The Campania Plain: Part 3, Vesuvius

It’s time to look out over the Gulf of Naples from Mount Vesuvius on a sunny Mediterranean day!

That landmass you see at the start, off in the distance and straight ahead, is Ischia (hi! Monte Epomeo!).

In between Ischia and the mainland is tiny Procida — volcanic too, but inactive for over 10,000 years. We’ll mention Procida in the next post, when we get to what is probably the world’s most dangerous known volcano.

This beast is in the video, too, keeping a fairly flat profile on land and also lurking under that curving bay in between Procida and the urban sprawl of metropolitan Naples below Vesuvius.

Yeah. The Pompeii-killer isn’t the worst volcanic threat facing Campania today.

But Vesuvius is bad enough, despite being such a popular tourist attraction during its present quiet mood.

When did Vesuvius last erupt?

Short answer: In April 1944, shortly after Allied forces had occupied Italy during World War II. Some experts suspect that activity at Mount Vesuvius comes in megacycles (De Vivo and others). If true, then the eruption shown below might have signalled the end of a longer cycle that began with a roar in AD 79. But there’s no way to be sure yet.

Details: There is a lot of intense music and narration in the following video, typical of the era, but no one was killed in San Sebastiano by this eruption.

Damage was bad, including the loss of almost a hundred Allied aircraft from tephra, but Vesuvius — ninth most deadly volcano in the world (Oppenheimer) — claimed a total of fewer than 50 victims this time (mostly through roof collapses, falling rocks, and asphyxiation from volcanic gases).

No travel videos from the summit THAT day! The cone that we now see in the volcano’s crater is only about a third of the size it used to be before this VEI 3 eruption. Believe it or not, some eruptions in the past have been even larger!

There are volcanoes around Naples for the same geologic reason that there is a bay — this region sits in a subduction zone, and the ground is being stretched here.

When you stretch rock, it cracks and moves along a fault. Magma from Earth’s mantle can then leak up to the surface, although the details of its movement and storage are complex, difficult to study, and not yet very well understood.

Geologic faults aren’t shown on this 19th-century map, but it does give you a general sense of where the important places are. “Somma” = “Vesuvius,” for reasons given later in the post. (“Popular Science” via Wikimedia, public domain)

Some geoscientists suggest that one such fault, miles down there in the limestone basement and trending southwest-northeast out into Naples Bay near the town of Torre del Greco, might be the reason why Vesuvius formed where it did. (Judenherc and Zollo)

Why are the volcanoes around Naples so explosive?

Because, a little over 60 miles underground, a subducting slab of sea floor is carrying water down into the Earth.

Under such high pressures and temperatures, that water leaves the slab and percolates into the surrounding mantle, lowering the mantle’s melting temperature and contributing volatiles to the melt, like CO2, sulfur, and chlorine, in addition to H2O.

The result?

. . . [E]xtraordinarily complex physical behaviour of molten rock with changing temperature and the additional complications that arise from its constitution by all three phases of matter: solid, liquid, and gas.

— Oppenheimer (see source list)

Let that complex mixture sit for a while in continental crust, polymerizing silica and picking up even more gassy stuff (limestone is a good source of carbon dioxide), and you’ve got, well, a volatile situation.

Something eventually gives way, for one reason or another, and “boom!”

We’ve already seen that Campania experienced a particularly big “boom!” about 39,000 years ago: the hurricane of ash that was the Campanian Ignimbrite eruption.

Believe what you see: that’s two volcanic structures, Somma Caldera (left) and Vesuvius cone (right). (Mentnafunangann via Wikimedia (Italian), CC BY-SA 4.0)

Vesuvius wasn’t involved in that one, although it has spewed at least two smaller pyroclastic deposits out onto the Plain. (Di Vito, 2008)

The volcano we call Vesuvius today actually has a double structure consisting of an older caldera (Somma) and a younger cone (Vesuvius).

This all took shape, starting with Somma, after the plain was buried under Campanian Ignimbrite.

In other words, underneath Vesuvius is ignimbrite and below that, limestone bedrock dating back to dinosaur times, when this part of the world was a tropical archipelago.

Early activity here was relatively quiet, compared to today, but the explosiveness of Somma-Vesuvius eruptions picked up around 25,000 years ago.

Geologists (Scandone and others) have identified at least four major plinian events (VEI 5-6) since then:

  1. Sarno/Pomici di Base, a little over 18,000 years ago
  2. Mercato/Ottaviano, around 8,000 years ago
  3. Avellino, roughly 3,800 years ago
  4. Pompeii, 79 AD (other places were buried, too, but scientists have named this eruption after its most famous casualty)

The last two plinian eruptions are especially noteworthy because they preserved evidence of human history in Campania.

What happened at Pompeii?

Short answer: This, more or less:

Details: No one really knows what Somma-Vesuvius looked like in early 79 AD, but eyewitnesses have described the lower part of the mountain being covered with orchards and vineyards, with a flat, fire-blasted rocky summit.

Inside that summit, there probably was a plain, rather than a cone, with room enough for thousands of Roman soldiers to maneuver during the Spartacus revolt in 73 BC, although livestock may have been grazing up there 106 years later, when the volcano woke up.

According to scientists who monitor the volcano:

On August 24 of the year 79 AD Vesuvius returned to activity after a period of quiet probably lasting about eight centuries, pouring on the surrounding areas, in little more than thirty hours, about 4 km3 [~1 cubic mile] of magma in the form of pumice and ash.

The eruption began around one o’clock in the afternoon of 24 August with the opening of the conduit following a series of explosions resulting from the immediate volatilization of surface water that came into contact with the rising magma. Subsequently, a column of gas, ashes, pumice and lithic fragments rose about 15 km [some 9 miles] above the volcano.

This phase of the eruption lasted until about eight o’clock the next morning, and was accompanied by frequent earthquakes. Taking advantage on the night of an apparent pause in the eruptive activity, many people returned to homes that had been left unattended. But they were surprised in the morning by the resumption of activity during which the complete collapse of the eruptive column occurred, which determined the formation of pyroclastic flows that caused the total destruction of the area of ??Herculaneum, Pompeii and Stabia.

In the terminal part of the eruption, which probably occurred late in the morning of 25 August, pyroclastic flows continued to form, whose deposits definitively buried the surrounding cities, while a dense ash cloud dispersed in the atmosphere until it reached Capo Miseno.

National Institute of Geophysics and Geology, via Google Translate

To modern eyes, Pliny the Elder was the most prominent victim; he died at Stabiae, near modern-day Castellamare (his nephew, Pliny the Younger, across the bay at Misenum, described the eruption so clearly that volcanologists now call such events “plinian”).

The remains of some 400 Pompeiians have been found in ruins, victims of structural collapse or asphyxiation, while at least 650 more were caught in the pyroclastic flows.

In Herculaneum, over 300 people were trapped in boat houses when those flows surged through town.

Herculaneum actually had escaped the earlier ash fall, thanks to the wind direction that day (it was blowing right over Pompeii). Historians suspect that residents here took advantage of this break to evacuate by sea; unfortunately, the eruption column collapsed before everyone could get away.

No one knows exactly how many human lives Vesuvius stole from Campania with this VEI 5 eruption. Only part of Pompeii, Herculaneum, and Oplontis have been excavated. Many people also probably fled into the countryside, which is still buried under deep pyroclastic deposits.

Amazingly, what we see today as a major historical disaster made hardly a ripple at the time.

The emperor made a brief visit; a few commemorative coins were struck; and the elite moved their playground resorts away from what was no longer Campania felix, “happy Campania.” Then apparently everyone let it go and moved on. (Oppenheimer)

That would never happen today, right? Well, it has happened.

In the Caribbean.

During the mid-1990s, a little over a decade after Paul McCartney and Stevie Wonder recorded “Ebony and Ivory” there.

And, unless you’re a volcanophile or are/know someone who was affected, you’ve probably never heard of that tragedy.

That’s what was left of Plymouth — former capital of Montserrat — about five years after the island’s volcano rumbled back to life.

Granted, the eruption on Montserrat was at least 100 times smaller than the Pompeii eruption, but still . . .

Well, perhaps it’s part of human resilience to not look very hard at such horrors unless we ourselves are in harm’s way.

But even then, our ties to the homeland are strong. Thousands of Montserratians have returned to their island, now that the volcano has quieted down (though Plymouth has been abandoned).

And, starting a couple of centuries after the Pompeii eruption, Campanians began to rebuild, too.

For example, here’s modern Ercolano, sitting atop ancient Herculaneum. Vesuvius looms in the background. (Big Albert, public domain)

But by then the human world was very different.

Human beings live in the moment — another facet of the resilience that has carried us through many hundreds of centuries’ worth of natural disasters.

But it takes the environment decades to centuries to become habitable again after volcanic devastation. Over such a long time, people do move on.

Perhaps there is no going back to the way things were before the volcano — any volcano — erupted.

After Pompeii vanished, recovery in this part of Campania might have followed the same general pattern that archaeologists have found in Bronze Age human reactions to an earlier plinian eruption of Vesuvius.

Pompeii wasn’t the first time Vesuvius preserved evidence of human history in Campania. These footprints are almost 3,800 years old. (Pierpaolo Petrone via Wikimedia, public domain)

They have uncovered thousands of footprints, belonging to livestock as well as people, in the lowest layers of the Avellino Pumice, which was laid down around 1780 BC.

These all lead away from the volcano, through mud as well as ash, proving that you can survive a nearby plinian eruption, if you get out early enough.

Mud, though?

Yes, Mount Vesuvius is also notorious for its floods. Naples emergency planners have set up a Blue Zone as well as Red and Yellow Zones.

There must be quite an aquifer up there. During an eruption, water from steam condensation can flow in such quantities that a 17th-century Spanish observer, who was watching yet another great Vesuvian paroxysm, thought that a river had burst out of the volcano.

Not everyone escaped the Avellino ash fall. Two skeletons — a young woman and a man in his 50s — have been found. They may have suffocated to death while struggling through very heavy ash fall.

Of course, no one would have survived the pyroclastic flows which are this volcano’s favorite murder tool. But the flows ran out of energy before wiping out everything.

Thick, relatively slow-moving layers of ash preserved a few villages in incredible detail, without heat or structural damage.

No human remains have been found there yet, but evidence suggests that people tried to come back soon after the Avellino eruption ended.

It didn’t work out for them. They abandoned the area for centuries, just as the Pompeii region was probably abandoned for a while after the AD 79 catastrophe.

But here’s the chilling part: some of these footprints are just 4 miles outside of metropolitan Naples. And the Avellino surge bed in Naples is almost 10 feet thick. (Mastrolorenzo and others)

It’s one thing to evacuate a Bronze Age village. It’s quite another to evacuate a modern city of many millions of people on roads that were not designed to handle that much traffic.

Of course, they’re making plans to do exactly that when Vesuvius reawakes, but it’s not easy. When authorities did a large-scale evacuation exercise in 2006, many people didn’t complete it because heavy rains blocked their access to the highway escape route. (Scarth)

Heavy rains.

Imagine what would happen under conditions like those encountered by the Bronze Age people who left their footprints in hot ash and mud for posterity to find and ponder.

The topic of managing volcanic risk in Naples requires its own post, but before we can get into that, we need to look at the third, and most hazardous, major volcano in the area; the one that’s lurking under the bay in that travel video up above; the “burning fields,” Campi Flegrei . . .

One of the first photographs of Vesuvius in eruption, in 1872, by Giogio Sommer via Wikimedia, public domain.

Some webcams and satellite data

INGV seismic monitoring

Toulouse VAAC (Volcanic Ash Advisory Center)

Featured image: Elisaboba, CC BY 2.0.


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