The U.S. Geological Survey employs a nationwide volcano alert-level system to characterize conditions at volcanoes in the United States. The agency categorizes these conditions as quiet, unrest, or eruption. Notifications about the status of activity at U.S. volcanoes are issued through five regional U.S. Volcano Observatories.
According to the U.S. Geological Survey, the alert-level system for volcanic activity has two parts. The first part uses ranked terms to inform people on the ground about the status of a volcano. The second part uses ranked colors to inform the aviation sector about hazards from airborne ash.
Ground hazard alert levels
The ground hazard portion of the system relies on ranked alert-level terms to describe a volcano’s current state. A Volcano Activity Notice is issued when the volcano alert level is changed.
The alert-level terms include Normal, Advisory, Watch, Warning, and Unassigned. A “Normal” state indicates the volcano is in a typical background, noneruptive state, or that volcanic activity has ceased and the volcano has returned to a noneruptive background state after a change from a higher level. An “Advisory” level means the volcano is exhibiting signs of elevated unrest above the known background level, or volcanic activity has decreased significantly but continues to be closely monitored for a possible renewed increase.
A “Watch” level signifies heightened or escalating unrest with an increased potential of an eruption with an uncertain timeframe, or that an eruption is underway but poses limited hazards. A “Warning” means a hazardous eruption is imminent, underway, or suspected.
The “Unassigned” term applies when there is insufficient information available to assess the current status of the volcano or volcanic activity.
Aviation color codes
The aviation color code system is used for aviation hazards. The system uses ranked colors to inform the aviation sector about the risks of airborne ash.
A Volcano Notice for Aviation is issued when the aviation color code is changed.