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.