Why volcanoes need a standardized alert system
Unlike earthquakes, which strike without warning, volcanic eruptions typically build through detectable precursory stages: increased seismicity, ground deformation, gas emissions, and thermal anomalies. Volcanic observatories monitor these signs and need a clear, consistent way to communicate the current threat level to the public, emergency managers, and aviation authorities.
The United States Geological Survey's Volcano Hazards Program uses a two-part alert system: a Volcano Alert Level for ground-based hazards and an Aviation Color Code for airborne ash hazards. Both use four tiers, and they often — but not always — match each other.
The four alert levels explained
Normal (Green): The volcano is at typical background levels of activity. Seismicity is low, there is no unusual deformation, and gas emissions are within baseline ranges. This does not mean the volcano is extinct — many volcanoes maintain low-level fumarolic activity at Normal status for decades.
Advisory (Yellow): The volcano is exhibiting signs of elevated or escalating unrest above known background levels. This might include increased seismic activity, changes in gas emission rates or composition, slight ground deformation, or minor phreatic (steam-driven) explosions. Advisory status means the volcano has the attention of monitoring scientists and could evolve toward eruption — or could return to Normal.
When the level goes to Watch and Warning
Watch (Orange): The volcano is exhibiting heightened or escalating unrest with increased potential for eruption, or an eruption is underway but poses limited hazards. At Watch level, monitoring is intensified, and emergency management agencies begin pre-positioning resources and reviewing evacuation plans.
Warning (Red): A hazardous eruption is imminent, underway, or suspected. Volcanic ash may be rising into the atmosphere at altitudes that threaten aviation. Ground-level hazards including pyroclastic flows, lahars, lava flows, or ashfall may be affecting populated areas. Warning level triggers the highest level of emergency response.
- Green/Normal: background activity, no threat
- Yellow/Advisory: elevated unrest, monitoring intensified
- Orange/Watch: significant unrest or minor eruption, prepare for escalation
- Red/Warning: major eruption underway or imminent, take protective action
Aviation color code: protecting flight paths
Volcanic ash is extremely dangerous to aircraft. Ash particles melt inside jet engines, coating turbine blades and potentially causing engine failure. Even at concentrations invisible to pilots, ash can damage windscreens, pitot tubes, and engine components. Nine Volcanic Ash Advisory Centers (VAACs) worldwide issue advisories when ash is detected or forecast in aviation airspace.
The aviation color code mirrors the ground alert levels: Green means no significant ash emission, Yellow means elevated unrest but no significant ash, Orange means heightened potential for ash emission, and Red means significant ash emission is occurring. Airlines route around ash clouds based on these advisories, and the economic impact of rerouting can be substantial — as demonstrated by the 2010 Eyjafjallajökull eruption that closed European airspace for weeks.
How observatories monitor volcanic unrest
The instruments that inform alert level decisions include seismometers (detecting volcanic earthquakes and tremor), GPS and tiltmeters (measuring ground deformation as magma moves), gas spectrometers (measuring SO2 and CO2 emissions), thermal cameras and satellite sensors (detecting temperature changes), and visual observation (webcams and field crews).
Each data stream tells a different part of the story. Increasing seismicity suggests magma is moving through rock. Ground inflation suggests magma is accumulating at shallow depth. Rising SO2 emissions suggest magma is degassing near the surface. Thermal anomalies in crater lakes suggest heat transfer from below. Observatory scientists synthesize all of these signals to assess the most likely near-term scenario.
International variations in alert systems
While the USGS four-color system is widely referenced, not all countries use identical schemes. Japan's JMA uses a five-level system. Indonesia's PVMBG uses a four-level system with different terminology. Italy's INGV uses a system adapted to its specific volcanic settings including Vesuvius, Etna, and Stromboli.
The International Civil Aviation Organization (ICAO) standardizes the aviation color code globally, but ground-level alert systems vary by country and volcano. This can create confusion when international media reports on eruptions using unfamiliar terminology. The underlying principle is consistent: escalating levels of monitoring confidence that eruptive activity is increasing.
What alert levels do not tell you
An alert level communicates the current state and trend of volcanic unrest. It does not predict exactly when an eruption will occur, how large it will be, or what specific hazards it will produce. A volcano can sit at Orange/Watch for weeks or months without erupting. Conversely, some eruptions escalate from Yellow to Red in hours.
The alert system also does not replace local hazard maps, which show the specific areas at risk from different volcanic hazards (lava flows, pyroclastic flows, lahars, ashfall) based on the volcano's eruptive history and topography. Alert levels tell you what the volcano is doing now. Hazard maps tell you what areas are threatened if it erupts. Both are needed for informed decision-making.