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. In Hawaii and other Polynesian islands, local people once attributed volcanic eruptions to the Goddess Pele. They believed Pele was moving from island to island as she sought to escape her evil sister, Na Maka O Kaha'i, the goddess of the sea. Today, scientists understand that volcanic eruptions are surface reminders of Earth's still hot interior.
The ash cloud formed by the May 18, 1980, eruption at Mount Saint Helens, WA. More images...
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". These floating pieces of crust are moving about very slowly on the hotter interior. 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.
Many kinds of volcanic activity can endanger the lives of people and property. Most of the activity involves the explosive ejection or flowage of rock fragments and molten rock. Volcanoes that erupt explosively can send particles as far as 20 miles high and many miles away from the volcano. The volcanic ash from these types of eruptions is a significant hazard to aviation. If an airplane flies into a volcanic cloud, it can lead to engine damage and malfunction and to many other kinds of aircraft damage. Volcanic eruptions near coastlines can generate damaging tsunami waves that can cause death and destruction among coastal communities.
To ensure safe navigation and monitor possible climatic impact, NOAA records global historic volcanic eruptions, tracks volcanic ash eruptions affecting the United States, issues volcanic ash advisories and provides ash cloud forecasts. For more information on volcanoes, visit NOAA's Volcano website