Geochronology of Colli Albani volcano
This paper reports on the main studies published on the chronological evolution of Colli Albani volcano activity and related carbonate deposition, from the earlier works of the nineteen century up to recent times. The first method that was systematically applied to dating volcanic products was K–Ar during the 1960s and 1970s. During successive decades, 39Ar/40Ar and U-series isochron methods were used to refine the chronology of different lithosomes. A careful and detailed revision of published data reveals some systematic discrepancies among ages calculated using the different methods. The main problems can be summarized as follows: (i) divergence of K–Ar age determinations from 39Ar/40Ar data because of the presence of excess argon; (ii) internal incongruity of K–Ar ratios measured on different crystals of the same mineral phase as a consequence of possible weathering processes; (iii) frequent disagreement between 39Ar/40Ar and U-series ages, mainly for older products, due to the presence of U-rich microphases within the fractures or along the boundaries of crystals, formed during late-stage hydrothermal activity. In general, the 39Ar/40Ar method seems to provide the most reliable and coherent data, even if it can occasionally be affected by problems due to a homogeneous distribution of excess 40Ar within crystals and/or by the occurrence of inherited crystals from the substrate. In conclusion, with reference to 39Ar/40Ar chronology, the activity of Colli Albani volcano can be constrained between c. 608 and 26 ka. In particular, available age determinations for lithosome IV (Vulcano Laziale caldera complex) range between c. 608 and 338 ka (most likely range >600–355 ka); for lithosome III (Tuscolano–Artemisio peri-caldera fissure system) from about 357 to 178 ka (most likely range <355–?180 ka); for lithosome II (Faete intracaldera stratovolcano) from 283 to <37 ka (most likely range <355–250 ka); and for lithosome I (Via dei Laghi maar field) from 372 to 26 ka (most likely range <200–26 ka). It is worth noting that a 14C age of about 5.8 ka has been obtained on a palaeosoil within the last lithostratigraphic unit (TAL).
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The Colli Albani volcano (also Alban Hills volcano) is the large quiescent volcanic field that dominates the Roman skyline. The Colli Albani is one of the most explosive mafic calderas in the world, associated with intermediate to large volume ignimbrites. At present it shows signs of unrest, including periodic seismic swarms, ground uplift and intense diffuse degassing, which are the main short-term hazards. New studies have discovered deposits related to previously unknown pre-Holocene and Holocene volcanic and phreatic activity. In the fourth Century B.C.E. Roman engineers excavated a tunnel through the Albano maar crater wall to keep the lake from breaching the rim and flooding the surrounding countryside, events that had previously destroyed this region several times.
The Colli Albani Volcano contains 21 scientific contributions on stratigraphy, volcanotectonics, geochronology, petrography and geochemistry, hydrogeology, volcanic hazards, geophysics and archaeology, and a new 1:50 000 scale geological map of the volcano. The proximity to Rome and the interconnection between volcanic and human history also make this volcano of interest for both specialists and non-specialists.