A Quiescent Giant's Restless Sleep
Just 20 kilometers southeast of Rome, the Alban Hills (Colli Albani) volcanic complex has been quiet for about 36,000 years. This apparent tranquility is deceptive. Scientific monitoring reveals a dynamic system beneath the surface. The complex is a broad caldera, a collapsed volcanic structure some 10 by 12 kilometers in diameter, formed by massive eruptions between 600,000 and 350,000 years ago. Later activity, from around 300,000 to 200,000 years ago, built the smaller, Monte Faete cone. The final eruptive phase, lasting until approximately 36,000 years ago, was characterized by violent interactions between magma and water. These phreatomagmatic explosions created the crater lakes, or maars, of Albano and Nemi that are visible today.
Geological studies suggest the volcano may operate on a 31,000-year cycle of eruption and dormancy, which indicates it is currently overdue for a new period of activity. While it is considered quiescent, the volcano is not extinct. Ongoing phenomena include seismic swarms, ground deformation, and the release of volcanic gases, all showing a live magmatic system.
Evidence from Above and Below
Modern instruments provide a clear picture of the unrest at Alban Hills. Satellite-based InSAR (Interferometric Synthetic Aperture Radar) measurements show the ground is uplifting at a rate of about 2 millimeters per year. Over the last 200,000 years, the total uplift in the area is estimated to be 50 meters. This swelling is consistent with the slow accumulation of magma in a chamber located 5 to 6 kilometers beneath the surface. The existence of this magma body is further supported by seismic tomography, which maps variations in the speed of seismic waves as they travel through the Earth's crust.
Earthquake swarms are another sign of activity. A significant series of tremors occurred between 1989 and 1990, and seismic events continue to be recorded. These earthquakes are generally shallow, with hypocenters located above the 6-kilometer-deep top of the magma reservoir. The ground releases large amounts of carbon dioxide (CO2) and hydrogen sulfide (H2S). At a single degassing site, Cava dei Selci, the CO2 output fluctuates between 5.6 and 24.8 tonnes per day. This gas release, sometimes lethal to animals in low-lying areas, originates from the deep magmatic source and rises through fractures in the rock.