The dome at Mount St. Helens is termed a composite dome
by scientists, because it represents the net result of many
eruptive events, not just one event. The dome-building process may be
pictured as the periodic squeezing of an
upward-pointing tube of toothpaste or caulking compound. The process is
dynamic, involving the upward movement of
new material, cracking and pushing aside of old material, sloughing of
material from steep surfaces of the dome, and
occasional, small but violent explosions that blast out pieces of the dome.
At the start of 1990, the composite dome was about 3,480 feet by 2,820 feet in
diameter and rose about 1,150 feet above the low point on the adjacent crater
floor. It has a volume of about 97 million cubic yards,
less than 3 percent of the
volume of the volcano (about 3.5 billion cubic yards) removed during the
landslide and lateral blast on May 18, 1980. If the dome resumes growth at its
average rate of the 1980s (about 17 million cubic yards per year), it would take
nearly a century to fill in the summit crater and more than 200 years to rebuild
Mount St. Helens to its pre-1980 size.
-- Excerpts from:
Tilling, Topinka, and Swanson, 1990,
Eruptions of Mount St. Helens: Past, Present, and Future:
USGS General Interest Publication
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