| 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. -- Simkin, 1994 |
