Colli Albani or Alban Hills volcanic complex, Latium, Italy

volcano number: 0101-004 (according to Volcanoes of the World, 1994 edition)

summit elevation: 949 m

location: 41.73�N, 12.70�E

The Colli Albani volcanic complex, also known as the "Vulcano Laziale" or the Alban Hills volcano, forms a prominent feature on the southeastern skyline of Roma. The structure of this "somma" type volcano (caldera with a central cone) is much more complex than it seems from a distance. Indeed, the volcano has two nested calderas and numerous more or less eccentric post-caldera vents, most of which are explosion craters. The highest point of the Colli Albani is Monte Cavo (949 m), a scoria cone sitting eccentrically on the SW rim of the younger Faete caldera. There are two crater lakes, Albano and Nemi which fill the most recent craters of the volcano. Lago Albano, with its lake level at 293 m elevation, is 170 m deep and thus the deepest of all volcanic lakes in the Central Italian volcanic region (also known as the Roman Comagmatic region). Lago di Nemi lies somewhat higher (lake level at 316 m) but is very shallow. Even these late-stage explosion craters, despite their apparently simple morphology, have gone through a complex evolution. Today, they constitute an extremely scenic landscape, which hosts the summer residence of the Pope at Castel Gandolfo, right on the rim of the crater that contains Albano Lake, and is a quite popular weekend destination for numerous Romans. The volcanic complex is relatively densely populated, and the outskirts of Rome extend to its northern base, including an airport. Much of Rome itself stands on the products of the Pleistocene explosive volcanic activity of the Colli Albani.

In spite of frequent seismicity (Amato et al. 1994), the Colli Albani have until recently been considered an "extinct" volcano - in the sense that no future eruptions had to be expected, in spite of historical documents indicating some kind of eruptive activity as recently as 114 B.C. This alleged eruptive activity was discredited by geological studies of the early 1960s (Fornaseri et al. 1963) to late 1980s (De Rita et al. 1988), which indicated that all deposits of the Colli Albani were older than Holocene (>10,000 years). A detailed study of historical documents by Stothers and Rampino (1983) led them to consider the reports on an eruptive event in 114 B.C. not trustworthy and thus seemed to confirm the apparent geological evidence.However, since the mid-1990s increasing evidence for ongoing volcanic unrest has been detected (Amato and Chiarabba 1995; Chiarabba et al. 1997; Anzidei et al. 1998), and the most recent research indicates that there has been eruptive activity during the Holocene indeed (Funiciello et al. 2003; Porreca et al. 2003), as indicated already a few years earlier by some researchers (Villa et al. 1999). Based on studies of the long-term behavior of the volcanic complex and on regional tectonic evidence, Karner et al. (2001a, b) and Marra et al. (2003) arrive at the conclusion that the volcano might be just at the beginning of a new eruptive phase, thus supporting the other evidence. The presence of a potentially active volcano at only 25-30 km from the center of Rome certainly is not exactly reassuring, and the new findings have stimulated a wave of intense study which will result in a completely revised concept of volcanic hazards in the area of the "Eternal City".

 

An completely different perspective of the same volcanic complex, seen here from southwest. The central Faete edifice with the conspicuous Monte Cavo lies at left, while the tallest portion of the Tuscolano-Artemisio caldera rim is seen at right. Photograph taken in mid-March 1992

Geological evolution

In recent years, the Colli Albani have been the subject of numerous geological studies, which focused mainly on the lithological, facies and compositional properties of certain eruptive events, and on unraveling the most recent geological history of the volcanic complex. Recently, competition between two simultaneously working groups of scientists makes it slightly difficult to summarize their findings, and the following basically summarized the findings of the two groups, trying to correlate them as precisely as possible.
The geological evolution of the Colli Albani as defined by De Rita et al. (1995) is subdivided into three major epochs (this term has recently been introduced to substitute the previously used "phases"): (1) Tuscolano-Artemisio, (2) Faete, and (3) the "final" hydromagmatic phase.
The Tuscolano-Artemisio epoch covers the period from 600 ka (561 ka, according to new ⁴⁰40Ar/³⁹Ar age determinations presented by Marra et al. 2003) until 350-360 ka (351 ka, Marra et al. 2003) when almost all volcanic activity occurred from a central volcanic edifice (Tuscolano Artemisio edifice). This epoch is comprised by four eruptive cycles each of which is representated by the emplacement of pyroclastic flows and air fall tephra followed by lava flows in the closing stages of each cycle. The first cycle lasted from about 600 until 500 ka and correlates strikingly with an eustatic lowering of the sea level. During this cycle, at four large pyroclastic flows were erupted in rapid succession, followed by an effusive phase concentrated mainly in the SW sector of the volcano. The deposit of the earliest of the pyroclastic flows, known as "Trigoria-Tor de'Cenci Tuff" and dated at about 561 ka, has a volume of up to 10 km³ and is described in detail by Palladino et al. (2001). The deposits of four pyroclastic flows (including the "Trigoria-Tor de'Cenci Tuff" and together constituting the "Pisolitic Tuffs"), produced by violent phreatomagmatic eruptions between 570 and 530 ka, were studied by De Rita et al. (2002), who attributed volumes of at least 10 km³ to each of the deposits. According to these authors the activity at that time was strongly controled by the presence of a large water volume present at the surface, possibly a major lake filling a volcano-tectonic depression similar to Lake Taupo in New Zealand. De Rita et al. (2002) believe that each of the four eruptions led to further caldera collapse. Little is known of the eruptive activity between these cataclysmic events, which produced leucite-bearing lavas.
The second cycle brought about the most significant eruption of the Colli Albani which led to the deposition of a major ignimbrite which has thicknesses of 90 m in paleovalleys exposed in the E sector of the volcano. De Rita et al. (1988) have calculated the minimum volume of this ignimbrite at about 34 km³. Outcrops exist up to 80 km from the eruption center where there is evidence that the pyroclastic flows climbed up to about 400 m elevation on the slopes of the Monti Tiburtini, that is, about 200 m above the valley floor. This unit is named "Pozzolane Rosse" or "Pozzolane di San Paolo".
The second cycle ended with a noteworthy effusive activity, dated at about 480 ka, which again correlates with a lowering of the sea level.
The third cycle includes another significant ignimbrite eruption of similar dimensions of the preceding one; much of the volume of the corresponding ignimbrite has been removed by erosion since there was no protecting cap of succeeding lava flows.
The fourth cycle, dated at about 350-360 ka (Karner et al. give an age of 535 ka for the termination of the Tuscolano-Artemisio epoch), is again characterized by the emission of an ignimbrite with two distinct flow units known as the "Tufo litoide" and the Villa Senni Tuff. Its volume has been calculated by Watkins et al. (2002) to be at least 30km³. With this major eruptive episode, the Tuscolano-Artemisio edifice of the volcano was affected by a major caldera collapse along regional tectonic fractures, resulting in a collapse structure 10 x 12 km in diameter, the Tuscolano-Artemisio caldera (see geological map below).

 

Left: simplified geological maps of the area of Rome including the Colli Albani (from De Rita et al., 1992). Key: 1=travertine; 2=Plio-Pleistocene sedimentary units; 3="final" hydromagmatic units; 4=air fall deposits; 5=lava flows; 6=pyroclastic flow units of the Colli Albani; 7=pyroclastic flow units of the Sabatini volcanic field (in northwestern part of the map); 8=Tortonian flysch; 9=caldera rims; 10=late explosion craters (a: Ariccia, b: Nemi, c: Albano, d: Giuturna, e: Valle Marciana, f: Pantano Secco, g: Prata Porci, h: Castiglione); 11=Meso-Cenozoic pelagic carbonate units (Sabina facies); 12=Meso-Cenozoic carbonate platform units (Latium-Abruzzi facies) Right: geological sketch map of the explosion crater that hosts the Albano Lake (right, from Villa et al., 1999). Key: a=alluvium and colluvium; b=cycle V products; c=cyvle IV products; d=cycle III products; e=cycle II products; f=cycle I products; g=products of the Faete eruptive phase of the Colli Albani; h=secondary emplacement flow directions; i=crater rims, the numbers corresponding to the supposed succession of eruptive centers within the Albano crater

After a brief period of volcanic quiescence, activity resumed in the central area of the caldera where a new small stratovolcano was constructed (Campi di Annibale or Faete epoch). This epoch is also subdivided into cycles, but the significance of the eruptions and erupted volumes were much less than during the Tuscolano-Artemisio epoch. The total volume of erupted products of the Campo di Annibale-Faete epoch is only 2 km³, compared to about 283 km³ of the Tuscolano-Artemisio volcanics. One of the most important products of this epoch is the Capo di Bove lava flow, which is morphologically very distinct and is cut by the GRA (Grande Raccordo Anulare, the outer ring freeway that goes all the way round the city of Rome). The Campi di Annibale-Faete epoch lasted from about 277 ka until about 250 ka (Watkins et al. 2002; Funiciello et al. 2003).

The most recent activity of the Colli Albani, previously coined the final hydromagmatic phase (De Rita et al. 1988), has been recently re-named "Phreatomagmatic epoch" by Funiciello et al. (2003). Karner et al. (2001a) named what was interpreted by them as the main deposit of this epoch"Peperino di Marino (Lapis Albanus)". It began after a period of repose of more than 200,000 years, at about 45 ka. During this epoch, hydromagmatic eruptions occurred from several eccentric craters (maars), mostly in the NW and SW sectors of the volcanic complex, such as the lake-filled maar craters of Nemi and Albano and several less-known dry depressions. Among these craters, the one now filled with the scenic Albano Lake seems to have undergone the most complex evolution (Villa et al. 1999), with at least five distinct eruptive centers (I to V; see geological sketch map of the Albano crater above) erupting in the area now occupied by the lake. The first of these eruptive centers (I) probably erupted about 45 ka ago, the third (III) about 26 ka ago, and the fourth might have been active about 16 ka ago. The youngest of the Albano crater vents, which coincides with the deepest portion of the lake, may be as young as 7.5 ka (Villa et al. 1999).
The most significant event of the Phreatomagmatic epoch was the emplacement of the "Peperino Albano", an ignimbrite deposit produced during a phreatomagmatic eruption about 25 ka ago, with a volume of about 0.2-0.5 km³, which is associated with lahar deposits (Giordano et al. 2002a, b). Differently from the other, minor, phreatomagmatic deposits from the Albano crater, which spread only up to 3 km from the vent, the "Peperino Albano" extended much further down the northwestern slope to a distance of 7 km (Giordano et al. 2002a).

A Holocene age of the latest eruptions and other geological events that would provoke catastrophic effects, were they to occur today, has been confirmed by fieldwork at the beginning of the new millennium conducted by Funiciello et al. (2002, 2003) and Porreca et al. (2003). These authors recognized that the ~25 ka "Peperino Albano" is not the youngest eruptive product of the volcano. During fieldwork in outcrops opened during the construction of new roads in the ever more populated northwestern sector of the volcano, they discovered the deposits of phreatomagmatic activity and lahars, which extend up to 15 km from the vent, right into the southeastern outskirts of Rome. Porreca et al. (2003) determined an emplacement temperature of more than 200°C for a pyroclastic density current deposit in the newly discovered succession. A paleosoil separating this deposit from overlying lahar deposits has been dated at 5100±100 years before present (Funiciello et al. 2003). Further deposits stemming from the catastrophic overspilling of water (convective rollover) from the crater lake (whose level stood much higher until the Roman era than at present; drainage work carried out by the Romans in the 4th century B.C. brought the lake level to its present elevation) make up the uppermost portion of the succession. Funiciello et al. (2002, 2003) suggest that these overspills were caused by the sudden release of large quantities of hot fluids rich in CO₂ into the lake, which occurred as a consequence of seismically induced microfracturing.

Until very recently, the Colli Albani has generally been considered an inactive volcano (especially after records of activity in historical sources were discarded by various researchers, such as Stothers & Rampino 1983). The recent research leads to conclusions that are contrary to that reassuring belief. While some authors fear that the lastest eruptions at the Albano crater and other explosion craters has just been the beginning of a new phase of activity (Karner et al. 2001a, b; Marra et al. 2003), others simply point out that catastrophic lahars and minor phreatomagmatic activity have continued until much more recently than previously assumed (Funiciello et al. 2003). Furthermore, the volcano is seismically active, with several notable earthquake swarms since the early 1980's, and it was only in the early 1990s that a significant amout of very recent ground uplift was detected (Amato and Chiarabba 1995), whose maximum coincides precisely with the central area of the volcanic complex. This uplift continued at least through 1997, as indicated on the Colli Albani web site of the Rome section of the Istituto Nazionale di Geofisica e Vulcanologia.
Emissions of toxic gas (CO₂) are occurring, especially since 1995, in some areas on the northwestern flank, most notably at Cava dei Selci (Carapezza et al. 2003). As of late 2001 29 cows, 8 sheep, and one person have been suffocated by these gas emissions. It is believed that the gas release is a reault of fracturing caused by the seismicity of 1989-1990 and 1995, and minor seismicity seems to have accompanied the major gas emission event in September 1999 which led to the death of the 29 cows (Beaubien et al. 2003). The most acutely problematic aspect of these CO₂ emissions is that they occur in an area that is now fairly densely populated, and several recent publications (Chiodini and Frondini 2001; Annunziatellis et al. 2003; Carapezza et al. 2003) are dealing with the hazard aspects of these phenomena.

 

Left: Monte Cavo, the tallest peak of the Colli Albani complex, casts its reflection onto the smooth waters of Albano Lake (Lago Albano) on a splendid 27 December 1994. The view is toward east, seen from Castelgandolfo, the summer residence of the Pope Center: northern portion of Albano Lake, with small harbor and nicely placed villas of wealthy people who happen to have their buildings placed on the rim of the most recent crater of the Colli Albani volcanic complex. The most recent eruptions from this crater may have occurred as recently as 7500 years ago Right: inside a potentially active volcano. The photograph, taken 27 December 1994, shows the eastern shore of Albano lake

Left: less famous but more suggestive, the lake-filled Nemi crater is smaller and shallower than the Albano crater. This is a view from the northern rim of the crater down onto its lake (Lago di Nemi), 27 December 1994 Center: The town of Nemi (center of image) sits on the northern rim of the Nemi crater, allowing for extremely beautiful views onto the crater lake below. Monte Cavo is in the background Right: view from the western rim of the Nemi crater across the crater lake, 27 December 1994

References

Amato A, Chiarabba C (1995) Recent uplift of the Alban Hills Volcano (Italy): evidence for magmatic inflation? Geophysical Research Letters 22: 1985-1988

Amato A, Chiarabba C, Cocco M, Di Bona M, Selvaggi G (1994) The 1989-1990 seismic swarm in the Alban Hills volcanic area, central Italy. Journal of Volcanology and Geothermal Research 61: 225-237

Annunziatellis A, Ciotoli G, Lombardi S, Nolasco F (2003) Short- and long-term gas hazard: the release of toxic gases in the Alban Hills volcanic area (central Italy). Journal of Geochemical Exploration 77: 93-108

Anzidei M, Baldi P, Casula G, Galvani A, Riguzzi F, Zanutta A (1998) Evidence of active crustal deformation of the Colli Albani volcanic area (central Italy) by GPS surveys. Journal of Volcanology and Geothermal Research 80: 55-65

Beaubien SE, Ciotoli G, Lombardi S (2003) Carbon dioxide and radon gas hazard in the Alban Hills area (central Italy). Journal of Volcanology and Geothermal Research 123: 63-80

Carapezza ML, Badalamenti B, Cavara L, Scalzo A (2003) Gas hazard assessment in a densely inhabited area of Colli Albani Volcano (Cava dei Selci, Roma). Journal of Volcanology and Geothermal Research 123: 81-94

Chiarabba C, Amato A, Delaney PT (1997) Crustal structure, evolution, and volcanic unrest of the Alban Hills, Central Italy. Bulletin of Volcanology 59: 161-170

Chiodini G, Frondini F (2001) Carbon dioxide degassing from the Alban Hills volcanic region, Central Italy. Chemical Geology 177: 67-83

De Rita D, Funiciello R, Parotto M (1988) Carta Geologica del Complesso vulcanico dei Colli Albani (Geological map of the Colli Albani volcanic complex), scale 1:50,000, Consiglio Nazionale delle Ricerche

De Rita D, Funiciello R, Rosa C (1992) Volcanic activity and drainage network evolution of the Alban Hills area (Rome, Italy). Acta Vulcanologica 2: 185-198

De Rita D, Giordano G, Esposito A, Fabbri M, Rodani S (2002) Large volume phreatomagmatic ignimbrites from the Colli Albani volcano (Middle Pleistocene, Italy). Journal of Volcanology and Geothermal Research 118: 77-98

Fornaseri M, Scherillo A, Ventriglia U (1963) La regione vulcanica dei Colli Albani (Vulcano Laziale). Consiglio Nazioale delle Ricerche Roma, 550 pages

Funiciello R, Giordano G, De Rita D, Carapezza ML, Barberi F (2002) L'attività recente del cratere del Lago Algano di Castelgandolfo. Rendiconti dell'Accademia dei Lincei (Scienze Fisiche e Naturali) ser. 9, 13: 113-143

Funiciello R, Giordano G, De Rita D (2003) The Albano maar lake (Colli Albani Volcano, Italy): recent volcanic activity and evidence of pre-Roman Age catastrophic lahar events. Journal of Volcanology and Geothermal Research 123: 43-61

Giordano G, De Rita D, Cas RAF (2002a) 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. Journal of Volcanology and Geothermal Research 107: 131-144

Giordano G, De Rita D, Fabbri M, Rodani S (2002b) Facies associations of rain-generated versus crater lake-withdrawal lahar deposits from Quaternary volcanoes, central Italy. Journal of Volcanology and Geothermal Research 118: 145-159

Karner DB, Marra F, Renne PR (2001a) The history of the Monti Sabatini and Alban Hills volcanoes: groundwork for assessing volcanic-tectonic hazards for Rome. Journal of Volcanology and Geothermal Research 107: 185-219

Karner DB, Marra F, Florindo F, Boschi E (2001b) Pulsed uplift estimated from terrace elevations in the coast of Rome: evidence for a new phase of volcanic activity? Earth and Planetary Science Letters 188: 135-148

Marra F, Freda C, Scarlato P, Taddeucci J, Karner DB, Renne PR, Gaeta M, Palladino DM, Trigila R, Cavarretta G (2003) Post-caldera activity in the Albani Hills volcanic district (Italy): ⁴⁰40Ar/³⁹Ar geochronology and insights into magma evolution. Bulletin of Volcanology 65: 227-247, DOI: 10.1007/s00445-002-0255-9

Palladino DM, Gaeta M, Marra F (2001) A large K-foiditic hydromagmatic eruption from the early activity of the Alban Hills Volcanic District, Italy. Bulletin of Volcanology 63: 345-359, DOI: 10.1007/s004450100150

Porreca M, Mattei M, Giordano G, De Rita D, Funiciello R (2003) Magnetic fabric and implications for pyroclastic flow and lahar emplacement, Albano maar, Italy. Journal of Geophysical Research 108, DOI 10.1029/2002JB002102

Trigila R (editor) (1995) The volcano of the Alban Hills. Tipografia SGS, Rome, 283 pages

Villa IM, Calanchi N, Dinelli E, Lucchini F (1999) Age and evolution of the Albano crater lake (Roman Volcanic Province). Acta Vulcanologica 11: 305-310

Web sites

The GPS network maintained by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) at the Colli Albani (in Italian)

Some information (in English) on the seismicity and deformation of the Colli Albani, by the Rome section of the INGV

An Italian web site describing the lakes of the Latium region (many of them filling volcanic depressions such as explosion craters and calderas) has information and photos of Albano Lake and Lake Nemi

Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"