurred October 2004 and consisted of steam and ash eruptions and
lava extrusions resulting in the significant expansion of the lava dome.
Understandably, Mount St. Helens has been studied
intensively, and more is known about its eruptive history than that of any other Cascade
volcano. It is a relatively "young" volcano, perhaps 40,000 to 50,000 years
old. The cone that partly collapsed in 1980 is only 2,200 years old. In the
last 515 years, it is known to have produced 4 major explosive eruptions and dozens of
lesser eruptions. An eruption in 1480 A.D., was about 5 times larger than the May 18, 1980
eruption,
The May 18, 1980 eruption came after nearly 2
months of thousands of local earthquakes and hundreds of steam eruptions, and the outward
growth of the volcano's entire north flank by more than 80 meters. A magnitude 5.1
earthquake struck beneath the volcano, sending the volcano's bulging north flank sliding
in the largest landslide in recorded history. This in turn triggered a lethal
lateral blast of hot gas, steam, and rock debris lasting only a few minutes, but which
swept across the landscape as fast as 1,100 kilometers per hour. The next 9-hours of
continuous eruption, were followed by 3 days of intermittent eruptions, then 5 smaller
explosive eruptions over a period of 5 months.
Altogether, about 4 billion cubic yards of new
and old lava material and about 170 million cubic yards of glacial snow and ice were
deposited over the landscape, as a result of the May 18 eruption. Some debris and
mud-flows, were so voluminous that they reached and blocked the shipping channel of the
Columbia River about 70 river miles from the volcano. The eruption itself blew
volcanic ash more than 15 miles into the air, within minutes. Winds carried about 490 tons
of ash generally eastward across the United States and, in trace amounts, around the
world. The ash fell in troublesome amounts as far east as western Montana. About 60
persons were dead or missing.
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Go Northwest!
Bookstore
Best selection of books on the
Northwest.
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A Complete Guide to Mount St.
Helens National Volcanic Monument
by Klindt Vielbig
April 1997, Mountaineers Book, Paperback, 256
pages, (non-fiction)
A well-organized guide providing all the historical,
geological, biological, and recreational information you'll need to take advantage of a
truly unique natural area.
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