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California Volcanoes

a map of active volcanoes in California

In addition to fires, floods, earthquakes, and tsunamis, Californians are also subject to volcanic threats from the 8 volcanic areas classified as moderate, high, and very high threat in the state. These threat level classifications are defined and assigned in the USGS National Volcanic Threat Assessment released in 2018.

Volcanic eruptions in the state are not currently imminent. Using the threat levels, these volcanic areas are monitored continuously by Cal OES, for emergency managment considerations and by the USGS California Volcano Observatory, for scientific observations of potential consequence. Almost all of the 8 significant volcanic areas are known for their local geothermal features, some of which produce energy for Californians.

The USGS has now released report 2018-5159 entitiled California's Exposure to Volcanic Hazards. Cal OES will use this important report along with the recently released National Volcanic Threat Assessment to initiate state level response planning.

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.

Normal/Green

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

Advisory/Yellow

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

Watch/Orange

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.

Warning/Orange

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).

Watch/Red

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.

Warning/Red

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.

Unassigned

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.