D De Rita

The Albano Maar Lake (Colli Albani volcano, Italy): recent volcanic activity and evidence of pre-Roman age catastrophic lahar events

Abstract The evaluation of volcanic hazard in the Roman hinterland related to the quiescent Colli Albani Volcano has recently been the subject of renewed attention and several interpretations by many authors. However, very little was known of the recent history of the volcano, making such interpretations rather speculative. The most recent activity of Colli Albani Volcano originated from the Albano polygenetic maar lake, which erupted several phreatomagmatic units, the most recent of which, the Peperino Albano ignimbrite, has been dated at around 25 ka. An area of several square kilometers centered around Albano Lake is presently the site of shallow and frequent seismic activity and gaseous emission as well as hydrothermal activity and is therefore considered the most prone to geologic hazards. This paper presents new stratigraphic and geomorphologic data as well as age determinations that allow rejuvenation of the most recent activity of the Colli Albani Volcano, and particularly the Albano maar lake, to the Holocene. This study allows for the first time to identify a potential hazard related to the Albano maar lake withdrawal interpreted to be related to endogenous causes, namely CO 2 emission. The main results of the study are: (1) the Peperino Albano is not, as is generally believed, the last phreatomagmatic eruption from the Colli Albani Volcano; a previously unrecognized phreatomagmatic surge deposit has been identified overlying the paleosol at the top of the Peperino Albano and related lahar deposits; (2) two lahar deposits separated by paleosols top the stratigraphic succession and are dispersed only to the NW, corresponding to the lowest point of the maar rim, indicating that catastrophic hydrologic events occurred at the Albano Lake in recent times; rapid and substantial lake-level variations and lake withdrawal are reported by Roman historians and recorded by the stratigraphy of the Albano Lake lacustrine sediments; (3) microfracturing related to seismic energy release is linked to sudden variation of CO 2 flow and upwelling of hydrothermal fluids. These occurrences across the lake are the likely causes that triggered during Holocene several episodes of lake withdrawal, rising the water table and probably triggering convective rollover of the lake water.

Valley pond and ignimbrite veneer deposits in the small-volume phreatomagmatic 'Peperino Albano' basic ignimbrite, Lago Albano maar, Colli Albani volcano, Italy: influence of topography

Abstract The ca. 23-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy) shows valley pond facies as well as veneer deposits along the maar rim and along topographic ridges. Valley pond facies is characterised mainly by massive structure and chaotic texture and can be up to 30 m thick. Veneer deposit facies is characterised by parallel to low-angle cross-stratified bedforms alternating fines-depleted lapilli-sized layers, and massive, matrix-supported beds. Occurrence of uncharred wood remains and accretionary lapilli suggests temperature of emplacement comprised between 246° and 100°C. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the pyroclastic flow. Ignimbrite veneer facies at the maar rim may reflect pulsatory eruption dynamics, whereas valley pond facies may reflect the bulking of the pyroclastic flow inside the valleys and consequent high sedimentation rates. Ignimbrite veneer facies at topographic ridges has been interpreted to reflect detachment processes of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic flow that climbs the ridges. The present interpretation suggests that processes of pyroclastic flow transformation downcurrent and induced by topography are not necessarily peculiar of hot, high-mobility pyroclastic density currents. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level. This is inferred by both the large amount of limestone xenoliths present in the Peperino Albano and the absence of vesicular juvenile clasts, the latter implying that magma–water interaction occurred before gas exolution processes were significant.

Facies architecture and origin of a submarine rhyolitic lava flow-dome complex, Ponza, Italy

The Islands of Ponza and Gavi (western Pontine Archipelago, Italy) preserve parts of a subaqueously emplaced, high-silica (75–77% SiO2) rhyolitic lava flow that overlies the margins of three older domes. Exposures of the lava alone cover 24 km2, and with a thickness of 150 m exposed in the cliffs, the preserved volume of the lava flow is 3.6 km3, and the original volume is likely to have exceeded 7 km3. The lava flow consists of coherent obsidian and several hyaloclastite facies, including in situ and clast-rotated breccias and sandstones, which all exhibit gradational relationships. The coherent obsidian occurs as separate domains and is often flow banded. Although flow folds occur, the banding is generally subhorizontal, dipping gently to the NNE. The hyaloclastite, representing 90% of the lava by volume, has a diffuse layering varying from 0.5 to 2 m in thickness. This layering also dips gently to the NNE and is marked in places by alternation of some coherent horizons, coarse and finely fragmented in situ hyaloclastite, pumiceous hyaloclastite, and clast-rotated hyaloclastite. The layering is laterally continuous and consistent in orientation over most of the 13 km length of Ponza: the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow. The resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones. The lava flow has been intruded and crosscut by rhyolitic bodies of various ages, including discordant, coherent to internally fractured contemporaneous rhyolite bodies, and late, penecontemporaneous dikes with glassy margins and alteration haloes.

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.

Magnetic fabric and implications for pyroclastic flow and lahar emplacement, Albano maar, Italy

[1]xa0The Albano polygenetic maar is the youngest eruptive center of the quiescent Colli Albani volcano, located near the city of Rome. The most recent activity of the Albano maar extends from ∼23 ka into the Holocene and produced the small volume, basic, phreatomagmatic Peperino Albano (PA) ignimbrite, and, more recently, phreatomagmatic surge and lahar deposits related to the overspill of the Albano maar lake. We have performed an anisotropy of magnetic susceptibility (AMS) study, in order to define the relationships between the magnetic fabric and the flow mechanisms of the PA and of the phreatomagmatic deposits and lahars. AMS results indicate different transport and/or depositional systems for the veneer and valley pond facies in the PA ignimbrite and for the lahar deposits. AMS also demonstrates that flow directions are mainly controlled by the paleotopography. The paleotopographic control has been interpreted in terms of talweg sedimentation even at proximal locations where deposition occurs from dilute pyroclastic flows. Furthermore, AMS results clearly evidence a southward provenance for Holocene post-PA ignimbrite units, cropping out in the Ciampino plain, and confirm their origin from Albano maar lake overspill. We demonstrate that AMS is a reliable marker to determine paleoflow directions also in small volume phreatomagmatic ignimbrites and in syneruptive lahar deposits and can be successfully used to define their depositional systems.

Emplacement processes of the mafic Villa Senni Eruption Unit (VSEU) ignimbrite succession, Colli Albani volcano, Italy

Abstract The Villa Senni Eruption Unit (VSEU) represents the last large explosive event of the Tuscolano–Artemisio phase of volcanic activity in the Colli Albani (C.A.) (Alban Hills) Volcanic District, situated about 25 km south east of Rome, Italy. The VSEU succession consists of pyroclastic fallout, density current deposits, covering an area of 1500 km 2 , reaching a distance of 35 km or more from vent, and having a volume of 30 km 3 DRE. The VSEU is unusual in that its juvenile products are K-foiditic and phono-tephritic in composition, with less than 50% SiO 2 content. The most important products of the VSEU eruption are two pyroclastic density current depositional units; the Tufo Lionato (TL) and Pozzolanelle (TP), with each deposit containing several recognisable internal facies types. A short time gap (hours to days) between the deposition of these two ignimbrites is inferred due to the lack of a palaeosoil or evidence of fluvial reworking at the contact between the two units. A new stratigraphic framework for the VSEU is presented based on field observations, component analyses and detailed lateral correlations, with several new stratigraphic subdivisions proposed. The eruptive sequence is subdivided into depositional facies types, each related to a discrete phase of eruption, and a reconstruction of the three-dimensional geometry and the internal facies association within the VSEU is presented. This reconstruction is used to assess the pyroclastic density current emplacement processes resulting from this ignimbrite forming explosive mafic eruption. The morphology of and internal structures preserved within the mafic VSEU are comparable to those seen in more silicic ignimbrite sequences, suggesting that similar processes of emplacement by pyroclastic density currents can be applied over the entire compositional spectrum. We interpret lateral and vertical facies changes in the VSEU ignimbrite deposits to be caused by changes in the palaeotopographic surface over which the depositing pyroclastic density currents were travelling, as well as spatial and temporal changes in the dynamics of the pyroclastic density currents responsible for their deposition. In particular, interaction between the emplacing pyroclastic density current and the underlying palaeotopography caused local changes in current dynamics, leading to a wide range of localised flow structures and internal facies complexity. These interpretations are made possible because the long axes of valleys into which the VSEU was emplaced, which occur as part of the drainage network surrounding the C.A. volcano, are aligned from 0 to 90° to the radial pyroclastic current direction. The erosive power of pyroclastic density currents is recorded by an erosive contact between the TL and TP ignimbrites in proximal exposures (