A century of observation
Systematic monitoring of Vulcano's Fossa crater began after its last major eruption in 1888-1890. That event, meticulously documented by volcanologist Giuseppe Mercalli, produced explosive bursts of viscous magma and became the type-example for what are now called "Vulcanian eruptions". Since then, the volcano has been in a state of repose, characterized by intense, high-temperature gas emissions from vents, or fumaroles, at its summit.
Continuous surveillance by Italy's National Institute of Geophysics and Volcanology (INGV) reveals long-term patterns in this activity. Temperature records of the hottest fumaroles show distinct cycles of heating and cooling. For example, between November 1998 and November 2000, the maximum temperature decreased from 420°C to 280°C. Afterward, a heating phase began, with temperatures reaching 470°C by September 2013. These fluctuations are not random; they appear to follow multi-decadal trends. These cycles are thought to show physical changes deep within the volcano's plumbing, such as the accumulation and subsequent massive escape of magmatic gases from a reservoir estimated to be 3.5 km deep.
A system in motion
The INGV maintains a dense network of sensors across the island to track its behavior in real time. This system measures seismicity, ground deformation, and the chemical composition and temperature of gases. The fumaroles vent a mixture of water vapor, carbon dioxide (CO2), and sulfur dioxide (SO2), among other gases. Changes in the ratios and temperatures of these gases can show shifts in the underlying magmatic-hydrothermal system.
A period of significant unrest began in September 2021, prompting authorities to raise the alert level to Yellow. This episode involved a sharp increase in micro-seismic events and changes in gas emissions. High-resolution thermal surveys conducted with drones mapped the extent of the heat anomaly. In October 2021, the area on the crater floor with temperatures above 100°C measured approximately 367 square meters. This area shrank to about 27 square meters by October 2023 but began to expand again by June 2024. This detailed data helps scientists model the dynamic processes of heat and gas transfer from the magma body below to the surface.