Volcano About

Earthquake, Tsunami & Volcano Programs
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About Volcanoes

Volcanoes can erupt explosively blasting hot solid and molten rock fragments and sending gases and ash into the air. Lava flows can occur on all sides of a volcano and ash can fall hundreds of miles downwind. Dangerous mudflows and flooding can occur in valleys leading away from volcanoes. If you live near a known volcano, active or dormant, you should be prepared to follow volcano safety instructions from your local emergency officials.

The California Governor’s Office of Emergency Services, Earthquake, Tsunami and Volcano Program is continuously researching, learning, and developing new ways to make California’s residents and visitors safer in the event of volcanic activity. Our staff strives to ensure everyone in California remains safe before, during and after a volcanic eruption.

​Causes of Volcanic Eruptions

Volcanic eruptions and earthquakes are a way for Earth to release pressure and heat, much like a safety valve.

A volcano is basically a vent, or opening, in the earth’s crust from which hot molten rock, gases, and volcanic ash escape to the surface. They may erupt explosively like Krakatoa in 1883, or effusively seep out lava like the Kilauea volcano. Formed as a result of plate tectonics, volcanic eruptions can result in the formation of mountains, craters, crater lakes, plateaus, and islands.

Volcanoes are classified as active, dormant or extinct. An active volcano is the one that erupts or causes seismic activity on a regular basis over hundreds or thousands of years. A dormant volcano is one that has not erupted for a very long time, but could erupt again in the future. And, a volcano that has been dormant for more than 10,000 years is considered an extinct volcano.

See our “Tools and Resources” Section below for more information.


​Signs of Danger

If you are near a volcano and watching for signs that it may erupt, here are some clues you might look for:

1. An increase in seismic (earthquake) activity occurs prior to an impending eruption as the magma and gasses move below the surface. The magnitude of the earthquakes may be small.
2. If you are looking at the cracks of a volcano’s surface, you may see vents (called fumaroles) that release pressure from the gasses below. If you see an increase in gases escaping from these vents, or a change in the temperature of the gases, this can signify a potential eruption.
3. Scientists can determine if the magma has thickened. If so, it may trap gases under it which can lead to an eruption
4. A sudden rise in the elevation of the ground and the appearance of new cracks may be a sign of an upcoming eruption.
5. Changes in the groundwater temperature may occur prior to eruption.

​Real Time Volcano Information

Scientists routinely monitor seismic activity near volcanoes which allows them to assess any unusual activity. They can also monitor gas, ground deformation and satellite imagery to determine if magma is rising toward the surface. This information can be assessed quickly and volcanic hazard information can be communicated as real-time warnings to prevent loss of life.

To sign up for email notifications,  go the Unites States Geological Survey, Volcano Notification Service website. It only takes a minute and there is no cost.

​Understanding the Volcano Alert Notification System

The U.S. Geological Survey (USGS) Volcano Hazards Program uses an alert-notification system to indicate the level of potential danger for volcanoes. Color-coded triangular signs are posted along with hazard details.



The normal/green icon is used when background activity is within the range of typical non-eruptive phenomena seen at the volcano.


The advisory/yellow icon is used when a volcano is exhibiting signs of elevated activity above normal ranges.


The watch/orange icon is used when heightened or escalating unrest is exhibited with an increase in potential for eruption. In this stage the timeframe is unclear or an eruption is occurring posing limited hazard with no or minor emissions of volcanic ash.


The warning/orange icon is used when a major eruption is suspected, underway, or imminent, but no or minor volcanic ash emissions to impact aviation (lava flows only).


The watch/red icon is used when an eruption occurs with limit hazard to ground based committees (little to no lava flow), but significant impacts to aviation with substantial emissions of ash in the atmosphere.


The warning/red icon is used when a major a major eruption is suspected, underway, or imminent, with significant hazards expected to both ground based communities, and aviation.


The unassigned icon is used when ground-based instruments are unable to determine that a volcano is within normal parameters.


Did You Know?

  • ​National Geographic has identified Kīlauea in Hawaii as the most dangerous volcano in the world. It has had 61 eruptions since 1893, which also makes it world's most active volcano. Perhaps that explains the name which means “spewing” or “much spreading” in Hawaiian.
  • The largest volcanic eruption on record was Toba (Sumatra, Indonesia), thought to have erupted 73,000 years ago. It released more than 1,000 cubic kilometers of material, and created a caldera 100 km long and 30 kilometers wide.
  • The term volcano originally comes from the name of the Roman god of fire, Vulcan. 
  • The United States ranks third, behind Indonesia and Japan, in the number of historically active volcanoes, according to the U.S. Geological Survey.
  • There are 65 volcanoes in the United States and its territories that scientists consider active, including Mount St. Helens.
  • The most destructive eruption in U.S. history was the May 18, 1980, eruption of Mount St. Helens.


​Tools and Resources


List of Volcanoes in California (USGS California Volcano Observatory)

List of Volcanoes in the United States (USGS CalVO)

Volcanic Monitoring (USGS)

Volcano Eruption Fact Sheet (multiple languages and large font) (Washington State Dept. of Health)

Volcano Hazards Program Feeds (USGS)

Volcano Hazards Program Fact Sheets and Posters (USGS)

Volcano Frequently Asked Questions (USGS CalVO)


Informational Websites

How Volcanoes Work (San Diego State University)

Learn about U.S. Volcanoes (USGS CalVO)

Planning and Preparedness for Governments and Tribal (CalOES)

Volcanic Ash Facts and Guidance (USGS)

Volcanoes: Earth's Fiery Power (National Geographic)

What Causes Volcanoes to Erupt? (Buzzle)


For Schools and Educators

How Volcanoes Work (San Diego State University, et al.)

Kids Tour to Mars - Volcanoes (Kid's Cosmos)

School Planning and Preparedness (CalOES)

Virtual Volcano Field Trips (Oregon State University)

Volcano Games (Oregon State University)

Volcano Resources for Educators (USGS)

For Businesses and Organizations

The Business and Industry Council for Emergency Planning and Preparedness (BICEPP)

Emergency Management Guide for Business and Industry (FEMA)

Emergency Preparedness Checklist for Small Businesses (FedEx)

Emergency Preparedness Resources for Business (FEMA)

Five Steps to Develop your Business Preparedness Program (CalOES)

Ready.gov for Business (FEMA)

U.S. Department of Homeland Security

U.S. Small Business Administration (SBA)

Volcano Preparedness

Volcano Safety Tips (ARC)

Key Facts about Preparing for a Volcano Eruption (CDC)

Key Facts about Protecting Yourself after a Volcanic Eruption (CDC)

Prepare for a Volcanic Eruption (US EPA)

Volcano Ready.gov (FEMA)


Volcano Recovery

Recover After a Volcanic Eruption (US EPA)



American Red Cross (ARC)

California Volcano Observatory (CalVO)

NOAA Volcanic Ash Advisory Center 

Seismic Safety Commission – Volcanoes

United State Environmental Protection Agency (US EPA)

United States Geological Survey: Volcano Notification Service 

​​Minute By Minute

The Eruption of Mt. St. Helens




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