Mount Vesuvius, a volcano towering over the Bay of Naples, is widely considered one of the world’s most threatening natural hazards due to its geological power and the sheer number of people living in its shadow. The stratovolcano is infamous for its catastrophic eruption in 79 AD, which buried the Roman cities of Pompeii and Herculaneum. Millions of residents currently inhabit the surrounding areas, making the question of when it will erupt again a matter of immense public concern. Scientists and civil authorities acknowledge that while the exact date of the next event cannot be predicted, the volcano’s activity is under constant surveillance.
Vesuvius’s Eruptive History and Patterns
Vesuvius has a history of alternating between long periods of quiet and highly explosive eruptions, defining its current risk profile. The 79 AD Plinian eruption was a catastrophic event, ejecting an ash and gas column over 20 miles high. High-energy eruptions like this are generally preceded by long dormancy periods, allowing pressure to build in the magma chamber.
Following 79 AD, Vesuvius entered a phase of more frequent, less intense activity, including the sub-Plinian event of 1631 and the effusive-explosive eruption of 1944. The 1944 eruption, which occurred during World War II, was the last time the volcano released lava, ending a cycle of open-conduit activity that had persisted since 1631. Since 1944, Vesuvius has been in a state of repose, which suggests the conduit is blocked.
This prolonged inactivity means the next eruption is likely to be an explosive event of medium to high energy, as pressure accumulates beneath the solidified cap. The longer the repose period, the greater the potential energy released. The current quiet phase has lasted over 80 years, suggesting a significant buildup of magma and volatile gases is possible.
Current Status and Scientific Monitoring
Vesuvius is currently classified at the “basic” or “green” alert level, meaning no anomalous phenomena are occurring. The Vesuvius Observatory (Osservatorio Vesuviano), part of the National Institute of Geophysics and Volcanology (INGV), maintains a sophisticated 24/7 monitoring network. This system tracks subtle changes that might signal a reawakening, detecting the movement of magma and the buildup of subsurface pressure.
Monitoring includes seismographs that track micro-quakes caused by fracturing rock as magma moves toward the surface. Geodetic instruments, such as GPS receivers and tiltmeters, measure ground deformation, indicating inflation or swelling of the volcano’s flanks as the magma chamber pressurizes. The observatory also employs geochemical monitoring, continuously analyzing the composition and temperature of gases emitted from fumaroles and the ground.
Changes in gas chemistry, particularly increases in carbon dioxide (CO2) or sulfur dioxide (SO2), can suggest new magma is rising. Researchers have noted a multi-year trend of decreasing hydrothermal activity within the crater area, interpreted as normal for a quiescent state. Any deviation from the current baseline—such as increased seismic activity, rapid ground uplift, or dramatic gas changes—would trigger a progression through the established alert levels (Green, Yellow, Orange, Red), prompting civil protection measures.
Potential Eruption Scenarios and Hazards
The next eruption of Vesuvius is modeled on two primary scenarios: a smaller, localized event or a large, Plinian-style eruption. Based on recent history, the most likely scenario is an explosive eruption of medium-low energy, characterized by an eruptive column several kilometers high and the fallout of ash and lapilli. The civil protection plan is conservatively based on a medium-energy explosive eruption scenario to ensure safety.
The greatest hazard is the pyroclastic flow, a superheated avalanche of gas, ash, and volcanic fragments that can move up to 435 miles per hour. Models show these flows could reach populated areas near the base in less than 10 minutes, making them unsurvivable. Preventive evacuation is the only effective measure to protect human life in the immediate surrounding areas.
Ash fall poses a secondary, geographically broader threat, as volcanic material can be carried tens of miles away. Heavy ash accumulation can cause the collapse of building roofs and severely disrupt transportation, potentially closing Naples International Airport. The combination of ash and water can also generate lahars, or destructive mudflows, which threaten areas farther afield along river valleys.
The Human Response: Evacuation Planning
The Italian Department of Civil Protection developed a comprehensive National Emergency Plan to safeguard the nearly 700,000 people residing in the high-risk zones. The plan focuses on two main geographical areas: the “Red Zone” (Zona Rossa) and the “Yellow Zone.”
The Red Zone
The Red Zone includes 25 municipalities considered most vulnerable to pyroclastic flows and is designated for mandatory preventive evacuation. The only measure for saving lives is to move all residents out before the eruption begins. This process is planned to take a maximum of 72 hours once the “alarm” operational phase is triggered. Municipalities in the Red Zone have been “twinned” with specific Regions and Autonomous Provinces across Italy, which will host the evacuees.
The Yellow Zone
The Yellow Zone covers 63 municipalities and parts of Naples, where the primary risk is heavy ash fall that could cause structural damage. Evacuation from the Yellow Zone is not mandatory before an eruption but may be necessary during the event, depending on wind direction and the ash cloud’s extent. The entire strategy hinges on the monitoring system providing sufficient warning time for this massive logistical undertaking.