Volcano Alert Levels and VEI: How to Read a Volcanic Warning

Direct answer: there are three separate systems, and they answer three different questions. Alert levels (Normal, Advisory, Watch, Warning — or numbered 0 to 5 in other countries) describe the current state of unrest and what people nearby should do. Aviation colour codes (Green, Yellow, Orange, Red) describe the hazard to aircraft from ash. The Volcanic Explosivity Index (VEI) rates the size of an eruption after it has happened. A volcano can be at Red for aviation while the ground alert is only a Watch.

Ground alert levels

Every volcano-monitoring country runs its own scheme, but they map onto the same four ideas. The US Geological Survey scheme is representative:

• Normal — typical background activity, or the volcano is dormant.
• Advisory — elevated unrest above known background. More earthquakes, more gas, ground deformation.
• Watch — heightened unrest with increased potential for eruption, or a minor eruption is under way.
• Warning — a hazardous eruption is imminent, under way, or expected imminently.

Indonesia, which monitors more active volcanoes than any other country, uses a numbered scale from Level I (Normal) to Level IV (Awas). Japan uses a five-level scale tied explicitly to actions — which is the better design, because a number that tells people what to do is far more useful than one that describes geology.

Aviation colour codes

Volcanic ash is a serious threat to jet engines, where it melts and fuses onto turbine components. The four-colour code exists purely for aviation:

• Green — normal, non-eruptive state.
• Yellow — unrest above background.
• Orange — heightened unrest with increased eruption likelihood, or an eruption with little or no ash.
• Red — eruption imminent or under way with significant ash emission.

Nine regional Volcanic Ash Advisory Centres worldwide issue the advisories that airlines route around. This is why an eruption on a remote island can cancel flights across a continent — as Eyjafjallajokull did across European airspace in 2010, from a comparatively modest eruption.

The Volcanic Explosivity Index

VEI runs from 0 to 8 and is logarithmic: each step up is roughly a tenfold increase in erupted volume. It is assigned after an eruption from the volume of material and the height of the plume, so it is a historical measure, not a warning.

• VEI 0 to 1 — gentle effusive eruptions. Most Hawaiian activity.
• VEI 2 to 3 — explosive, common; several per year globally.
• VEI 4 — large. Eyjafjallajokull 2010.
• VEI 5 — very large. Mount St Helens 1980, Hunga Tonga 2022.
• VEI 6 — colossal, with measurable global climate effects. Pinatubo 1991 cooled global average temperature for about two years.
• VEI 7 — Tambora 1815, which caused the "year without a summer" and widespread crop failure across the northern hemisphere.
• VEI 8 — supereruptions, tens of thousands of years apart.

VEI measures explosivity, not deaths. Nevado del Ruiz in 1985 was a relatively small eruption that killed an estimated 23,000 people, because meltwater generated lahars that buried the town of Armero.

What actually kills people

Lava flows are the least of it — they are usually slow enough to outwalk. The lethal hazards are:

• Pyroclastic density currents — avalanches of hot gas, ash and rock moving at highway speeds and hundreds of degrees. Not survivable; the only defence is not being there.
• Lahars — volcanic mudflows of ash mixed with water from rain, snowmelt or a crater lake. They follow river valleys, travel tens of kilometres, and can arrive years after an eruption when heavy rain remobilises deposits.
• Ashfall — wet ash is extremely heavy and collapses roofs; it also contaminates water, ruins crops, wrecks machinery and causes respiratory harm.
• Volcanic gases — carbon dioxide pools in hollows and can asphyxiate silently; sulphur dioxide creates volcanic smog and acid rain.
• Flank collapse and tsunami — as at Anak Krakatau in 2018.

How volcanoes are monitored

• Seismicity. Swarms of small earthquakes track magma moving; a shift to continuous volcanic tremor often precedes eruption.
• Ground deformation. GPS and satellite radar (InSAR) detect swelling of millimetres as magma accumulates.
• Gas emissions. Rising sulphur dioxide indicates magma nearing the surface.
• Thermal and satellite observation. Hotspots, plumes and ash clouds.

Precursors usually appear days to months ahead, which makes volcanoes considerably more forecastable than earthquakes. But unrest frequently fades without eruption, so authorities face a hard choice between costly evacuations and the risk of being too late.

What to do if you live near one

• Know your volcano's alert scheme and what each level requires of you — they differ by country.
• Know the designated exclusion zones and evacuation routes, and never enter a closed zone.
• Keep N95-type masks and goggles for ashfall, and plan to clear roof ash before it accumulates — but only where it is safe to do so.
• Understand your lahar risk. If you live in a valley draining a snow-capped or crater-lake volcano, that is your main threat, and it can arrive with no eruption at all.
• Store water; ash contaminates open supplies quickly.

Read the volcano safety guide and follow activity on the volcanic activity page.

Frequently asked questions

What is the difference between a volcano alert level and an aviation colour code?

The alert level describes ground hazard and what people nearby should do. The aviation colour code describes the ash hazard to aircraft. They are issued independently and often differ — a volcano can be Red for aviation while the ground alert is only a Watch.

What does VEI mean?

Volcanic Explosivity Index, a 0 to 8 logarithmic scale of eruption size based on erupted volume and plume height. Each step is roughly a tenfold increase. It is assigned after an eruption, so it is not a warning.

Can volcanic eruptions be predicted?

Better than earthquakes. Seismic swarms, ground swelling and rising gas emissions typically appear days to months before an eruption. But unrest often fades without eruption, so timing remains uncertain.

What is the deadliest volcanic hazard?

Pyroclastic density currents and lahars cause most volcanic deaths, not lava. Lahars in particular can travel tens of kilometres down river valleys and can occur years after an eruption when rain remobilises ash deposits.

Is volcanic ash dangerous to breathe?

Yes. Volcanic ash is fine, sharp, abrasive rock and glass. It irritates the airways and is especially dangerous for people with asthma or other lung conditions. Use an N95-type mask and goggles, and stay indoors during ashfall where possible.

Sources

• USGS Volcano Hazards Program — Alert levels and aviation colour codes
• Smithsonian Global Volcanism Program
• ICAO — Crisis management and the International Airways Volcano Watch
• Indonesian Center for Volcanology and Geological Hazard Mitigation (PVMBG)
• Japan Meteorological Agency — Volcanic Warnings and Alert Levels

Found an error? See our corrections policy.