The structure of the Colli Albani, about 20 km south of Rome, is the southernmost of a chain of volcanoes, which runs along the Tyrrhenian coast of the Lazio region. The evolution of volcanism in this area is closely linked to the distensional tectonics affecting the western margin of the Apennine chain during the last two million years.
The Colli Albani represent a complex central apparatus, the result of predominantly explosive activity alternating with long phases of inactivity. The morphology resembles that of Somma-Vesuvius, with an edge enclosing a flat area about 8 km in diameter, within which is another volcano. The products of older activity form the outer rim of the Monti Tuscolani and Artemisio. Within this semi-circular structure rises then the most recent volcano, the Faete cone. Standing 932 m high, it slopes toward the southwest, where it is interrupted by the craters that now host Lake Albano and Lake Nemi.
The eruptive activity began 600,000 years ago and continued until at least 20,000 years ago. At present this volcanic complex is considered quiescent, still presenting evidence of hydrothermal and seismic activity
The origin of the Colli Albani volcanoes is not very different from that of the others bordering the western coast of the Italian peninsula. The earth's crust of the area in which they grew is crossed by deep and long faults that reach as far as the Tyrrhenian Sea. On these faults are triggered others, with almost perpendicular trends, produced by the stretching of the earth's crust. The Colli Albani arose at the intersection of the two fault systems, preferential pathways for magma to rise to the surface.
The eruptive activity could be distinguished into three main phases.
The first phase, called the "Tuscolan-Artemisian," refers to a period between 600,000 and 350,000 years ago and was characterized by a large volume of erupted magma (>280 cubic kilometers). During this phase, there were a series of large eruptions, at least four in number, with emission mainly of pyroclastic flows, fall pumice, and lava flows. These products, which covered a very large area, about 1600 square kilometers, including the area on which much of the city of Rome stands, were later intensively exploited for construction. The last of these eruptions, called the Villa Senni eruption, caused the collapse of the central portion of the volcanic apparatus, now identifiable with the horseshoe-shaped area extending from Mount Tuscolo to Mount Artemisio.
In the second phase known as the "Faete," between 350,000 and 270,000 years ago, volcanic activity resumed within the caldera, where the Faete cone was built. The activity was characterized by both Strombolian-type explosions and the emission of large lava flows, such as that of Capo di Bove (280,000 years ago), which covered the volcano's northwest flank.
The third and final phase known as the "Hydromagmatic" phase, between 270,000 and about 20,000 years ago, was characterized by violent explosions and led to the formation of a series of isolated craters.
The products emitted during this phase are characteristic of explosive events caused by contact between magma and water (hydromagmatic or phreatomagmatic). The eruptions of this type, are usually of short duration and form broad volcanic structures (Maar), with low elevated edges and with the crater floor at a lower elevation than the surrounding area. The numerous eruptions of this phase, formed a series of craters that later became lake basins: in the northern part of the Alban Hills district, the small centers of Prato dei Porci, Pantano Secco and Valle Marciana, while in the western sector Albano and Giuntura and almost simultaneously the centers of Ariccia and Nemi.
The last eruptions in the area date from about 20,000 years ago, and since then the volcanic complex has been in a state of quiescence, characterized by seismic swarms, ground deformation, and gaseous emissions distributed along the outer margins of the volcano.
Currently, the Colli Albani volcanic complex is in a quiescent condition, with volcanic manifestations represented mostly by ground deformation, seismic and hydrothermal activity, the latter characterized by sulfur springs and gaseous emissions distributed along the outer margins of the volcano.
Currently, the main volcanic risk in the Colli Albani area is toxic gas emissions-CO2, H2S, SO2, CH4, and Rn-that rise from soil fractures. The main emanation points are located in Cava dei Selci, Santa Maria delle Mole, Marino, Frattocchie, Ciampino and Tor Caldara. In the past, these areas have been affected by the sudden release of gas from soil or aquifers, sometimes in conjunction with seismic events or as a result of excavations made for wells. An additional aggravating factor in terms of the amount of gas released may be the lowering of the water table. In fact, the economic and urban development that the area has undergone over the past 50 years has caused excessive exploitation of the water resource for both private and agricultural-industrial uses. The conspicuous lowering of the piezometric level of the aquifer results in a reduction of carbon dioxide dissolved in the aquifer, with the consequent increase of this gas in the air.
The emission occurs preferentially at fractures, along which gases in the deep layers rise more easily to the surface. The phenomenon can vary in space and time: in fact, earthquakes can produce the opening of new fractures and thermal water circulation can seal others. Gases rising from the ground can form bubbles in standing water or mud pools and can damage vegetation or cause animal deaths.
In particular, carbon dioxide, which is denser than air, in the absence of wind tends to accumulate at ground level where it can reach very high concentrations, and being colorless, odorless, and tasteless is difficult to detect. At low concentrations it can cause increased respiratory activity, nausea, visual disturbances and for high concentrations asphyxiation.
Concentrations of gas can also be detected in dwellings as these can flow along small fractures in the ground, or through pipes and ducts, and stagnate down near the floor in basement rooms.
The facility in charge of monitoring volcanic activity in Colli Albani is the Rome Section of the National Institute of Geophysics and Volcanology (INGV). The network consists of seismicity and ground deformation monitoring systems. Periodic water and gas measurements and sampling are also carried out.