Italy is a fascinating country, and when it comes to volcanology, Italy has been arguably the most influential location in the planet. The first ever detailed description of a volcanic eruption came from Pliny the Younger writing about the eruption of Vesuvius in 79 AD. Vesuvius was also the first volcano to be watched over by a volcanological observatory, in 1841. Three types of eruptions plinian, strombolian and vulcanian were described in and named after Italian volcanoes. The very name volcano comes from the island of Vulcano. While In Sicily, according to legend, the philosopher Empedocles jumped into Mount Etna believing he was immortal. And was this before or after Zeus threw Mount Etna on top of the monstrous Typhon?
In such an exuberantly volcanic region it is easy to miss some of the other volcanoes that haven’t done so much historically, but are still there, and some of them are genuine wonders. Here I’m going to talk about Colli Albani. A large caldera right next to Rome. The arguably weirdest caldera in the world in terms of chemical composition.
The Roman Volcanic Province
Colli Albani is part of a remarkable cluster of large caldera volcanoes. The Roman Magmatic Province. The cluster includes four caldera systems generally classified as separate. These are from north to south: Vulsini, Vico, Monti Sabatini, and Colli Albani. The calderas started their major caldera forming eruptions at about the same time 600,000 years ago. They have become inactive in the past 100,000 years, except for Colli Albani which still shows signs of intrusive activity.
The exact number of caldera forming events produced by these volcanoes seems unclear. The Vulsini Complex had at least 5 ignimbrite caldera-forming events. Vico had four caldera-forming ignimbrite eruptions. The largest eruption of the Roman Magmatic Province may have been the 300,000 BP event that formed the Bolsena Caldera of Vulsini, which had a bulk volume of 460 km3, the caldera is now filled with Lake Bolsena.
The weirdness of Colli Albani
So what makes Colli Albani so special? A unique chemistry. Colli Albani has done foidite ignimbrites, something as far as I know no other calderas in the world have. Of course, throughout the history of the planet there must have been other calderas similar to Colli Albani, but none of them remain active. Remember foidites? My last article was about foiditic magmas, and then Jesper’s Nyiragongo series also touched this magma type. Foidites are some of the most potassic magmas in the world, very rich in certain elements such as potassium, phosphorus, barium, among many others, and depleted in silicon and aluminium. Foidites include magmas such as melilitites, nephelinites, leucitites, and kimberlites, among many other less common designations. Carbonatites are also related to foidites, given that carbonatites come from nephelinites through a partition between silicate and carbonate melts. Colli Albani’s magmas would be classified as leucitites.
Last time, I was talking about Kaiserstuhl, an extinct nephelinite-carbonatite stratovolcano in Germany, and nearby Urach, a field of melilitite maar volcanoes. However, this is not the only province of foiditic magmas in Europe. One of the most important active foidite volcanoes of the world is in Italy, and it is the only one that has made it into a caldera.
We could distinguish four series of magmas given the way that they are classified in the TAS diagram:
-Low-K series (low potassium): Subalkaline/tholeiite basalts, basaltic-andesites, andesites, dacites, and some rhyolites.
-Medium-K series: Alkalic basalts, trachybasalts, basaltic-trachyandesites, trachyandesites, trachytes, and some rhyolites.
-High-K series: Basanite, tephrite, phonotephrite, tephriphonolite, phonolite, and in some highly evolved cases trachyte.
-Extreme-K series: The rocks clasified as foidites in the TAS diagram.
Foidite volcanoes are usually isolated in terms of their chemical composition. The nephelinitic Ol Doinyo Lengai is the black sheep among the trachytic volcanoes of the Ngorongoro complex. Nyiragongo nephelinites contrast with the basanites and phonolites of the other Virunga volcanoes. Colli Albani is a similar case.
Roman Magmatic Province calderas have compositions that are less potassic than Colli Albani. Vico is a medium-K volcano, that has erupted trachytes and rhyolites. Monti Sabatini and Vulsini are more potassic than Vico, and straddle the boundary between the medium-K and high-K series. To the south of Colli Albani lies the Campanian Volcanic Province where volcanoes have mainly medium-K compositions, with the notable exception of Vesuvius which is a high-k phonolitic-phonotephritic volcano.
There are other foiditic volcanoes in Italy appart from Colli Albani, however they are monogenetic. These monogenetic volcanoes make up the so called Umbria-Latium ultra-alkaline district and are located to the east of the Roman Magmatic Province. They are San Venanzo, Acquasparta, Polino, Cupaello, Oricola, and others. Umbria-Latium volcanoes erupted melilitites and carbonatites. Eruptions were mainly explosive forming maars/diatremes and pyroclastic cones. Some lava flows are also present but usually of very small volume. These eruptions have been usually attributed to hydromagmatism. However, I think they were probably driven, at least partly, by the very high gas contents of melilitites, because wherever there is melilitite you find maars and diatremes.
The Umbria-Latium ultra-alkaline district might still be active, however eruptions here are extremely rare and minuscule in volume. Melilitites are some of the most geochemically interesting magmas but also some of the rarest. It is not unusual to find a lonely melilite maar or tuff cone volcano with no other volcano nearby, which probably formed in one day, was a VEI-2, and effused a 200 metre long lava flow. The way of melilitites is modesty.
Caldera-forming Colli Albani.
Colli Albani is 10-km wide caldera that has collapsed repeatedly generating massive pyroclastic flows. The first dated activity of the volcano was at 608 ka (ka means thousands of years before present). This was also at about the same time explosive activity started in nearby Sabatini volcano, which happened at 591 ka. It has been noted that the eruption histories of Colli Albani and Monti Sabatini appear to be connected, and that they have often entered eruption phases at the same time.
The first caldera-forming ignimbrites of Colli Albani are less well known. There are 4-5 ignimbrites, depending on the publication, that took place during 2 eruption periods, at 561 ka and 530 ka. The volume is not well constrained, but they seem to have been at least VEI-5 eruptions. One of the 561 ka explosive eruptions was the Tor de Cenci ignimbrite, that is shown in the map below.
A series of larger caldera forming eruptions followed: the >34 km3 bulk Pozzolane Rosse eruption at 456 ka, the 9 km3 DRE Pozzolane Nere at 407 ka, and the >50 km3 bulk Villa de Senni ignimbrite at 365 ka, which ended the caldera-forming period. As such ignimbrite eruptions occurred at intervals of about 50,000 years, although sometimes there may have been two ignimbrites that occurred in more rapid succession. It is not too easy to know how much time elapsed between certain explosive eruptions, and it is difficult to know which ash deposits formed in separate eruptions or phases of the same event, making things unclear.
The eruptions, as typical of caldera-forming ignimbrites, were very powerful. A caldera-forming ignimbrite is a particular type of pyroclastic flow of enormous extent which is thought to issue from the ring fault of a caldera through a ring dike. Each Colli Albani ignimbrite started with a plinian/subplianian eruption which rained scoria around the volcano, this phase was relatively small in volume. Each eruption then intensified and a massive pyroclastic flow issued from the caldera, spreading over a radius of 20-30 km all around the volcano, obliterating everything in its way. It would have blown trees to the ground and suffocated animals to death and finally buried them under meters or tens of meters of ash and scoria. Caldera-forming ignimbrites reign supreme among eruptions in terms of intensity. If one of these pyroclastic flows were to occur today, then it would devastate most of Rome. Luckily for Rome, Colli Albani doesn’t seem to be in the caldera business any longer.
In caldera-forming ignimbrites, the magma type doesn’t seem to matter much. Doesn’t matter if it’s leucitite like Colli Albani, water-rich basalt like Okmok, or rhyolites from the Altiplano-Puna, the eruption intensity is relatively similar. The leucitites of Colli Albani are silica undersaturated. None of the ignimbrites has had more than 50 wt% SiO2. The Pozzolane Rosse eruption, for example, had only 40-44 wt% SiO2. A magma like this is very fluid and gas rich. Normal eruptions will usually be explosive but still have low eruption rates. However, a caldera-collapse is different. Leucitite has enough gas to fragmentate the magma into tephra. The caldera collapse brings in enormous eruption rates that generate the violent pyroclastic flow-style of activity. The only magmas that seem incapable of producing caldera-forming ignimbrites are the gas-poor basalts from intraplate or mid-ocean ridge environments, probably because of the very little volatile content that they carry.
Following the final ignimbrite, the Villa Senni eruption, Colli Albani entered a very active period from ~365 ka to ~350 ka in which many vents erupted from a ring-like area around the caldera, making scoria cones. These eruptions were effusive and explosive. They produced lava flows, but also generated subplinian eruptions that deposited scoria for tens of kilometres to the east of the volcano. Nearby Sabatini volcano also has a similar pattern of concentric fissures which envelop the Lago di Bracciano caldera.
The distribution of fissures around the caldera suggests to me that they were fed from cone sheet intrusions. Cone sheets are intrusions which face inwards towards the centre of a volcano and, together, multiple cone sheet intrusions make the shape of a cup or an inverted cone, and where they reach the surface, concentric fissures open up that usually run parallel to the rim of the caldera. Such intrusions are typical of caldera systems worldwide. There is a link with caldera resurgence. In order for magma to intrude, it must be able to push away the rock. Cone sheets happen by pushing the rock upwards. Because they face the caldera, they must push the caldera floor up, and if the caldera is already rising up because of an expanding magma chamber below, then the cone sheets will intrude effortlessly towards the surface, taking up the stresses generated by this resurgence.
Thick successions of scoria fall deposits and lava flows separate the major ignimbrites of Colli Albani. Many scientific publications focus only on the ignimbrites, so that it may seem that the 561-365 ka period of Colli Albani activity simply involved a couple of ignimbrites and there was little intervening activity. In reality, I suspect that very roughly half of the volume erupted from 561 ka to 365 ka, was possibly erupted from circum-caldera fissures fed by cone sheets, similar in style to the activity that followed the Villa Senni eruption. This activity probably would have accompanied caldera resurgence that eventually culminated in the ignimbrite eruptions.
The “reverse” of a cone sheet is a ring dike. While cone sheets thrive in emerging calderas, ring dikes intrude in collapsing calderas. Ring dikes work by pushing the ground downwards. They need a trigger to depressurize the caldera. Most caldera-forming ignimbrite eruptions of Colli Albani started with a scoria fall deposit, which corresponds to a subplinian/plinian eruption. These eruptions would have depressed the floor of the caldera by extracting a large amount of magma from it. The subsidence allows a ring dike to intrude, in turn the dike lubricates the caldera ring fault with magma, and the roof collapses faster, magma shoots out of the ring dike in a caldera-forming ignimbrite eruption, the more the roof collapses the more the dike opens up and the faster it erupts.
Post-caldera Colli Albani
Following the ignimbrite eruptions, Colli Albani became less active, even so it has kept some activity to recent times. After the ignimbrites, activity became centred inside the caldera were it built a small stratocone, the Faete stratovolcano. Faete is known to have been active in 290-260 ka, although may have started earlier. The volcano was mainly constructed from lava flows and subplinian eruptions, but also involved maar style activity that blew away the top of the cone into a 2 kilometre wide crater.
Since 200 ka, Colli Albani has been dominated by maar eruptions. Several maars have been constructed, including three large polygenetic maars southwest of Faete stratovolcano. The largest maar is Albano, which erupted 7 times starting 45 ka. Albano makes up a massive crater of 3 x 4 km, and 200-400 meters deep in relation to the rim of the crater. The explosions of Albano ejected large metre sized blocks of old lava and carbonate rocks and deadly pyroclastic surges swept across the landscape to distances of 15 kilometres to the northwest and southeast of the vent.
The last dated event of Lake Albano in 4.8 ka may have been non-volcanic, and consisting of a lahar. This seemingly repeated in 406 BC, when, as reported by Plutarch, the lake surged over the surrounding hills, unleashing a flood of water that destroyed fields and vineyards, and then poured into the sea. This event may have been related to hydrothermal activity.
Will Colli Albani erupt again?
It seems to me that Colli Albani is a waning volcano well past its prime. However, it will probably erupt again.
The Global Volcanism Program reports that Colli Albani experienced inflation between 1993 and 2000, which might still be continuing. The inflation rate in 1993-2000 was about 2-4 mm per year. This is, of course, very little compared to other volcanoes of the world. The inflation is, however, in an ominous location. Inflation is centred in the Albano and Remi maars. Likely there is a magma storage under the large polygenetic maars which is refilling with magma. There is also active seismic activity. 1100 earthquakes took place under the Albano maar in 1989-1990 at depths of 4-5 km.
Should a large maar eruption take place, consequences would be devastating. There are well over 100,000 people living in the area that was affected by earlier pyroclastic surges of the maar eruptions. Two of the larger towns in this area amount to 80,000 people. Many other towns amount together to a probably greater number. So even though an eruption is unlikely to happen anytime soon, Colli Albani should be carefully watched.
One publication has also suggested that Lake Albano could produce a limnic eruption given the CO2 that is being dissolved into the lake. This is, however, considered by them to be an unlikely possibility, given that the concentration of CO2 is presently far from saturation.
Colli Albani has it all. It has bizarre leucitite magmas, the power of a large caldera, and a bit of a mystery regarding Lake Albano, with its earthquakes, inflations, and floods. It is also a volcano that can teach us about calderas.
Calderas are calderas, regardless of their magma composition. It is true that rhyolite calderas usually erupt larger volumes. However, leucitite, trachyandesite, or basalt calderas, all of them are frightening systems capable of VEI-6+ eruptions. The ignimbrites of Colli Albani would each have destroyed areas of 1000-2000 km2 within a very short time-span.
In reality, volcanic eruptions are very complex. A lot of factors. There is viscosity, yes, but there is also volatile content, magma chamber size, conduit width, or groundwater interactions. Simple rules like: the more a volcano pressurizes, the worst the eruption will be, or the more viscous the magma, the more explosive, fall short to explain the enormous diversity of volcanic phenomena out there. And in a world full of volcanoes you can still find a volcano that is a one of a kind gem, like Colli Albani.
Discussion on ignimbrites: