M Fabbri

Large volume phreatomagmatic ignimbrites from the Colli Albani volcano (Middle Pleistocene, Italy)

Abstract In this paper we describe four large volume, ash- and accretionary lapilli-rich, phreatomagmatic compound ignimbrite units, mafic in composition, from Colli Albani volcano, south of Rome. The four units, that form the ‘Pisolitic Tuffs’ succession, are separated by paleosols and represent the earliest explosive large volume eruptive episodes from the Quaternary Alban Hills volcano. The occurrence of large volume phreatomagmatic–phreatoplinian eruptions implies the availability of large quantities of water interacting with the rising magma. The paleogeography of the area below the volcano has been reconstructed by the analysis of stratigraphic data from more than a thousand bore-holes distributed around the volcano that allowed to identify NW-trending and NE-trending paleotopographic lows that underlie the central area of the volcano and interpreted as extensional tectonic basins. These lows are filled with Lower to Middle Pleistocene, pre-volcanic lacustrine and fluvial deposits and suggest that at least the central part of the Colli Albani volcanic area hosted a large lake or lagoon. The absence of sedimentary xenoliths in the Pisolitic Tuffs and the low vesicularity of scoria and shards suggest that water interacted with a poorly fragmented magma at very shallow level, triggering the large explosivity of the eruptions. We suggest that water interacting with magma was mostly surficial water related to the presence of the lagoon or lake. In this environment, it is likely that vents were subaqueous allowing a continuous access of water to the conduit. Considering that the minimum calculated volume of products for the Pisolitic Tuffs succession is >37 km3 and each eruption unit averages approximately around minimum volume of 10 km3, we suggest that each eruption was related to a caldera collapse, which would have allowed the persistence of a Taupo-like lake in the central area and of phreatoplinian activity. After the last phreatomagmatic eruption, however, the fragmentation style of large volume ignimbrites from Colli Albani became magmatic, suggesting the extinction of the lake after that date. Each of the four units show a basal phreatoplinian fallout level, overlain by a complex association of low aspect ratio surge deposits and ignimbrites. Phreatomagmatic pyroclastic flow deposits are found at distances of more than 40 km from the central area of the volcano, and show important facies variation according to the paleotopography. To the west of the volcano, pyroclastic flows reached the Tyrrhenian coast and emplaced mostly stratified facies on a flat topography, interpreted to reflect both the spreading of pyroclastic flows on an unconfined topography and their interaction with lacustrine and lagoon areas. By contrast, to the east of the volcano, where pyroclastic flows were confined within paleovalleys, the main facies is thick and massive.

Facies associations of rain-generated versus crater lake-withdrawal lahar deposits from Quaternary volcanoes, central Italy

Abstract Two syneruption lahar deposit successions from Quaternary Italian volcanoes are presented, displaying different facies associations interpreted to reflect different water sources. The lahar deposits associated with the White Trachytic Tuff Cupa (WTTC) ignimbrites from the Quaternary Roccamonfina volcano, located 150 km to the southeast of Rome, have been interpreted in terms of rain-generated lahars. The WTTC ignimbrites are made of more than 1 km 3 of loose pumice and lava lithic debris emplaced along the hyperbolic slope of the volcano at ca. 300 ka during an interglacial period characterised by mild and wet climate. The lahar deposits are organised in a coarsening-upward, aggradational, and back-stepping succession of medium- to thick-bedded, progressively juvenile-poorer, non-cohesive debris flow to fluvial deposits. Box-shaped channels cut the WTTC ignimbrites along the steep upper slopes. Channels are filled with lava lithic-rich fluvial to hyperconcentrated-flow sand and conglomerate, which are interpreted as lag deposits related to processes of bulking due to the removal of light pumice and ash debris from the upper slope. Along the lower slopes of the volcano and in the surrounding ring plains where the average slope inclination decreases to few degrees, lahars emplaced an aggradational succession of bedded, ash-rich, hyperconcentrated-flow deposits entirely derived from WTTC components. The succession coarsens upward with increasing presence of lava-rich conglomerate lenses, fluvial in origin, interpreted to record the progressive restoration through time of the drainage network. The succession is cut by incised gullies filled with polygenetic fluvial deposits which indicate the restoration of intererruption condition. By contrast, the ca. 23-ka, small-volume, Peperino Albano phreatomagmatic eruption from Colli Albani volcano, located 30 km to the southeast of Rome, emplaced a valley-ponded, block and ash ignimbrite, which, along the western slope of the volcano, grades laterally into a single, far-reaching, thick lahar deposit. The lahar deposit coarsens upward from coarse-ash, hyperconcentrated-flow deposit into a lithic-block-rich, debris-flow deposit. This lahar deposit has been interpreted to be directly derived from a pyroclastic flow and particularly related to the entrance of the pyroclastic flow into a pre-existing maar crater lake along the pyroclastic-flow path. The basal sand-size, hyperconcentrated-flow deposit is interpreted to represent early deposition from the fast frontal flood wave, whereas the coarse lithic-rich debris-flow deposit at the top may represent the rear of the lahar. The separation of the two facies can be related to processes of ‘hydraulic sieving’ operated by the lake water, which couples with ash particles, leaving behind the coarser fraction.

I Geositi Urbani di Roma Capitale. Fra geologia, paesaggio e memoria

About four years ago, the Lazio chapter of the Italian Society of Environmental Geology (SIGEA) launched a survey of Urban Geosites within the municipality of Rome, employing specially-designed data sheets to be compiled primarily by the Societys members. The goal of the survey was to define a certain number of sites, areas or elements affected by urban development, where it was possible to establish geological value worthy of conservation.The project was devised in order to highlight the fact that geological studies in urban areas are adversely affected by a number of circumstances, first and foremost the scarcity of geological outcrops destroyed, excavated or otherwise hidden beneath the continuum of asphalt and cement. It was thus decided to create a record of the outcrops intentionally left within urban areas, as well as those presumably destined to disappear and those that have already disappeared but for which we possess historical/photographic documentation.

The smart use of the Valle Murcia geology in the construction of Circus Maximus

ABSTRACT The Murcia Valley is one of the largest tributaries to the Tiber river; south-east the Tiber Island. The valley, between the Palatine and Aventine hills, was a wide waterlogged area. Despite attempts to land reclamation started as early as the 7th century BC under Lucius Tarquinius Priscus, the swampy nature and difficult management, suggested to the romans to destine the area, for its proximity to the Foro Boario, to ritual activities and socialization, such as games and competitions in the largest building for the show of all time that is the Circus Maximus. Through surveys ERT (Electrical Resistivity Tomography) we investigated the geometry of the original valley to understand how the romans had operated to drain the valley. Results suggest that the Murcia Valley, where the Circus Maximus is, probably was a anastomosed flood plain. The romans forced the waters in two side channels at an embankment realized through the union of sandy-clayey bars. This latter over time was monumentalised with the Spina around which ran the quadrigas. This interpretation well agreement with respect for nature that Romans observed. Furthermore, we recognize an original oblique NW-SE trend of the Spina, according to the orientation of the Imperial palaces on the Palatin hill. KEY WORDS: Circus Maximus, anastomosed valley, Rome, paleogeography.