USGS volcano alerts have two separate parts: a word-based Volcano Alert Level for hazards to people and infrastructure on the ground, and an Aviation Color Code focused on volcanic ash and aviation. Read the accompanying official notice and local emergency instructions; the label alone is not a universal evacuation order or a countdown to eruption.
What the USGS alert system measures
The U.S. Geological Survey (USGS) uses a four-level word scale to describe a volcano’s status and the potential for hazardous effects on the ground. It also uses a four-color aviation scale to communicate the risk that volcanic ash poses to aircraft. These systems are designed for volcanoes within USGS responsibility and are not a worldwide standard. Other countries and observatories may use different names, levels or actions.
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| System | Primary audience | What it describes | What to read next |
|---|---|---|---|
| Volcano Alert Level | People, communities and infrastructure on the ground | Overall unrest, eruption status and potential ground hazards | Volcano Activity Notice (VAN) and local emergency-management instructions |
| Aviation Color Code | Pilots, airlines and aviation authorities | Volcanic-ash conditions and the likelihood of significant ash emissions | Volcano Observatory Notice for Aviation (VONA) and aviation advisories |
A status notice supplies the context that a single word or color cannot. It explains what has changed, which hazards are possible or occurring, and what scientists think may happen next. Ground hazards can include lava flows, lahars, ashfall, airborne ash, volcanic gases and other effects specific to the volcano.
Volcano Alert Levels: what each word means
NORMAL
NORMAL means the volcano is in its typical background, non-eruptive state. If it was previously at a higher level, NORMAL can indicate that activity has ceased and the volcano has returned to its background condition. NORMAL does not mean that every hazard is impossible; it describes the current status relative to the volcano’s usual behavior.
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ADVISORY
ADVISORY means unrest is elevated above the known background level. Instruments or observations show a change that merits closer attention, but the level does not by itself say that an eruption is certain, imminent or even likely. After a higher alert, ADVISORY can also mean that activity has decreased substantially while monitoring continues for a possible renewed increase.
For a reader near the volcano, the important question is not simply “Is it ADVISORY?” but “What evidence and hazards does the current notice describe, and do they affect my location?”
WATCH
WATCH means unrest is heightened or escalating and the potential for an eruption has increased, although the timing remains uncertain. USGS also uses WATCH when an eruption is underway but the hazards are limited. WATCH therefore is not a precise time-to-eruption estimate and does not automatically mean that an evacuation has been ordered.
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WARNING means a hazardous eruption is imminent, underway or suspected. The word signals a serious situation, but it still does not identify the specific threat to every location. The accompanying notice and instructions from the responsible local authority determine whether the relevant danger is lava, lahar, ash, gas, a fast-moving flow or another hazard.
UNASSIGNED
UNASSIGNED is used when there is not enough information to assess the volcano’s current status. It is not a reassuring level below NORMAL. It means scientists cannot assign a current alert level with the available information, so readers should look for an explanatory notice and local guidance rather than interpreting the label as “safe.”
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Aviation Color Codes: GREEN through RED
The aviation colors address airborne ash, which can damage aircraft engines and affect flight routes. They are not a color-coded public evacuation order.
| Color | USGS meaning | What it does not mean |
|---|---|---|
| GREEN | Typical background, non-eruptive conditions | That every flight near a volcano is risk-free under all circumstances |
| YELLOW | Elevated unrest above known background, or a significant decrease after a higher code while monitoring continues | That an eruption is certain or that ash is currently affecting every route |
| ORANGE | Heightened or escalating unrest with increased eruption potential and uncertain timing, or an eruption with no or minor volcanic-ash emissions | A universal ground warning for residents |
| RED | Significant ash emission is likely with an imminent eruption, or an eruption is underway or suspected with significant ash emissions | A stand-alone evacuation instruction for communities on the ground |
When possible, an aviation notice also gives the observed or expected ash-plume height. Pilots and aviation authorities use that technical information with flight planning and other advisories.
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The word and color often change together, but they measure different consequences. Ground hazards and aviation ash hazards do not always rise at the same time.
WARNING with ORANGE
A major eruption may be imminent, underway or suspected, creating severe danger on the ground, while ash emissions remain limited enough that aviation risk is comparatively lower. The combination can therefore be WARNING/ORANGE.
WATCH with RED
An eruption may have limited ground effects while producing significant ash emissions that threaten aircraft. In that case the combination can be WATCH/RED. Neither example is a contradiction: the first word describes the ground situation and the color describes airborne-ash conditions.
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What to do when an alert changes
There is no single action table that applies to every volcano, because danger depends on the volcano’s shape, eruption type, wind, drainage channels, roads, infrastructure and your distance from the hazard. Use the alert as a prompt to read the current notice and local instructions.
- Identify the volcano and the update source. Confirm that the notice applies to the volcano nearest you and note its issue time and date. Conditions can change quickly.
- Read the complete Volcano Activity Notice (VAN). A VAN explains why the level changed or why significant activity occurred within the same level. Look for named hazards such as lava, lahars, ashfall, gases or airborne ash, along with areas that may be affected.
- Match the hazard to your location. Check whether your home, workplace, travel route or outdoor activity lies in the area described by the notice. A level is not a map and cannot show your personal exposure by itself.
- Check local emergency-management messages. The authority responsible for the affected area issues evacuation orders, shelter instructions, road closures, access restrictions and other protective directions. Follow those instructions even if the alert word has not changed.
- Monitor subsequent official updates. Use the USGS Volcano Notification Service or the observatory’s official channels for new notices, but do not treat an email or text notification as a replacement for the full notice or a local emergency warning.
If your concern is aviation, read the VONA rather than relying on the ground alert word. A VONA is issued for a color-code change or significant activity within a code and contains the information aviation operators need.
How scientists assign and change levels
Observatories use continuous monitoring and scientific judgment rather than a fixed countdown. The Hawaiian Volcano Observatory, for example, describes round-the-clock use of seismometers, cameras, satellites and other instruments to detect magma movement and active lava or ash. Scientists consider the monitoring record, the volcano’s past unrest and eruption patterns, and the physical processes they believe are occurring.
There are no universal numerical thresholds that automatically switch every volcano from one level to the next. Natalia Deligne of the Hawaiian Volcano Observatory wrote, “Knowing exactly when to change the Volcano Alert Level and Aviation Color Code can be challenging.” She also wrote, “We do not have set thresholds: each situation is different.” Notices explain the reason for a change and the possible next developments based on the current scientific consensus.
This approach means an alert can rise quickly when several observations point to escalating unrest, or remain unchanged while activity evolves within the same level. A lower level after a peak does not erase the need to read the notice: ADVISORY and YELLOW can indicate that activity has declined while monitoring continues.
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VAN versus VONA
Volcano Activity Notice (VAN)
A VAN is the ground-focused notice. It is issued when the Volcano Alert Level changes or when significant activity occurs within an existing level. It summarizes observations, current or potential hazards and likely outcomes so residents, emergency managers and other agencies can understand what the status label means in practice.
Volcano Observatory Notice for Aviation (VONA)
A VONA is the aviation-focused notice. It is issued when the Aviation Color Code changes or when significant activity occurs within the existing code. It addresses ash emissions and other information needed for aviation decision-making, including plume details when they can be determined.
Observatory scientists prepare both types of notice from monitoring networks, direct observations and satellite sensors. Reading the notice that matches your situation prevents a common mistake: using an aviation color as if it were a public evacuation order, or using a ground word as if it fully described aircraft exposure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common misunderstandings
“ADVISORY means an eruption is about to happen.”
No. ADVISORY records unrest above background or a monitored decline after a higher level. It does not establish that an eruption is certain or imminent.
“WATCH gives me a number of hours or days.”
No. WATCH indicates increased potential or a limited-hazard eruption, with timing uncertain. It is not a schedule.
“WARNING tells everyone to evacuate.”
WARNING identifies a hazardous eruption that is imminent, underway or suspected. Evacuation zones and deadlines come from the local emergency authority and the hazard information in the current notice.
“The color is just a simpler version of the word.”
No. The color is an aviation ash assessment. A word and color can diverge because the threats to people on the ground and to aircraft are different.
“UNASSIGNED is the lowest-risk status.”
No. UNASSIGNED means there is insufficient information to assess the current status. Treat it as an information gap, not as reassurance.
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Using alerts safely in real time
- Prefer the newest notice from the responsible observatory and the current message from local emergency management.
- Check the issue time, because an older status may not reflect rapidly changing activity.
- Read the geographic details and hazard descriptions instead of making decisions from the alert word alone.
- For travel by air, check airline or airport instructions and aviation advisories informed by the VONA; a ground-level alert does not by itself determine whether a particular flight will operate.
- Do not wait for a level change if local officials have already issued an evacuation, shelter or access instruction.
Key points to remember
The word tells you about the volcano’s status and hazards affecting the ground; the color tells aviation users about airborne-ash conditions. ADVISORY is elevated unrest, not a promise of eruption. WATCH signals heightened potential or a limited-hazard eruption with uncertain timing. WARNING means a hazardous eruption is imminent, underway or suspected. Because the two systems measure different hazards, their labels can differ. For any decision that affects your safety, use the latest VAN or VONA together with instructions from the local authority responsible for your area.
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