What Are Volcanoes?

A volcano is a fissure that opens downward to a pool of melted rock deep below the earth’s surface. As pressure builds up due to continental movement, eruptions occur. Molten lava, gases and ash shoot up through the opening and spill over or fill the air with lava fragments. The cooling lava and ash pile up to form a cone, eventually becoming a mountain. A volcano can erupt many times over a period of thousands of years. It is extremely difficult to predict just when an eruption might occur.

Eruptions can be sudden and violent blasts, or they may be slow flows of lava or hot ash. An eruption often triggers earthquakes, mudslides, avalanches, rockfalls or floods. Volcanic eruptions have been known to topple entire forests of trees. An erupting volcano in or near the ocean can trigger a tsunami. An underwater volcano may grow until its top sticks out of the water, becoming an island.

Most of the world’s active volcanoes are located along the coasts surrounding the Pacific Ocean, in what is known as the "Ring of Fire." In these areas the edges of the continental plates are constantly in motion, grinding against one another. The danger area surrounding a volcano typically covers about a 20-mile radius, though a volcanic blast may cause ash falls in a much larger area.

Volcanic Eruption Preparedness Resources

Preparing for Volcano Emergencies

Types of Volcano Hazards

What to Do During Volcanic Ashfall

Volcano Preparedness Safety Tips 

USGS Volcano Warning Schemes in the United States

The Deadliest Volcano Eruptions

USGS Weekly Volcanic Activity Report