The Campania Plain: Part 2, Ischia Volcano

No, Italy’s beautiful Monte Epomeo, shown above, is not the volcano. This mountain is “just” a block of 50,0000-year-old greenish ignimbrite pushed up by volcanic processes out of its submarine caldera to an elevation of about half a mile above sea level.

This entire island of Ischia — older than the other two hazardous volcanoes near Naples, and the largest and westernmost of all Naples Bay islands — is the volcano.

And tourists (particularly Germans) really love this place!

So do volcanologists, not so much for the hot springs and resort living as for the insights Ischia gives them about restless calderas and large-volume volcanic eruptions.

You don’t need to speak German to enjoy this travel video: there’s no narration and the titles are mainly Italian place names. A helpful map at the start shows you where Ischia sits in relation to the City of Naples, and “Vesuv”–the Pompeii-killer–makes a cameo appearance at around 0:30. (Next week, we’ll take a look at Ischia from the summit of Vesuvius!) The ferry ride to Ischia is short in the video and takes only about 40-60 minutes in person.

Ischia’s thermal resorts, which feature hot springs like those shown at 4:45, draw up to 6 million visitors a year, generally from Europe and Asia. Check out the cooking eggs! There’s also a close-up of the ignimbrite (formally known as Monte Epomeo Green Tuff) at 7:10, and a fumarole at 9:08, though apparently there aren’t many volcanic gases escaping from it, since the sign only warns visitors away from “hot sand.”

Hot, indeed! Ischia’s hydrothermal system can reach as much as 250-300° C (482-572° F). This volcanic field is complex, but as far as the experts can tell, its heat source sits a little over a mile underground.

What’s so special about Ischia?

Short answer: It’s one of Earth’s smallest known restless calderas. Uplift of Monte Epomeo and a long history of volcanic activity on Ischia provide geoscientists with data they need to study similar structures worldwide, as well as to better understand how volcanism works on the complex and densely populated Campania Plain.

Details: “Horst-and-graben” is not the name of a comedy team working the resorts visited by those German tourists above.

It’s one of the more delightful terms in geology.

That’s pretty self-explanatory. This type of landform happens when the Earth’s crust stretches enough to develop some steep-angled cracks, which are technically known as faults because the ground moves along them.

“Basin and range” is another name for horst-and-graben.

What does this have to do with Neapolitan volcanoes?

One of the few straightforward pieces of the intricate geological puzzle that is Campania is the fact that the plain itself is a graben.

It has filled in with volcanic deposits and sediments over time and is still sinking very slowly at around 1.5 to 2 mm/year.

The plain contains grabens, too, like the complex system of basins and half-basins that we call Naples Bay.

What’s pulling the ground apart here?

At least one of the factors behind this is fairly straightforward, too. It’s back-arc spreading.

Shallow Sub Zone
Campania hasn’t actually split into a spreading axis yet–it’s just stretched. (Zyzzy2 via Wikimedia, CC BY-SA 3.0)

In this case, subduction is happening to the south of Campania, at the Aeolian Islands. (Sartori)

The slab of subducted seafloor is descending into the mantle underneath the Apennine Mountains.

Campania, say the experts, sits above the slab, too. They say many other things as well, but the devil is in the details.

There’s very little consensus yet on Campanian geology and tectonic processes.

Fortunately, we just need to know that the ground here began to stretch apart some 780,000 years ago, with what is now Naples Bay starting to open up into grabens around 730,000 years ago.

Also, there’s a lot of magma down there, from the slab subduction.

Not surprisingly, many cubic miles of that molten rock have reached the surface, exploding out of the ground as ignimbrite flows, probably through both volcanic vents and along faults.

At least five flows besides the Campanian Ignimbrite have covered the plain over the last 300,000 years. (De Vivo and others, Di Vito and others) But since each new eruption buries the last one, geologists have a very hard time working out exactly what happened here, and when.

The oldest rocks on Ischia are about 150,000 years old. At least two of the nearby vents in Naples Bay — yes, the Bay is volcanically active in a few places, too — are the same age.

Little is known about those ancient times, but Ischia may have had some big explosive eruptions then, judging from the rock types. It then settled down into relatively quiet dome-building activity around 75,000 years ago and maintained a fairly low profile in geologic terms until about 53,000 years ago.

Then, while continental ice sheets sat atop Scandinavia, and cave lions and sabercats prowled Europe’s prairies and tundra, Ischia had a huge eruption, spewing out the Monte Epomeo Green Tuff (MEGT) and then collapsing into a caldera that has been estimated to be 6 x 4 miles wide. (Carlino and others)

Since so much ash went into the sea and has been lost, it’s not easy to estimate the size of this eruption, but tuff does cover the island to a depth of over half a mile and is almost 200 feet thick on parts of the mainland.

It was a big eruption, though not anywhere near “super.”

One volume estimate (Brown and others) is 9-15 km3 of erupted magma, which puts the MEGT blast close to or a little bigger than Krakatau’s 1883 eruption (10 km3), which left a 5-mile-wide caldera.

No evidence of ancient Ischia’s pre-MEGT appearance remains, but perhaps it had a stratovolcano like Krakatau’s before the 1883 eruption there.

Caldera-forming eruptions with a volume of 100 km3 or less often do. (Cashman and Giordano)

Library of Congress

Whatever it looked like before this huge eruption, the new Ischia caldera and its filling of Green Tuff were soon beneath the waves. Geochemical studies prove that the rock stayed underwater for some 20,000 years.

Then it began to rise, at an average rate of about 1.3 inches per year.

Now, as Monte Epomeo, this tuff sits over half a mile above the sea. Geologists have mapped it thoroughly to clarify its history and figure out what happened during the eruption (which they say was probably a series of big explosions). (Brown and others)

They have also done many analyses and run a number of computer models to try to understand exactly what has raised this block of rock up from the sea bottom.

Yes, this thing is a horst, and yes, it does sit in the extensional tectonic setting of the Campania Plain. But there is obviously heat down there, too, and also magma.

Yet there is no vent atop Monte Epomeo.

At least 40 eruptive vents surround the mountain, though.

Some researchers (Carlino and others) suggest that a magma body, roughly 6 miles across, sits 1-2 km (0.6 to 1.2 miles) below the island and has pushed Monte Epomeo’s Green Tuff up into its present position.

They think that this batch of molten rock first showed up some 30,000 years ago and built up pressure while leaking around Monte Epomeo’s edges through those 40 vents.

Today, though, Monte Epomeo is going down, not up. It’s sinking at a rate of 8 mm (about a third of an inch) per year. (INGV)

This is very un-horst-like behavior (remember: horsts go up, graben go down).

But it fits right in with Ischia’s volcanic past, if you think of it as a restless caldera — one that shows ground deformation and other low-level activity.

Throughout the world, well over a hundred restless calderas, as big or bigger than Ischia, have been found. That’s a lot.

And since humans have been around for such a short span of geologic time, it’s likely that most caldera volcanoes are restless over their lifetimes.

Sometimes these episodes lead to an eruption, as happened on a relatively small scale in 1994 at Papua New Guinea’s densely populated Rabaul Caldera.

That event at Rabaul was a little unusual in that two vents opened almost simultaneously. Scary!

Sometimes no eruption happens.

For example, Campi Flegrei is a younger and much bigger caldera that sits on the mainland just east of Ischia. This sleeping giant is so restless that they’ve coined a word for it: “bradyseism.”

How extreme is that “slow earthquake”?

At Ischia, Monte Epomeo is slowly subsiding now. Researchers (Vezzoli and others) have identified possible pulses of uplift that include:

  • From 7200 to 6800 BC
  • From 4100 to 2300 BC
  • From 1800 to 1000 BC
  • From 650 BC to 355 AD

They had to study the island’s old eruptions to get those dates.

At nearby Campi Flegrei, all you have to do is look for the old Roman town of Port Julius — it’s now underwater.

That’s how much the surface of this restless caldera moves up and down in a comparatively short time!

Geologic evidence also suggests that bradyseism has been going on at Campi Flegrei ever since its last big eruption some 15,000 years ago.

What hazards exist at Ischia Volcano?

Short answer: Earthquakes and landslides are more likely than an eruption, given the ongoing subsidence of Monte Epomeo. Any input of new magma would presumably stop that downgoing motion, at the very least, and possibly even cause uplift.

Details: It must be nice to have scenic Monte Epomeo in the neighborhood and know that there’s no volcanic vent up there to give you trouble.

But that’s a lot of weight in motion. Even though it’s sinking very, very slowly, stresses are building up, especially in the north near this huge horst’s highest elevation.

When something gives way, there’s an earthquake.

There have been at least six destructive quakes on Ischia since the early 13th century.

The most recent one, with a magnitude of around 4, happened in 2017.

Two people died in Casamicciola Terma, which is near the northern flank of Monte Epomeo and is also the most seismically active part of the island. Forty-two people were injured and some 2,600 became homeless.

Ischia’s deadliest earthquake happened in 1883,when most of Casamicciola — a big resort town back then — was leveled and over 2,000 people lost their lives.

After that disaster, Giulio Grablovitz (link is in Italian) founded a seismic observatory at Casamicciola in 1885, and scientific study of the island’s geophysics began.

Giulio Grablovitz

A modern seismic network was first established on Ischia in 1970 and updated over time. Several mobile stations were added after the 2017 quake.

Today, stations are monitored in real time by the National Institute of Geophysics (link is in Italian).

Seismic data show that Ischia earthquakes are related to the subtle movements of Monte Epomeo, not to underground volcanic activity. (De Novellis and others)

The island’s last eruption happened in 1302, when magma leaked around the Epomeo block near Cremate, at first explosively and then more effusively (more like a Hawaiian-style eruption, though the lavas are very different).

Over two months, that lava flowed into the sea, about 2 miles from the vent, and added some 50 acres of new land to Ischia as well as forming Capo Molino.

The eruption apparently caught people by surprise, though.

Records are incomplete, but reportedly “many” people died, and there were also heavy livestock and property losses.

Landslides are another concern on Ischia because of the combination of earthquakes and steep slopes, particularly since those slopes are made out of volcanic deposits: not the most stable of natural materials.

The 2017 quake caused some ground cracking and landslides. But Ischia has also had four major debris avalanches in the last 9,000 years, including one that carved Epomeo into a horseshoe shape and now covers the seafloor off the southern coast with a hummocky apron of rocky material.

Everywhere on the Campania Plain, the location of volcanoes and the nature of their eruptions are controlled by the complex dance of ground movements and magmatism.

No one really knows why Ischia Volcano first formed where it did 150,000-plus years ago. Most of its history before the Monte Epomeo Green Tuff caldera-forming eruption remains a mystery.

But everyone, from tourists to PhD-mentoring experienced volcanologists, appreciates the rugged beauty of this Neapolitan volcano that is constantly shaped and renewed by earthquakes, landslides, avalanches, eruptions, and caldera restlessness.

Edited June 10, 2020.

Featured image: Roberto De Martino, via Wikimedia


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