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Speaking Of: : The Ring of Fire

June 11, 1991

Much of the Earth's surface has been shaped by volcanic activity. Mt. Unzen, which erupted eight days ago in southwestern Japan, had been dormant for about 200 years before its recent activity. In 1792, the same volcano caused landslides and tidal waves that killed 15,000 people in Japan's worst volcanic disaster. It rumbled to life again late last year, with the eruption that began June 3 killing at least 38 people, including Harry Glicken, 33, of Los Angeles, a noted volcanologist.

Volcanoes have made their mark on human affairs from the beginning of civilization, producing death and destruction, fear and superstition. Deities and supernatural events linked to volcanic activity play important roles in the legends and myths of civilizations that developed around volcanic regions.

History records eruptions from more than 500 volcanoes around the globe, about 75% of them located within the "Ring of Fire"--an area roughly encircling the Pacific Ocean. Most areas of volcanic activity are located along the boundaries of the dozen or so geological plates that make up the Earth's solid surface.

An eruption occurs when magma--molten rock, minerals and gases at temperatures as hot as 2,700 degrees Fahrenheit--is forced by pressure under the Earth's crust through a fissure or crack on its surface. Magma erupted onto the Earth's surface is called lava. Eruptions are also characterized by the ejection of ash, rock fragments and gases from a volcano.

In an average year, between 50 and 60 volcanoes erupt. Most of the eruptions are minor; one comparable in severity to the 1980 eruption of Mt. St. Helens in Washington state tends to occur only once or twice a decade. Larger eruptions, such as the ones that destroyed Pompeii in the year 79 and ravaged Krakatoa in 1883, occur about once every one or two centuries.

Major Eruptions

Major eruptions Deaths: AD 79 Mt. Vesuvius, Italy 16,000 1669 Mt. Etna, Sicily 20,000 1792 Mt. Unzen, Japan 15,000 1793 Laki, Iceland 10,000 1815 Tambora, Indonesia 92,000* 1883 Krakatoa, Indonesia 36,000 1902 Mt. Pelee, Martinique 30,000 1980 Mt. St. Helens, United States 60 1985 Nevado del Ruiz, Colombia 22,940

* Includes 80,000 killed in ensuing famine

Sources: Global Volcanism Network, Smithsonian Institution, Associated Press. McGraw-Hill Science and Technology Encyclopedia.

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