Drilling into Campi Flegrei

Naples is a bustling shipping and cultural center and home to several million people, all living in between Vesuvius on the city’s eastern skyline and the much less famous complex of craters, cinder cones, and steaming sulfur pools known as Campi Flegrei on its west side.

The volcanic risk is high here.  While experts are certainly keeping a close eye on Vesuvius, research since the 1980s has shown that a Campi Flegrei eruption could cause the worse disaster.

♦The Campi Flegrei Caldera

Vesuvius, a stratovolcano, has erupted frequently during recorded times, last in 1944.  Campi Flegrei, which is known outside of Italy as the Phlegrean Fields, actually used to be considered the lesser of the two threats, since it hasn’t erupted in over 450 years.  It was more famous for the way its ground goes slowly up and down, leaving some Roman ruins submerged, and at other times, standing on dry ground.

Italian geologists call this movement a bradyseism, or “slow earthquake,” as compared to the more typical earthquakes that happen frequently throughout the region, which is in a very active geologic zone where the European and African tectonic plates come together.

Starting in the 1980s, experts noticed that this bradyseism was very similar to movements that were also happening just then in two calderas, Rabaul and Long Valley.  Careful investigation showed that Campi Flegrei is a caldera, too.

Calderas are the largest and least understood type of volcano known today.  They form when most of the magma erupts out of the chamber, causing the remaining volcanic edifice to collapse into the now empty space.  This eruption can happen fairly quietly, as with some Hawaiian volcanoes, or with spectacular violence, as it has happened a few times in the very distant past at Yellowstone caldera.

At some 8 miles wide, the Campi Flegrei caldera is only about one-fifth as big as Yellowstone, but its location and ongoing deformation make it one of the most high-risk volcanoes in the world.  Yellowstone is a national park located in the mountains far from any major urban center.  At least a million live inside the Campi Flegrei caldera and two or three million more live right next to it.

♦Campi Flegrei erupts

Once its true nature had been recognized, emergency planning and scientific research shifted into high gear, and today Italy’s national, provincial and local emergency managers are prepared for an eruption like the last one there, which happened in the 16th century.  It wasn’t a super-eruption, but rather more along the lines of what Vesuvius, an “ordinary” volcano, is capable of.

Early in the evening of September 29, 1538, fissures and a “horrid hole” opened up in Campi Flegrei near the port city of Pozzuoli, spewing lava, ash, and pumice up into a big eruption column.  Pozzuoli was almost totally destroyed. The city of Naples was protected by the Posillipo ridge, though it got a heavy coating of ash; indeed, much of the region was temporarily uninhabitable, just as areas near Mount Merapi had to be abandoned for a while when that Indonesian volcano erupted in late 2010.

Pozzuoli was rebuilt as soon as possible afterwards, and today it is part of the vast urban sprawl that covers the shores of Naples and Pozzuoli bays.  The forested Monte Nuovo cinder cone nearby is the most prominent residual from the 1538 eruption and is now one of the famous tourist landmarks in Campi Flegrei, along with the bubbling sulfur waters and hot springs of Solfatara crater.

Italy and the world were much simpler and less interconnected back in 1538.  If an identical eruption occurred today, it would devastate all of Naples Province and have repercussions throughout much of the world.  Italians have prepared for it, as best they can.  However, their plans can’t cover a super-eruption.  Is one likely to happen there again?

♦A violent past

Evidence for two super-eruptions at the Phlegrean Fields has been identified so far.  The biggest one happened some 39,000 years ago, when huge pyroclastic flows swept the entire region and ash fell as far away as modern Moscow and Greenland.  The second happened 15,000 years ago, covering the Naples region in thick deposits of yellow and gray tuff.

Between then and 1538, the volcano erupted over 60 times, but not again on such a massive scale.  Some of these smaller eruptions, including a big explosion at Solfatara in 1198, have involved steam, because Campi Flegrei has an active hydrothermal system.  Scientists are not yet sure exactly how the volcano’s hot water system interacts with and affects its magma “plumbing” and subsequent eruptions, but they do know that the presence of water makes those eruptions much more explosive.

♦Scientists look for answers

The violent reworking of the caldera’s floor so many times makes it hard for geoscientists to get a good picture of Campi Flegrei structures and their relationship to each other down through time.  This stratigraphic work, though, is of key importance in understanding the caldera’s eruptive style well enough to get a reasonable idea of what it may do in the future, and when.

It also doesn’t help that 60% of the caldera is currently submerged in Pozzuoli Bay, and that what is above water has been affected by human activity for almost 3,000 years.  Too, the population density in the Naples area is some 21,200 people per square mile, and very little ground is exposed anywhere in the region for geologists to study.

They do the best they can, boring down into the bedrock underneath city streets, for example, or studying highway and railroad rock cuts.  Radiocarbon dating has helped somewhat, but the high carbon dioxide content in this volcanic terrain makes it difficult to interpret the results.

In addition, researchers have indirect methods that can tell them much about hidden structures and processes, including instruments sensitive enough to measure how gravity changes over a denser or lighter underground rock formation; assays of the volcano’s water that show what it is doing and how its chemistry and temperature change over time; and satellite-based interferometry that measures even very slight ground movement.

The pressure is on:  Emergency planners, local citizens, and businessmen would like to know if an eruption of Campi Flegrei is “due” soon, and if so, how big it will be.  Unfortunately, the volcano is so difficult to study that scientists, despite their best efforts, can say little about it with any certainty.

However, that may soon change.

♦The Campi Flegrei Deep Drilling Project

In early February, scientists will start drilling into parts of Campi Flegrei and its environs at locations where they hope to find the most information with the least amount of difficulty or risk.

The project will start with two initial bore holes on land, in the grounds of an unused factory near Bagnoli.  These will be relatively shallow, less than 2 km, and should provide data on ground structures as well as measure the temperature gradient with depth, which tells geologists just how far down the top of the magma chamber is without actually having to approach it with the drill.  Experimental devices will also be placed to monitor heat flow and to investigate the possibility of using some of Campi Flegrei’s heat for geothermal energy. 

The rest of the bore holes planned for this year-long project will be done off shore both in the caldera, to correlate with the land drilling results, and further out in areas where the stratigraphy of the first, great eruption has been less affected by the more recent one 15,000 years ago.  The last three of these drills will take place in seabeds off southern Italy’s coasts, where sediments have been undisturbed for many millennia, in order to get a complete picture of the entire eruptive event that happened 39,000 years ago.


With its history of colossal eruptions, unpredictability, and ongoing deformation, Campi Flegrei caldera represents high risk to Naples and its environs, but exactly how much is not yet known.

The past is the key to the future.  Scientists plan to protect Naples by studying the volcano closely in order to learn everything they can about the dangerous caldera’s structure and eruptive history.  With a clearer idea of Campi Flegrei’s past, they will be in a better position to predict its future behavior and thus provide at least some of the information that authorities and residents need in order to prepare for whatever the volcano throws at them next.


Volcanoes of Southern Italy:  Campi Flegrei.  John Guest

The Campi Flegrei Deep Drilling Project.  De Natale, et al.

Campi Flegrei.  Italian National Civil Protection Department (Google Translate Italian to English)