The sedimentary substrate of the Colli Albani volcano
The structure, age and facies distribution of the sedimentary substrate of the Colli Albani volcano has been investigated by analysing sedimentary xenoliths enclosed in pyroclastic and phreatomagmatic deposits from different eruptive centres of the volcano, for comparison with the stratigraphies of deep wells drilled in the area and the known surface geology of the areas surrounding the volcano. Thin-section analysis has been carried out on the same xenolith samples collected and discussed by Funiciello and Parotto (1978. Il substrato sedimentario nell’area dei Colli Albani: considerazioni geodinamiche e paleogeografiche sul margine tirrenico dell’Appennino Centrale. Geologica Romana, 17, 233–287.) and Amato and Valensise (1986. Il basamento sedi- mentario dei Colli Albani: risultato di uno studio degli ejecta dei crateri idromagmatici di Albano e Nemi. Memorie Società Geologica Italiana, 35, 761–767.), largely from the maars deposits of the most recent Via dei Laghi period of activity of the volcano. The pre-orogen marine stratigraphic sequences range from Late Triassic?–Lower Lias to Middle Miocene, with carbonate platform-to-basin facies. These sequences are very similar to those cropping out in the Sabina area to the north and NE of the volcano. Upper Cretaceous carbonate platform units, similar to those cropping out in the Lepini Mts, to the east of the volcano, have been recognized only in the xenoliths from the Nemi maar. Similar to that observed in the outcropping geology of the Sabina region, no evidence of siliciclastic xenoliths related to the Upper Miocene foredeep units have been found. The Upper Miocene?–Lower Pliocene to Upper Pliocene post-orogen marine sedimentary cycle has been recognized in almost all the eruptive centres, and has similarities with the sedimentary units present in the Rome area. The absence of marine Quaternary sediments among the xenolith types suggests that the Colli Albani area was continental before the onset of volcanism.
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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.