Mount Vesuvius, the volcano that destroyed Pompeii in 79 AD, is classified as active and extremely dangerous. While it has been quiet for decades, its location on the Gulf of Naples makes it one of the most closely watched volcanoes on Earth. Its potential for explosive eruptions, coupled with the dense population on its slopes, necessitates continuous monitoring. The threat posed by Vesuvius is not a matter of if it will erupt again, but when, making surveillance a priority for Italian Civil Protection.
Vesuvius’s Geological Classification
Vesuvius is categorized as a Somma-stratovolcano, characterized by the younger volcanic cone, the Gran Cono, growing within the caldera of the older, collapsed Mount Somma. This composite cone is built up from alternating layers of lava, ash, and other volcanic materials, giving it a distinctive steep profile. Volcanologists describe its current status as “active but quiescent,” meaning it is capable of erupting but is currently resting.
The geological setting, where the African plate subducts beneath the Eurasian plate, feeds Vesuvius with magma rich in silica and gas. This composition causes the magma to be highly viscous, trapping vast amounts of gas and leading to a significant pressure buildup. This stored energy results in highly destructive Plinian-style eruptions, characterized by massive, vertical columns of ash and gas reaching tens of kilometers into the atmosphere.
The Eruption History Since 79 AD
The catastrophic eruption that buried Pompeii and Herculaneum in 79 AD was one event in Vesuvius’s long history. Following that event, the volcano entered a cycle of frequent, smaller eruptions punctuated by larger, destructive episodes. Activity often shifted between periods of open-conduit, low-level activity and longer periods of quiescence that ended with a violent explosion.
One devastating post-79 AD eruption occurred in December 1631, ending a long quiet period and killing an estimated 3,000 people as pyroclastic flows and lahars swept down the slopes. The volcano then remained semi-continuously active until the most recent significant eruption in March 1944. This event, which took place during World War II, marked the end of the volcano’s last eruptive cycle and began the current period of repose. The long quiet period since 1944 concerns volcanologists, as the longer the rest period, the more pressure is likely building in the magma chamber, increasing the probability of a high-energy, explosive eruption.
Current Monitoring of Vesuvius
The Vesuvius Observatory, part of the National Institute of Geophysics and Volcanology (INGV), maintains a sophisticated surveillance network to track the volcano’s internal status. Seismic monitoring uses a network of sensors to detect and locate minor earthquakes beneath the structure. These tremors indicate rock fracturing caused by the upward movement of magma and pressurized fluids.
Geodetic monitoring tracks physical deformation of the volcano’s surface, signaling magma intrusion and swelling. This is achieved using high-precision GPS stations and tiltmeters, which measure minute changes in ground inclination and elevation. Geochemical monitoring samples and analyzes gases emitted from the crater and soil, especially carbon dioxide and sulfur dioxide. An increase in the volume or change in the composition of these gases serves as a precursor, suggesting that fresh magma is rising and degassing closer to the surface.
Assessing the Risk of Future Eruptions
A future eruption of Vesuvius poses an extreme risk due to its explosive nature and proximity to the Naples metropolitan area. The most severe threat comes from pyroclastic flows—superheated avalanches of gas, ash, and rock fragments that travel at hundreds of kilometers per hour. These flows are unsurvivable and define the boundaries of the high-risk area.
Italian Civil Protection has designated a “Red Zone,” encompassing 25 municipalities and parts of Naples, placing approximately 700,000 residents at risk from pyroclastic flows. A secondary hazard involves the heavy fall of ash and lapilli, which can cause roof collapse and infrastructural damage in the surrounding “Yellow Zone.” The national emergency plan mandates a preventive evacuation of the entire Red Zone population, with a target of completion within 72 hours once an alarm is declared.