THE VOLCANIC ROCKS OF ROME: ERUPTIVE HISTORY, EMPLACEMENT MECHANISMS, AND ENVIRONMENTAL CONDITIONS

The city of Rome was founded, for the most part, on pyroclastic rocks erupted during the Middle to Upper Pleistocene from neighboring Monti Sabatini and Colli Albani volcanic districts. These volcanic rocks formed the substrata of ancient Roman houses and monuments and provided the first sources of stone building material for the early inhabitants of Rome. Indeed, Roman builders employed a large variety of lithological types, ranging from hard lavas to light and resistant tuffs to loose, often pumiceous, granular volcanic ash, for paving slabs, dimension stones, and the reactive components of pozzolanic mortar in ancient concrete. In recent years, numerous ⁴⁰Ar/³⁹Ar method age determinations - performed at the Berkeley Geochronology Center (CA, U.S.A.) in collaboration with the Istituto Nazionale di Geofisica e Vulcanologia (Rome, Italy) - have allowed a detailed reconstruction of the eruptive histories of the Monti Sabatini and Colli Albani volcanic districts, and a revision of their stratigraphy. Furthermore, the dated volcanic layers provide time constraints for the coastal sedimentary deposits of the Paleo-Tiber River, among which they are intercalated, and an investigation of the interplay of sedimentation, eustatism and volcanism in the Roman area. The diverse petrographic features of these pyroclastic rocks are the direct consequence of original magma composition, eruptive mechanism, modality of emplacement, environmental conditions at the moment of deposition, and post-depositional physical-chemical alteration and weathering processes. In particular, glacio-eustatic fluctuations coupled with tectonic uplift provided continuous paleogeographic and environmental changes that strongly influenced the stratigraphic setting and alteration of the volcanic rocks. Techniques developed by R. L. Hay for analysis of primary volcanic and secondary authigenic processes give new information on the complexity of these events.