Abstract The quiescent Colli Albano volcano is presently characterized by moderate-intensity earthquakes, seismic swarms, gas emissions and ongoing uplift that reflects the current evidence of its residual activity. An uplift of −30 cm over the last 43 years was recently detected by levelling surveys performed in the period 1950–1993 along a levelling line that crosses the highest elevation area of the western flank of the volcano. Space-based GPS and synthetic aperture radar interferometry geodetic observations confirm that this uplift is distributed in a wide area around the craters of Albano and Nemi, where the most recent volcanic activity occurred. GPS data from continuous monitoring stations indicate that both horizontal and vertical deformations do occur and can be addressed to a shallow magmatic source. All the geodetic observations are in agreement and high- light that the Colli Albani is still a potentially active volcano. Being located in a densely populated area close to Rome, the volcano should deserve the same monitoring and hazard assessment effort of any active volcano within urbanized areas. Here we review the geodetic results obtained during the last decades for the Colli Albani volcano.

Abstract Lake Albano is situated in the Colli Albani volcanic district, about 20 km SE of the city centre of Rome. It is 287 m above sea level and is the deepest of the volcanic crater lakes of Italy, presently being 167 m deep. It is 3.5 km long and 2.3 km wide with an area of about 6 km 2 . The crater has a long history, which starts with the formation of the Albano crater c . 70 ka BP, and shows evidence of human settlements since pre-historical times. Geological evidence indicates that a catastrophic overflow of the lake occurred in 398 BCE due to a rapid increase in the water level. This phenomenon persuaded the Romans to excavate an artificial outlet though the crater wall to control the lake level. The lake is thought to be a hazard for the surrounding human settlements and the city of Rome, so high-resolution multibeam bathymetry of Lake Albano was carried out for the Italian Dipartimento della Protezione Civile in order to evaluate the potential for CO 2 storage and eruption from the lake. The shape of the crater floor was mapped in two and three dimensions. Here, we show the main submerged morphological features and a brief history of the changes in lake level, which still affect the basin today.

Abstract The Colli Albani volcano has been active since c . 600 ka and is presently quiescent. Rock stratigraphy indicates that the activity of the volcano has undergone major changes in terms of eruption style, average eruption rate and location of active vents. The chemistry of the Colli Albani products is remarkably mafic, K-rich and silica undersaturated. Nevertheless, the volcano has experienced all types of eruption styles, from plinian explosive paroxysms, to milder strombolian and hawaiian eruptions, to effusive, including large- and small-scale phreatomagmatism. The first period of activity of the volcano is termed the ‘Vulcano Laziale’ period, and lasted from c . 600 ka to c . 355 ka. During this period, the volcanism was predominantly explosive, with an average eruption rate of 1 km 3 ka −1 . At least seven intermediate- to large-volume ignimbrites (VEI 5–7) were erupted and emplaced over an area larger than 1600 km 2 , forming an extensive ignimbrite shield around the central, continuously forming c . 8 × 8 km 2 caldera. The caldera complex and the ignimbrite shield are named the ‘Vulcano Laziale edifice’. The Vulcano Laziale edifice can in turn be subdivided into a lower ‘Pisolitic Tuffs succession’ ( c . 600–500 ka), in which ignimbrites are dominated by large-scale phreatomagmatism associated with the likely presence of an early caldera lake, and an overlying ‘Pozzolane Tuffs succession’, in which ignimbrites show a dominantly magmatic fragmentation style, probably in response to progressive exhaustion of the caldera lake. The typical succession of these mafic ignimbrites is composed of a sub-plinain to plinian basal scoria fall deposit covered by the main dark scoria and ash tabular ignimbrite sheet found as far as >30 km from the caldera rim and across ridges several hundreds metres in elevation, and is characterized by co-ignimbrite breccias at proximal locations. Major ignimbrites erupted with an average interval of c . 40 ka. After each paroxysmal ignimbrite eruption, volcanic activity was predominantly effusive to mild explosive, and was concentrated along peri-caldera fissure systems, forming continuous scoria cone and lava ridges, together with more explosive eruptions from intracaldera vents. The last major caldera-forming eruption of the Vulcano Laziale period occurred at c . 355 ka, emplacing the ignimbrites of the Villa Senni formation. Following this eruption, the complex Tuscolano-Artemisio peri- and extracaldera fissure system, predominantly composed of scoria cones and lavas, formed in response to the deflation of the caldera and peri-caldera area, together with formation of the intracaldera Faete stratovolcano. These edifices were emplaced between c . 355 and c . 180 ka, an interval termed the ‘Tuscolano–Artemisio–Faete period’. Although similar peri-caldera and intracaldera activity occurred earlier, that is, after each major caldera-forming eruption during the Vulcano Laziale period, the Tuscolano–Artemisio–Faete period was subject to a significant reduction in average eruption rate, by one order of magnitude, of 0.1 km 3 ka −1 , which can be related to a consistent reduction in the deep recharging of the plumbing system, and suggesting why no further ignimbrite eruptions occurred after 355 ka. Peri-caldera activity began along the northern and eastern peri-caldera ring fractures (Tuscolano and Artemisio sections, respectively) and after c . 300 ka progressively migrated outwards to extracaldera positions (Pantano Borghese section) and to the western peri-caldera fractures (S. Maria delle Mole section). The activity of these latter fracture systems ended almost simultaneosly, together with that of the Faete intracaldera stratovolcano, between c . 280 and c . 250 ka. After 250 ka, activity migrated to the south (Monte Due Torri section). The most recent activity along this latter peri-caldera area interfingers (between c . 200 ka and c . 180 ka) with phreatomagmatic products, which instead became dominant in the most recent activity of the Colli Albani volcano. Beginning from c . 200 ka (Via dei Laghi period), the western section of the peri-caldera area has been the site of repeated very small- to small-volume, maar-forming phreatomagmatic eruptions, which formed both monogenetic and polygenetic maars, collectively named the Via dei Laghi maar field. The most recent of these maars is the polygenetic Albano maar, which was formed after c . 70 ka by at least seven eruptions migrating along a NW–SE-trending, 3.5-km-long fracture. The last eruption of the maar occurred at <23 ka. Subsequent phreatic activity occurred throughout the Holocene, with lahars originating from dramatic withdrawals of the deep maar lake, at least up to the Eneolithic time (6000–5000 years ago) and probably up to Roman times (fourth century BCE), when the Romans dug a tunnel drain to keep the lake at a constant low level. The Albano area is currently the site of volcanic gas emissions, ground uplift and periodic seismic swarms, which may indicate persistent activity of a magmatic body at depth.

Abstract 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.

Abstract The quiescent Colli Albani volcano is composed of overlapping edifices, with each showing changes in volcanic style over the 600-ka history of the volcanic field. The edifices and changes in eruption style through time are indicated in the field by the major unconformities, which have been identified from surface geology and borehole stratigraphies. This reconstruction of the major unconformities allows for three-dimensional geometric reconstruction of the relevant rock packages, computation of the deposit volumes, and evaluation of syn-volcanic tectonic or volcano-tectonic activity. Furthermore, some rock packages, such as the Pisolitic Tuffs, the Tufo Lionato and the Albano maar deposits, are characterized by relatively low permeability, and are therefore essential for modelling of the hydrogeological circulation and flow of volcanic gases. The reconstructions of the isopachs of the three main caldera-forming ignimbrites erupted between 460 and 355 ka have allowed the calculation of their minimum preserved extracaldera volumes at 59 km 3 (Pozzolane Rosse), 20 km 3 (Pozzolane Nere) and 30 km 3 (Villa Senni Fm.) respectively. The total volume of volcanic deposits erupted after the last caldera collapse (<355 ka) is calculated as 34.7 km 3 . The preserved deposits of the most recent Albano polygenetic maar have been calculated to be 0.9 km 3 . The total minimum deposit volume of the Colli Albani volcano is 208.1 km 3 . This volume does not include the intracaldera ignimbrite volumes which may add some 80 km 3 to the total volume.

Abstract Following the final phase of Pleistocene volcanism in the Latium region, the main craters of Albano and Nemi in the Colli Albani volcano started to accommodate a sedimentary sequence in both lakes of variable thickness. In the mid-1990s, an EU-funded interdisciplinary project (PALICLAS) investigated the palaeoenvironmental record of the sedimentary sequences of Lakes Albano and Nemi using a multi-proxy approach. A set of up to 14-m-long cores was recovered from the two maar lakes following a seismic survey. Detailed petrophysical (magnetic), sedimentological and geochemical analyses, combined with a large palaeoecological dataset including algal and bacterial pigments, biological remains such as pollen, diatoms, Cladocera, chironomids and ostracods were carried out in three selected sites in Lake Albano and one site in Lake Nemi. A robust chronology was established by integrating accelerator mass spectrometry (AMS) radiocarbon datings, pollen analysis and secular variation record of the magnetic field together with the identification of two tephra layers correlated with the Late Pleistocene Etnean eruption of Biancavilla (Y1; 17.2 cal ka BP) and the late Holocene Pomici di Avellino (4.1 cal ka BP). The compiled results of these investigations provide a detailed chronicle of the response of both lacustrine basins to climate- and anthropogenically triggered environmental changes in central Italy for the past c . 28 cal ka. The Lake Albano record further demonstrate that at least the earliest changes reflect distinct warm/cold cycles that triggered major lake level changes of millennial to centennial duration as a major response of the lacustrine basin to climate forcing. Alternatively, these dramatic lake level variations – also identified during the Holocene – could have been driven by CO 2 injections of possible magmatic origin. However, flickering interannual to interdecadal variations further identified within these cycles can be correlated to oscillations of the North Atlantic (NAO) as observed in Greenland ice cores, marine and other lacustrine records. The latter, thus, would favour the climatic rather than volcanic cause for these changes. The Holocene record in both lakes is characterized by organic-rich sediments with a variable development of lamination. Although human activity in the catchment is evident since the mid-Holocene, the global signal indicates that changes in climatic variables such as wind intensity, precipitation and temperature are the most probable factors producing these environmental changes. The response of both lake systems to probable warm/cold episodes during the late Holocene, however, is difficult to disentangle from the often-contemporaneous human impact on their catchments.

Abstract Although controversy exists about the age of its most recent eruption (either 36 ka or <23 ka), Colli Albani volcano is unanimously considered to be quiescent and not exinct. During the Holocene, several lahars were generated by overflows from Albano crater lake up to the fourth century BCE, when the Romans excavated a drainage tunnel to keep the lake level below the crater rim. Such recent activity, together with the frequent occurrence of seismic swarms underneath the crater zone, the ongoing uplift of the volcanic edifice and the magmatic affinity of the emitted gas, indicate the presence of an active magma chamber. The most likely site for a new eruption is the deep crater hosted in the southern part of the Lake Albano, where the last eruptive events occurred. Any eruption would have a strong explosive character enhanced by the interaction of magma with the water of the lake and would endanger a densely inhabited area up to the outskirts of Rome. The hazard of a new overflow from Lake Albano is very low because of the present low level of the lake. There is instead a potential for CO 2 release from the deep lake water following the occurrence of rollovers, which would threaten the lake shore, a site where thousands of people spend their vacations in the summer. However, the content of dissolved CO 2 is presently far from saturation and no Nyos-type events will occur today. Presently, the main hazard is related to strong gas emissions (CO 2 , H 2 S and Rn) from fractured zones and gas blowouts from wells reaching shallow gas-pressurized aquifers.

Abstract 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.

Abstract The Colli Albani volcano (also known as Alban Hills volcano) is the large quiescent volcano that dominates the southern Rome (hereon Roma) skyline (Fig. 1 ). It covers almost 1600 km 2 and its K-rich rocks have attracted the attention of geologists since the nineteenth century. The Colli Albani is one of the most, if not the most, explosive mafic volcanoes in the world. The state of activity of this volcano and its influence on the history of the Roma foundation has been heavily debated since Roman times by historians, scientists and archaeologists. As a result of recent detailed field surveys aimed at the redaction of the new Geological Map of Italy and of detailed studies prompted by the occurrence of gas eruption events and seismic swarms, new deposits related to previously unknown pre-Holocenic and Holocenic volcanic and phreatic activity from the most recent maar were revealed. This has changed the classification of the volcano from extinct to a still active and quiescent volcano, with important implications for the hazard to the city of Roma. The prehistoric and historic settlement history of the region and the foundation of Roma have been deeply influenced by this activity, as reported by several classical authors (e.g. Plutarch, Dionissus, Livy). In the fourth century B.C.E., the Romans dug a 1.5-km-long tunnel through the Albano maar crater wall to lower the crater lake level and prevent the dramatic overflows that had previously destroyed this region several times. This tunnel, which is still potentially functional, has to

Abstract Albano Lake is within the youngest polygenetic crater of Colli Albani, from which several lahar-generating water overflows occurred up to early Roman times. The area has anomalous gas emissions and is affected by seismicity and uplift. The geochemistry of the lake have been systematically investigated since 2003 by measuring physico-chemical parameters along vertical profiles with a multiparametric probe and by collecting water samples for chemical and isotopic analyses. The lake is thermally and chemically stratified, with an anoxic hypolimnion from −70 m to the bottom (−167 m). The isotopic composition of dissolved helium and total carbon is similar to that of the main gas emissions of Colli Albani and of the phenocryst inclusions of the Alban volcanics, suggesting that an endogenous gas of deep provenance is injected into the lake water. The dissolved CO 2 content is, however, far from saturation, and no Nyos-type hazardous gas cloud emission may presently occur in the lake. Temperature and chemical time variations indicate that water rollover episodes occur in harsh rainy winters when the surface lake temperature cools below 8.5 °C. Such rollovers tend to homogenize the physico-chemistry of the lake water and reduce the dissolved CO 2 content. They may cause an environmental hazard because of related toxic algal blooms.

Abstract Lake Albano is deep and has a small surface area, which reduces the potential for mixing across its full depth. This can lead to dangerous accumulations of carbon dioxide in the bottom waters of the lake deriving from subterranean magmatic processes. It has been proposed that limnic gas eruptions emanating from these deep waters represent the most significant present-day natural hazard of the Colli Albani volcano. Although recent evidence has shown that the lake is not meromictic and that rare full mixing events do occur, reducing to some extent the risk of limnic eruptions. At present the lake volume and water quality is rapidly decreasing due to over-abstraction and uncontrolled inputs of waste and sewage. These conditions may be responsible for large growths of toxic cyanobacteria in the lake. Lake water concentrations of microcystins (a cyanobacterial toxin) are well above the recommended limits set for drinking water, and some trace groundwater contamination has also been determined. These toxins can cause the death of animals and pose a significant risk to human health, representing a secondary hazard of the lake.

Abstract The polygenetic Albano maar is the most recent centre of the Colli Albani volcano. Phreatic activity at the maar occurred throughout the Holocene. This paper summarises the close relationships between the activity of the maar and the history of settlement in the Roman region. Repeated lahars associated to the lake overflows occurred along the northwestern slope of the maar. The last catastrophic overflow occurred in 398 B.C.E., after which the Romans excavated a 1.5 km long drain-tunnel through the maar crater wall, which has since kept the lake 70 m below the lowest point of the rim. This tunnel drain may be regarded as the first construction made to mitigate a volcanic hazard in history. The surprising and still largely unknown results of this study are very important to understand the history of settlement of the area and to assess the hazard of the Roman region.

Abstract The Torre Spaccata valley, situated in the southeastern suburbs of Rome, was investigated between 1997 and 1998 and again in 2006. It lies about 15 km from the Colli Albani volcanic complex, the recent evolution of which incorporates deposition of the volcanic-sedimentary sequence of the Tavolato Formation (including the Albano lahar deposits). During the excavations, a limited strip of lahar was uncovered and dated archaeologically to the fourth century BCE. Other main features include several intercrossing channels with a sandy-pebbly fill, evidence of intermittent hydrological activity. In this area there is evidence of a prolonged interaction through time between human activity and the valley’s varying environments. The prehistoric evidence (third to second millennium BCE) is well documented in a concentration of sites on the eastern outskirts of Rome. The prehistoric evidence (third to second millennium BCE) has a good context in the concentration of sites in the eastern outskirts of Rome. Other reliable traces date to the fourth to third century BCE, soon after local lahar deposition. During the Late Republican and Imperial period, the hilltops were occupied by a dense system of villae. Later remains provide evidence for agricultural use of the valley floor during the Middle Ages.

Abstract This paper analyses the xenoliths found in the phreatomagmatic products of the most recent Via del Laghi maar field at Colli Albani volcano in order to investigate the nature of the substratum and its interaction with the magmatic system. The pre-volcanic sedimentary xenoliths are from the same rock formations that form the pre-orogenic Mesozoic–Cenozoic carbonatic pelagic succession cropping out in the nearby Apennine mountain belt, and the Pliocene post-orogenic marine sedimentary succession. Along with the sedimentary xenoliths, a large variety of thermo-metamorphic, metasomatic and magmatic xenoliths are present. Skarn xenoliths have been studied to estimate peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism. The presence of hydrous minerals such as phlogopite and amphibole and the carbonatic nature of the substratum suggest a binary H 2 O–CO 2 mixture as the dominant fluid. Microscale wollastonite filling fractures indicates a temporal link between fracturing and fluid infiltration. The occurrence of retrograde hydrous textures superimposed on anhydrous associations indicates at least a two step process: metamorphism with a T peak at dry conditions (CO 2 saturated), followed by fracturing of the substratum in the late stage, allowing abundant aqueous fluids to penetrate the thermally modified rocks. A significative amount of rare earth minerals crystallized in the late stage.