The Recent Activity of the Lake Albano Maar (Colli Albani Volcano, Roma, Italy).

Lake Albano is a composite maar that fed the last phases of activity of the Colli Albani volcano, located just south of Roma, Italy. The study of several new stratigraphic sections opened by archeological escavations and civil works has revealed the existence of previously unknown phreatomagmatic deposits, and of several lahar deposits which infill the Last-Glacial fluvial network to the northwest of the maar. 14C radiometric dating indicate an age of 5000+-100 years for the youngest syn-eruptive lahar deposit and indicate that the Colli Albani volcano must be considered quiescent. The distribution of the Protostoric human settlements in the area and the revision of the ancient history and myths of Roma, are coherent in indicating that the activity of lake Albano is much younger than previously believed. Until the IV Century b.C. catastrophic exondations occurred from the lowest rim of the crater lake, with lahar emplaced mainly in the Ciampino plain and extending to the southern suburbs of Roma. The repetition of these phenomena was prevented by a drain-tunnel dug by ancient Romans. The overflows were likely triggered by sudden injections, at the bottom of the lake, of hot and CO2-rich fluids that are present underneath the volcano. The presence of several gas emission sites, the high CO2 flux in zones corresponding to structural highs of the carbonate basement, the existence of pressurised aquifers also at shallow depth and the reported sudden increase of water temperature and gas release in relation to earthquakes, indicate that a similar hazard persists nowadays.