Volcanoes/Volcanic Eruptions – Introduction

Volcano Structure and Hazards[5]
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A volcano is a mountain that opens downward to a reservoir of molten rock below the surface of the earth. Unlike most mountains, which are pushed up from below, volcanoes are vents through which molten rock escapes to the earth’s surface. When pressure from gases within the molten rock becomes too great, an eruption occurs.[1]


The word volcano comes from the island of Vulcano in the Mediterranean Sea. Long ago people thought this island mountain was the chimney of the blacksmith forge of the Roman God Vulcan. The steam and ash that came out of the vent was a sign that Vulcan was working at his forge making weapons for Jupiter and Mars.[2]


Sixty percent of all active volcanoes are found at crustal plate boundaries such as the Pacific Plate. Earth’s crust, like the cracked shell of a hard-boiled egg, is broken into a number of “plates”. Where the plates are moving apart or colliding with one another, volcanoes may form. Volcanoes also form oceanic islands in the Pacific Ocean or Mediterranean Sea where “hot spots” occur in the crust and mantle.

Active volcanoes in the U.S. are found mainly in Hawaii, Alaska and the Pacific Northwest. The danger area around a volcano covers approximately a 20-mile radius however some danger may exist 100 miles or more from a volcano.[3]

Volcanoes can actually erupt within one week after the first signs of activity. The first steam eruption at Mount St. Helens on March 27, 1980, was preceded by only 7 days of intense earthquake activity. The climactic eruption, on May 18, followed seven weeks later. An eruption of Redoubt Volcano in Alaska on December 13, 1989, was preceded by only 24 hours of intense earthquake activity. But other volcanoes have been restless for months or years before an eruption occurred, and sometimes a period of unrest doesn’t produce an eruption at all.[4]