Volcanoes guide
Volcano Alert Levels and the VEI: How Eruption Danger Is Measured
When a volcano starts to stir, the observatory watching it raises its alert level. Here is what those levels and colours mean, how scientists measure the size of an eruption, and how many volcanoes are erupting at any time.
Updated 4 November 2026
Two kinds of alert
Many observatories give each volcano two ratings: one for people on the ground and one for aircraft, because volcanic ash can damage jet engines far from the volcano itself.
In the United States, the US Geological Survey uses these:
| Ground alert level | Aviation colour code | Meaning |
|---|---|---|
| Normal | Green | The volcano is in its usual quiet state |
| Advisory | Yellow | Signs of unrest above the usual level, such as more earthquakes, ground swelling or gas |
| Watch | Orange | Unrest is increasing and an eruption is possible, or a minor eruption is under way |
| Warning | Red | A dangerous eruption is under way or expected soon |
The aviation colour codes are used around the world. The ground levels differ from country to country: Indonesia, Japan, the Philippines, Italy and others have their own systems, some with four levels and some with five. A level is always set by the national observatory, which knows that volcano best.
What observatories watch
- Earthquakes: swarms of small quakes as rising magma breaks rock.
- Ground movement: the volcano swelling or sinking, measured by GPS and by satellite radar to within millimetres.
- Gas: more sulphur dioxide and carbon dioxide coming out as magma nears the surface.
- Heat: hot spots seen by thermal cameras and satellites.
Unrest doesn't always end in an eruption. Many volcanoes rise to yellow or orange and calm down again without erupting.
How big was the eruption? The VEI
The Volcanic Explosivity Index (VEI) rates explosive eruptions from 0 to 8, mainly by how much rock and ash they throw out and how high the plume rises. From VEI 2 upwards, each step means about 10 times more material.
| VEI | Ash and rock erupted | Example |
|---|---|---|
| 0–1 | Less than 0.001 km³ | Lava fountains and flows in Hawaii and Iceland |
| 2–3 | 0.001–0.1 km³ | Frequent explosive eruptions at Etna and Sakurajima |
| 4 | At least 0.1 km³ | Eyjafjallajökull, Iceland, 2010, which closed much of Europe's airspace |
| 5 | At least 1 km³ | Mount St Helens, USA, 1980; Vesuvius, AD 79 |
| 6 | At least 10 km³ | Pinatubo, Philippines, 1991, which cooled the planet by about half a degree |
| 7 | At least 100 km³ | Tambora, Indonesia, 1815, followed by the "year without a summer" |
| 8 | At least 1,000 km³ | Toba, Sumatra, about 74,000 years ago |
Most eruptions are VEI 2 or smaller. A VEI 6 happens a few times a century; nothing of VEI 8 has happened in recorded history.
How many volcanoes are erupting?
About 1,350 volcanoes on land are considered potentially active, and roughly 50 to 70 of them erupt in a typical year. At any moment a few dozen are in eruption, many of them for months or years at a time. The Smithsonian Institution's Global Volcanism Program and the USGS publish a weekly report of new and ongoing activity, and the Volcano app shows it alongside each volcano's latest alert level.
Not an official warning service. In an emergency, follow your local authorities.