The Campania Plain: Part 4, Campi Flegrei


But close up, Campi Flegrei (the “Burning Fields”) is actually quite pretty.

Look for saddle-shaped Vesuvius in the background from time to time, starting at around 1:21. Downtown Naples is hiding on the other side of Posillipo Hill, which rises in the middle distance and stretches out seemingly at the foot of Vesuvius (look close and you’ll get a little blue glimpse of the great Gulf of Naples beyond that really separates Posillipo and that volcano; you could also check out the view from Vesuvius).

True, the ground rises and falls in disconcerting ways (bradyseism), as though everyone, from 6th century BC Greek colonists onward, built their civilization on the chest of a slumbering giant.

And there are occasional one-shot (technically, monogenetic) volcanic eruptions here. The last one of these built Monte Nuevo, a huge spatter cone, in 1548 AD (check it out in our video at 0:42, coming in from the left).

Much older than this centuries-old “New Mountain” are the explosions and other steam-driven geological activity that constructed the Miseno peninsula, where this video begins, thousands of years ago.

For that matter, the island you see just behind Miseno, called Procida, is made up of old volcanoes, but those haven’t shown any sign of life for well over 10,000 years.

Only the Solfatara — literally a burning field of fumaroles — bears any resemblance to what most of us would call a volcano.

I couldn’t see Solfatara in our travel video above; the steamy crater is northeast of town (which is called Pozzuoli, by the way) and sits on this side of Posillipo Hill.

Nevertheless, Campi Flegrei is a volcano, and a very important one, too.

Depending on which source you ask, Campi Flegrei was either the vent responsible for the Campanian Ignimbrite eruption (at least 150 km3 of magma erupted) that buried the whole plain about 39,000 years ago or else it contributed to that cataclysm — the biggest Mediterranean eruption in the last 200,000 years.

As well, Campi Flegrei definitely was the source, some 15,000 years ago, of the Neapolitan Yellow Tuff (almost 50 km3 of magma) that Naples is now built on (and with).

So, where’s the volcano?

Short answer: It’s everything you see in that video: all the land around and underneath the bay. More than 1.5 million people live in it.

Campi Flegrei caldera
Many of the world’s caldera volcanoes have fairly clear-cut outlines, but scientists haven’t quite made up their minds about the Campi Flegrei structure. At the time of writing, the red and blue lines look like the most likely walls for the Campanian Ignimbrite and Neapolitan Yellow Tuff calderas, respectively. (De Natale and others, 2016, per source list, CC BY-NC-ND)

Details: We’ve all seen a partial caldera collapse, but most of us didn’t realize it because it happened slowly and not at the point that was drawing world attention in 2018, when fiery red lava poured out of the flank of Kilauea volcano, ruining the lives and property of many Hawaiians.

But Kilauea’s summit collapse was dramatic, nonetheless.

There was a lava lake at the Kilauea summit for years. When that lava drained into the East Rift and headed for the Puna District, there was nothing left to hold up the summit and it collapsed.

Now imagine that happening with a “gray-lava” volcano — one that explodes rather than having runny red lava flows.

Exactly. This shot is from the 1991 caldera-forming eruption at Mount Pinatubo in the Philippines (at least 10 km3 of magma). (USGS, via Wikimedia, public domain)

Was there ever a big volcano like Pinatubo or Kilauea where Campi Flegrei sits now?

Years ago, some volcanologists thought so and christened it Archiflegreo, but evidence for this structure has never turned up.

Nowadays, many experts point out that it’s not necessary. The Campania Plain, while one of the most complex geological regions on the planet, exists because the Earth’s crust began to stretch here, many hundreds of thousands of years ago, in response to, among other things, tectonic plate subduction a little farther south, at the Aeolian Islands.

As the ground stretched it cracked, and molten rock made it to the surface in some places. The real-life process, of course, is much more intricate and involved, but it is still true that all of these Neapolitan volcanoes, and the many ignimbrite flows that came before them (of which the Campanian Ignimbrite was the largest), are the result of that ongoing interplay between magmatism and regional tectonic stresses.

Such geologic dances can produce anything from a Vesuvius, towering over the Gulf of Naples, to a hole in the ground, formally, a caldera.

Campania has both a tiny caldera — nearby Ischia — and a whopper, i.e., Campi Flegrei.

Video excerpt
This screenshot from the travel video shows what many consider to be the center of Campi Flegrei caldera: La Starza marine terrace, that small hill just behind town. Beyond that is the larger Posillipo Hill, which is the eastern caldera rim according to some volcanologists. It forms the red line just west of Naples on that caldera map above. And there in the background is Vesuvius, which you might have considered the worst volcanic hazard in Naples until you read this post.

Are Ischia, Vesuvius, and Campi Flegrei related?

That’s hard to say, both because their plumbing systems are miles underground and because of the geological complexity here. Many volcanologists include Campi Flegrei, Ischia, Procida, and a few smaller, older islands nearby in a single volcanic district.

Procida, for example, used to erupt fairly frequently until around 14,000 years ago. It stopped just before the Neapolitan Yellow Tuff eruption, and hasn’t been heard from since.

Too, it’s not completely impossible that Vesuvius and Campi Flegrei, at least, might share a common magma source, despite the distance between them and the geochemical differences in their eruption products. (Zollo 2008) Let’s look a little more closely at such things in a separate post.

Campi Flegrei is especially difficult to study because about half of the known caldera is underwater. But the whole region is heavily urbanized and also contains the residual of thousands of years of human habitation, not to mention burial under massive volcanic deposits and weathering detritus over hundreds of thousands of years.

Geologists can find very few useful outcrops of rock to study here. The scientific literature is filled with references to discoveries made in cellars, tunnels, and other sites that people have dug up.

In 2012, scientists drilled a borehole some 1,600 feet into the ground near Posillipo Hill. They needed to know more about the stratigraphy of the area, the properties of rocks underlying Campi Flegrei, temperature gradients, and the local stress fields.

Of course, this produced highly technical data, but there were two points that most of us can grasp easily:

  1. The caldera rim probably does not underlie downtown Naples (the black dotted line on the map up above is from an earlier study that suggested it did). That’s good news, since Posillipo Hill can, to some extent, protect the city during an eruption.
  2. Campanian Ignimbrite deposits were smaller than expected, lending support to those who suspect that Campi Flegrei was “only” a peripheral vent in that eruption.

Outside of academia, local and national emergency management people are aware (Italian) of the danger.

So why don’t many of those 1.5 million Neapolitans move out of the caldera?

Well, it’s home to them and their ancestors, going thousands of years back in some cases.

And it’s beautiful here, most of the time. Good living is possible, when the volcanoes and the weather cooperate.

It’s the usual complex human story.

All told, some 6 million people are at risk from Neapolitan volcanoes. Next time, we’ll see what authorities are doing to protect them.

Edited June 15, 2020.

Featured image: NASA, via Wikimedia


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