USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Volcanoes: Their Occurrence and Geography
--
Tom Simkin, 1994,
Volcanoes: Their Occurrence and Geography:
IN: Casadevall, Thomas J., (ed.), 1994,
Volcanic Ash and Aviation Safety:
Proceedings of the First International Symposium on Volcanic Ash and Aviation
Safety:
U.S. Geological Survey Bulletin 2047, 450p., p.75-79.
Abstract
The explosive volcanoes that threaten air safety tend to be arranged in long,
linear belts near boundaries where crustal plates are converging. These belts
cover less than 0.6 percent of the Earth's surface. At least 1,300 volcanoes
have erupted in the last 10,000 years and, because the lifetimes of most
volcanoes are very long, they are likely to erupt again in the future. Among
these 1,300 volcanoes, however, only about 60 are active in a typical year, and
that activity may range from a mild pyrotechnics of Italy's Stromboli to
prehistoric catastrophes that dwarf recent eruptions such as Mount St. Helens,
1980; Krakatau, 1883; or Pinatubo, 1991. Like earthquakes, the bigger eruptions
happen less often than the smaller, with St.Helens-sized events occurring
perhaps once per decade and events such as Ruiz, 1985 and Redoubt, 1989, several
times a year. During the years 1975-85, more than 63 eruptions penetrated the
altitude range of air traffic and at least nine passed into the stratosphere
where volcanic products are easily dispersed around the world.
Eruption durations are often not recorded historically, but they are known to
range from minutes to thousands of years. The median duration is 7 weeks, and
few eruptions last longer than 3 years. Pulses of activity mark all but the
shortest eruptions, and the paroxysmal event may come at any time from the first
day (as occurs in 45 percent of all eruptions) to months or even years after the
start of eruption. Of particular importance for volcano hazards is the fact
that unusually violent eruptions commonly occur after unusually long periods of
repose. The historic record in many parts of the world is far shorter than the
hundreds or thousands of quiet years that precede violent eruptions, meaning
that some of our most dangerous volcanoes may be those not currently recognized
as "active." Of the 16 largest explosive eruptions in the 19th and 20th
centuries, all but four were the first historic eruption known from the volcano.
These factors emphasize the importance of communicating reports of new
eruptions. We at the Smithsonian hope for more news from "eyes in the sky" and
will do our best to disseminate volcano reports as we have been doing for nearly
a quarter century.
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06/20/02, Lyn Topinka