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Geology of the Bay of Naples

(I acknowledge the  kind comments and  suggestions of a dear friend, Peter Humphrey, geologist and member of the U.S. Foreign Service, for his revisions and his particular ability to make science accessible. Any mistakes are, of course, mine.)

I recommend that you read this from the beginning, but you may also click through to the following subheadings:

general geology
Italy in particular
The "Ring of Fire"
Monte Nuovo
The Islands in the Bay
On Predicting Eruptions and Earthquakes


There are a number of obvious features of the landscape here in the Bay of Naples that are of extreme geological interest. In order of "obviousness," the ones that stand out are:

1) Mt. Vesuvius;

2) The intense geothermal activity at the western end of the bay, centered near the town of Pozzuoli in an area called The Flegrean Fields ("Fiery Fields”);

3) The presence in that same area of Monte Nuovo, (literally, "New Mountain," so-called because it appeared in a single week in the mid-1500s, just yesterday on the clock of geologic time);

4) The on-going small changes in sea level in that area, caused by so-called "bradiseisms" (the ground is bouncing up and down);

5) The presence in the Bay of the islands of Capri, Ischia, and Procida, two of which were formed by volcanic activity; and, finally,

6) The cliffs along the Sorrentine peninsula, which give you, the spectator, a good view of how mountains are thrust up above the surface by subterranean activity and then worn away and eroded into the shapes we see today.

General Geology

All of the above items, except erosion, are manifestations on the surface of activity below us. For the last forty years, geologists have been refining the theory of "plate tectonics" to describe the phenomena of "sea-floor spreading," and "continental drift," phenomena that are the direct cause of earthquakes and volcanoes.

The outer solid mineral crust of the earth is called the "lithosphere". It is a rocky layer underlying the continents and ocean basins, varying in thickness from almost zero at the mid-ocean ridge crest to over 100 km when carrying an embedded continent. It is helpful to visualize this layer as relatively thinner compared to the earth than the skin of an orange is to the fruit, itself. Below the crust lies the mantle, a layer of rock extending to a depth of about 3,000 km, or halfway to the center of the Earth. Parts of the mantle get so hot that rock becomes molten and moves slowly in vertically rotating currents. This is convection, the force that drives continental drift. Below the mantle is the core of the Earth, a ball about 2,500 km in diameter consisting of a fluid outer layer and a solid center, both mostly of iron and some nickel.

map of tectonic plates

he continents and ocean basins
are the upper portion of the lithosphere. The lithosphere is fractured at various points around the planet, giving us a global jigsaw puzzle of tectonic plates. There are about a dozen major tectonic plates and several dozen small—even tiny—ones (some of them are only the size of a big county and are often called “mobile terrenes”).

The continental configuration that we see today on the surface of the Earth is the result of these broad, thick rafts of oceanic crust and mantle shifting slowly to come together into a single primordial super-continent (nicknamed "Pangaea" by geologists) and then to start breaking apart again about 200 million years ago, first into two chunks ("Laurasia" and "Gondwana") and then into the configuration that is familiar to us today. (Young geology students are occasionally seen sporting t-shirts with messages calling for the “Reunification of Gondwanaland”. These kids need more homework.)

When tectonic plates move, they do so along fracture lines, the borders of each plate that define the actual pieces of the gigantic jigsaw puzzle. The plates move apart undersea and form large mid-oceanic rifts and then ridges, true undersea mountain ranges, formed over the course of millions of years as hot magma flows from below the lithosphere up into the rift. It is this sea-floor spreading—driven by the convective movement of the internal heat of the earth—that drives the entire process of continental drift.

Tectonic plates have existed since Earth's molten inception 4.65 billion years ago. It was the cooling of the crusts that made for the first tectonic plates, a model that can be seen on any cooling lava lake. (I have been told that it is great fun to put on a good pair of hiking boots and go running across the cooling crust of a lava lake. I have also been told that it is important not to trip and fall.) A particularly dramatic example of very ancient tectonics is the Ural Mountains, a classic collision plate boundary. Likewise, the Appalachians mark a very ancient closure of a proto-Atlantic; the continents then severed again, shearing the old Appalachian plate boundary between the US and Scotland.


The heat within the earth—the heat that drives continental drift—is due to three things:

(1) Heat from when our planet formed and accreted, and which has not yet been lost; the amount of heat that can arise through simple accretionary processes, bringing small bodies together to form the proto-Earth, is large (on the order of 10,000 degrees kelvin—about 18,000 degrees Fahrenheit);

(2) Frictional heating, caused by denser core material sinking to the center of the planet; descent of the dense iron-rich material that makes up the core of the planet to the center would produce heating on the order of 2,000 kelvins (about 3,000 degrees F);

(3) Heat from the decay of radioactive elements; the magnitude of this third main source of heat—radioactive heating—is uncertain. The precise amount of radioactive elements (primarily potassium, uranium and thorium) in the deep earth is poorly known.

In other words, there was no shortage of heat in the early earth, and the planet's inability to cool off quickly results in the continued high temperatures of the Earth's interior. In effect, the earth's plates act as a blanket on the interior, and even convective heat transport in the solid mantle does not provide a particularly efficient mechanism for heat loss. Our planet does lose some heat through the processes that drive plate tectonics, especially at mid-ocean ridges. For comparison, smaller bodies such as Mars and the Moon show little evidence of recent tectonic activity or volcanism.

Meanwhile, back on Earth, as new material is pumped into the rifts formed by sea-floor spreading, adjacent plates shift along the fracture lines causing the global jigsaw puzzle to slowly reassemble itself into ever-different configurations. Important in this view of the dynamics of the earth's surface is the fact that during the process of sea-floor spreading and rift formation, spread to both sides then causes some plates to come together elsewhere with varying results. The most important force in this spread is the pull of old, thick, relatively cold lithosphere into the trenches, with a little help from drag along the bottom of the plate. Ridges start as passive cracks opened by plates being dragged away to either side. Nature, abhorring a vacuum, then fills the cracks with lava. Again, the entire process of plate tectonics and continental drift is driven by convection—the enormous heat within the earth drives molten material towards the surface. Some of this material may escape to the seafloor, itself, to add to the great undersea mountain ranges; the rest cools and sinks to be recycled into a later round of convection.

When a relatively new (and therefore thinner) oceanic plate hits an older oceanic or a continental plate (both thicker than the youngster), a trench forms along the tectonic fault. Then, one of two plates coming into contact can subduct—go into the trench and under the other plate—forcing it up and producing great mountain ranges such as the Rockies, Andes, Alps, and Himalayas. (It is helpful to think of tectonic collisions as agonizingly slow car crashes!) (Note that some mountains, however, are caused also by direct volcanic activity, huge bursts of solid and molten material vented through fault lines at great pressure onto the surface.) The heavier subducting layer will eventually cycle back down into the hot magma below the lithosphere. The process of spreading on one end and subduction on the other suggests the picture of a continuously manufactured, one-way conveyor belt.

Plates can also strike and slip, i.e., rub together along the fault lines (faults are surface manifestations, often visible on the surface, of the actual plate boundaries far below), and cause considerable earthquakes. The San Andreas Fault is one example of  strike-slip movement. One-half of California is moving north, and the other south, as two plates slide past each other. Western California will one day be off the coast of Alaska, which is fine with me. (Note: There is a fortunate item called "afterslip," movement along a tectonic fault that causes little or no perceived surface quake, but dissipates energy.)

This relatively new theory of "plate tectonics" is a beautiful one, because it explains so much at once, which is what good science is supposed to do. The theory takes sea-floor spreading, continental drift, mountain building, earthquakes and volcanic activity and ties them together. Indeed, the theory explains why the continents exist at all. Without plate tectonics creating rock piles, most of our planet would erode below sea level in a few tens of millions of years.

Though mid-oceanic ridges were discovered through primitive string soundings by the H.M.S. Challenger in the late 1800s, refining a theory of plate tectonics depended on figuring out what kind of powerhouse energy source could possibly drive continents around the globe. That problem was solved with the development of underwater mapping techniques in the 1960s and the actual observation of planetary convection at work, basaltic magma flowing up onto the seabed from below. Remember that two-thirds of the surface of the earth is sea floor, made up entirely of sediment-covered basalt. Water conceals from our direct view such wonders as the great mid-ocean ridges, the combined lengths of which are some forty-thousand miles long. In places the ridge is 600 miles wide and two miles high, an uninterrupted, mammoth line of magma venting up to the seafloor for hundreds of millions of years. To get an idea of that, go out and look at Mt. Vesuvius; imagine it twice as high, then twice as wide as Italy —and stretching almost twice around the world!

Superswells: A new wrinkle in the surface of the earth

As if plate tectonics and shifting continents weren’t enough, current research is probing what are termed “superswells” in order to explain some of the planet’s most massive surface features. Southern Africa for example, has  an expansive plateau, more than 1,000 miles across and almost a mile high. Geologic evidence shows that southern Africa and the surrounding ocean floor have been rising slowly for the past 100 million years, even though that part of Africa has not experienced  a tectonic collision for nearly 400 million years. Such vertical movement of continents requires other explanations than standard plate tectonics. Research is focusing on, among other things, the existence of “superplumes,” massive blobs of molten material that rise faster than surrounding heavier material, moving upwards with enough power to lift the landmasses, themselves.  (Further information on this particular topic can be found in “Sculpting the Earth from Inside Out” by Michael Gurnis in Scientific American, March 2001.)

Italy in Particular

Much earthquake and volcanic activity on the surface of the earth is found along the lines of tectonic fractures. Italy is at the meeting point of a few of these tectonic plates. The movement below has formed the Italian peninsula and is the source of Italy’s great natural beauty and much of its considerable history of natural catastrophe.

In broad terms, the entire Mediterranean Sea was brought into existence about ten million years ago by the coming together of the African tectonic plate and the plate that makes up the greater Eurasian landmass to the north. In the case of Italy, the prominent mountain range, the Apennines, the "backbone" of Italy, resulted from the collision of the smaller Apulian plate with the Iberian plate. The mountains formed, and islands such as Sardinia and Sicily surfaced. The entire fault line that runs the length of Italy was then primed to produce volcanoes and geothermal activity on the surface along the line of the tectonic fracture, an obviously weak chink in the earth's armor and a natural place for internal energy and heat from the inner earth to vent to the surface. Today’s earthquake and volcanic woes along the west coast of Italy, on the Aeolian Islands, and on Sicily are a direct result of on-going subterranean activity as the great African Plate, which contains the Mediterranean and Italy, subducts below Europe via movement somewhat slower than the growth of a fingernail. The Alps mark the collision of Africa and Europe. Putting additional crunch on Italy is the plate that bears the Balkans; it is subducting beneath the eastern side of the Italian peninsula, yea, even as we speak.



A volcano is a conical mountain built up around a vent in the crust of the earth. Imagine the structure of a large tree below the surface; it is rooted below the crust of the earth in a magma chamber and vents through the main trunk up to the surface. This magma chamber is the bulge of molten material from below the lithosphere that has worked its way up into the actual crust, forming a deposit close to the surface, from whence it will ultimately vent. The trunk vents and forms the main crater; also, there are side branches coming off to form what are called "parasite cones." (Other types of volcanoes, such as "fissure" volcanoes, don't fit that conical configuration, but the principle of venting from a magma chamber through the lithosphere to the surface is the same).

Here, a word about "lava." Though it is a synonym for “magma,” it generally  refers to the flow of magma from a volcano and not to the stored magma in the chamber beneath a volcano. Apparently, Neapolitans were the first to use the word “lava” in its volcanic sense. The magma issuing forth from Vesuvius offered the analogy to a flow of water (lavare means "to wash").  Again, magma is the molten mineral material stored below the lithosphere in a stratum called the asthenosphere, a few hundred miles thick, material that is then ejected by an erupting volcano. The viscosity of magma and the speed and surface appearance of a lava flow are determined by the silica and water content. A high silica content makes the lava very thick so that it flows very slowly or just piles up above the vent to form a “dome”. Geologists take great care to measure the minute changes in a volcanic dome in order to determine the swelling that is often a harbinger of impending eruption. Less viscous lava flows more rapidly, such as Ol Doinyo in Tanzania, which erupts like muddy water. Thus, lava types vary greatly from place to place on earth and, indeed, throughout our solar system. The dominant type of lava flow underwater is called “pillow” lava and is characterized by long tube-like structures that in cross-section look like elliptical pillows. It is this kind of lava that forms the great mid-ocean ridges.

Types of Volcanoes

Volcanoes can be classified in various ways: e.g., as extinct, dormant, or active; or according to the viscosity of magma that they eject. One convenient classification is based on the way they erupt: explosive, effusive, or intermediate.

Explosive volcanoes are those that give us great catastrophes: the eruption on the island of Santorini, which destroyed the ancient Minoan civilization on nearby Crete in the second millennium before Christ; the famous eruption of Vesuvius, which destroyed Pompeii and Herculaneum in the first century, a.d; that of Krakatoa in the 1880s;  the recent eruptions of Mt. St. Helens in the 1980s and Pinatubo in the Philippines in 1991; and the recent eruption on the island of Montserrat. These eruptions are sudden, throwing up massive amounts of magma and solid material quite rapidly. The volcanoes do, literally, explode, often tearing off the top of the volcano, itself.

Explosive volcanoes typically have steep cones due to the rapid settling of the great amount of heavy material onto the surface. In the cases of very large explosive volcanoes, you may even wind up with a "caldera": i.e., the explosion is so massive and forceful that it collapses the cone of the volcano down into the magma chamber deep below, producing a gigantic rim on the surface. If such an explosion occurs on an island, such as Thera (Santorini), the sea may rush in and fill the crater, producing a characteristically shaped "rim" island above the surface. Encrusted with coral and submerged (by plate tectonic movement downhill, or just slow sea level rise), these rims form the atolls of the world's warm oceans. (Fresh rainwater produced a similar body of water in Crater Lake, Oregon.)

[Also see this entry on the Archiflegrean caldera.]

Effusive volcanoes, such as those on the island of Hawaii, give off a constant slow flow of magma. Over the course of many eruptions, the lava flows build up and, little by little, produce a large, gradual slope. Mt. Etna on Sicily is another volcano of this kind.

Some volcanoes are termed "intermediate" because in the course of their history they have been both explosive and effusive. Vesuvius is now classified as an intermediate volcano because the most recent eruptions—going back to 1631—have been effusive. Before that, Vesuvius had a more violent history. There were, for example, explosive eruptions in much of the 1600s, a century of such considerable seismic activity that a mountain, Monte Nuovo (New Mountain) surfaced in nearby Pozzuoli. 


Atmospheric Effects

Large volcanic eruptions have significant effects on the atmosphere and on global climate. It is true that eruptions produce major quantities of carbon dioxide (CO2), a gas that contributes to the greenhouse effect, but human activities generate more CO2 than do volcanic eruptions—about 10,000 times as much! By far the greatest climatic effect from volcanoes comes from the production of atmospheric haze. 

Large eruptions  inject ash particles and sulfur-rich gases into the atmosphere; these clouds can circle the globe within weeks—even days—of an eruption. The ash particles decrease the amount of sunlight reaching the surface of the earth and lower average global temperatures. The Krakatau eruption on August 26 and 27, 1883 put about 20 cubic kilometers of material in an eruption column almost 40 kilometers high, and by the next day the haze had reached South Africa; two days later it had circled the globe. 

Global temperatures are affected, not so much by the volume of ash in the atmosphere as by the chemical composition of  the gases thrown up by the eruption. A smaller eruption in terms of explosiveness and volume of ash produced can have a longer-lasting effect if it injects more sulfur into the air. These so-called "sulfate aerosols" can take several years to settle out of the atmosphere, thus producing greater global cooling. A sulfur-rich eruption such as El Chichòn in 1982 can lower the temperature by half a degree centigrade in the entire hemisphere for a number of years.

The Indonesian volcano Tambora erupted in 1815 and gave North America and Europe a "year without a summer" with snowfalls as late as August and massive crop failures, all of which inspired Lord Byron to write:

"The bright Sun was extinguish’d, and the stars
Did wander darkling in the eternal space
Rayless and pathless, and the icy earth
Swung blind and blackening in the moonless air;
Morn came and went—and came,
And brought no day…"

And the great explosion of Krakatoa in Java in 1883 produced atmospheric effects on a global scale and even more poetry — this time by Tennyson in a poem entitled "St. Telemachus", from 1892: 

"Had the fierce ashes of some fiery peak 
Been hurl'd so high they ranged about the globe? 
For day by day, thro' many a blood-red eve, 
In that four-hundredth summer after Christ, 
The wrathful sunset glared against a cross…"



Mt. Vesuvius, 1944 eruption. Photo
courtesy of Herman Chanowitz. Photo
restoration by
Tana A. Churan-Davis.

Vesuvius 1944 eruptionThe most famous volcano in the world is known to geologists as the Somma-Vesuvius volcanic complex. It is a composite, really made up of an older volcano, Monte Somma, the activity of which ended with a summit caldera collapse, and of a more recent cone, Vesuvius, contained within the caldera.

In a generally accepted chronology of the volcanic history of this area, eight main eruptive cycles of Vesuvius within the last 17,000 years are recognized. Each cycle started with a highly explosive eruption that occurred after a long quiescent period measured in centuries. The a.d. 79 "Pompeii" eruption opened the last cycle, which went up to 1944, the year of the last eruption of Vesuvius. The period between 1631 (the year of the last explosive-type eruption) and 1944 is characterized by relatively mild activity (lava fountains, gases, and vapor emissions from the crater), frequently interrupted by short quiet periods never longer than seven years.

There have been so many documentaries about the eruption that doomed Pompeii and Herculaneum that what follows may not be new information to you. Simply note, however, that the victims were not overrun by lava. Although people have been killed by very liquid lava flowing very fast, most of the time that is not what kills victims of volcanic eruptions. Indeed, Pompeiians were killed by what is now called the "surge and flow" of an exploding volcano.

The eruption of Mt. Vesuvius in 79 a.d. was highly explosive. This type of eruption is caused (1) by the enormous pressure from the magma chamber, and (2) by the relatively high water content of the magma (3-5% by mass), itself. The high pressure and the rapid decompression of the water cause the erupting mixture to burst from the vent at 300-700 feet (100-230 meters) per second, a true explosion compared to effusive volcanoes, which vent at speeds as slow as one foot per second and not more than 150 feet per second. As the dense mixture of ash, pumice (bits of cooled lava) and gas rise from a Vesuvius-type eruption, the surrounding air will heat and expand. Thus, the rising column of vented material becomes so much heavier than the atmosphere that it will eventually collapse

When Pompeii "blew,” it did so for about eleven hours, first exploding its top into oblivion and then venting a 12-mile high column of noxious gas and pumice into the stratosphere. The column hung in the air and then collapsed back down onto the slopes and "surged,” producing a very fast (in excess of 100 mph) avalanche of superheated gases, pumice, and rock rushing down the slopes.

Behind that surge came the somewhat more slowly moving "pyroclastic flow," a ground-hugging mass of more solid molten material and gasses. Vesuvius surged and flowed at least four times within a few hours after the eruption. The inhabitants of Pompeii and Herculaneum suffocated, many of them because they figured they were safe. It is almost common sense that if the eruption doesn't get you —if a hot boulder doesn't land on you— then you're safe, right? Wrong. You cannot outrun a surge. Bear in mind that this description of the lethal surge and flow after a main eruption is a recently arrived at (19th century) description of the dynamics of volcano behavior, a scientific topic the ancient Romans knew little about.

Long before the ideas of wandering continents and sea-floor spreading became accepted, the connection between volcanic activity and earthquakes was recognized. Volcanic eruptions are often preceded and accompanied by earthquake activity. The eruption of Vesuvius that destroyed Pompeii was preceded by years of earthquakes. One of them apparently started a fire in nearby Pozzuoli and destroyed portions of the city, a fact that has only recently come to light though excavation of the old city. (More on Pozzuoli, below.)

Interestingly and very recently (late 2001), archaeology around Vesuvius near the town of Nola has shed light on the fate of a so-called “Bronze Age Pompei.” In about 1800 b.c. (roughly about the same time as Hammurabi was formulating his exemplary Code in far-off Babylon) a little village on the slopes of the volcano was buried by an eruption. The site is already recognized as one of the world's best-preserved prehistoric villages, found only because someone decided to build a supermarket with an underground parking structure. Thus far, no human remains have been uncovered, indicating that the inhabitants had enough time to avoid the fate of the some 2,000 victims of the Pompeii eruption.

Again, when tectonic plates move, various things can happen on the surface. Mountains can form as subduction occurs; also, when plates rub together, or strike and slip, energy is released upward along fault lines. This produces earthquakes and volcanic eruptions.  Most volcanoes erupt along or near tectonic lines. The rest, far from tectonic zones, result from still mysterious and relatively fixed "hot spots" (such as Hawaii) which scar the moving plate with mountains while hiding their deep-seated roots (much deeper than 100 km, possibly even a core-mantle boundary source). To add to the complexity, deep hotspots can erupt through tectonic zones, themselves (Iceland, for example, is the result of such activity).

(Also see "Recent eruptions of Mt. Vesuvius") & about a dozen entries on Vesuvius under -V- in the index.

(click here to link to a separate item on the the town of Pozzuoli)

(and here for a separate item on the port of Pozzuoli)

Moving around the bay to the west, we come to Pozzuoli and the aptly named Campi Flegrei —Phlegrean ("Fiery") Fields. That part of the Bay of Naples is pock-marked with extinct volcano craters from millions of years ago and some craters from as little as 35,000 years ago (the great Pozzuoli caldera). It is also the site of venting fumaroles, hot springs and thermal baths, and one big bubbling sulfur pit called Solfatara, all signs of geothermal activity. Heat from below the surface—either by conduction through the rock or by direct infusion of magma—heats the groundwater contained in reservoirs below the surface.  Depending on the make-up of the surface rock—how permeable it is, for example—surface activity will manifest itself as bubbly hot springs, or venting steam, or even slight seismic shifts as the ground rides up and down on the geothermal activity below it.

These slight shifts are called "bradyseisms," meaning "slow shaking." Though we say "slight," these small tremors are enough to raise or drop the ground surface by as much as a few meters in a single decade and cause considerable shift in sea level. The port facility of Pozzuoli has had to be rebuilt in the last few years to adjust to the perceived drop in local sea level —the ground rose, actually— caused by bradyseismic activity in the early 1970s. The famous tourist site, the Roman market in Pozzuoli, which was partially under water in 1970, is now totally on dry land. The bradyseisms in the area came at about the same time as the big Naples (Irpinia) quake of 1980, so they may be geologically connected. For example, the same energy release that triggers a quake along a fault might also vent heat energy into adjacent groundwater reservoirs, raising the temperature, increasing the bubbling, and causing the ground on top to jiggle.

Add: July 2015 - More recent reseach indicates that the special composition of the minerals in the ground beneath Pozzuoli may have had something to do with the sudden flurry of 'micro-quakes' in Pozzuoli in the 1980s.        

[Also see entries under "earthquake(s)" in the main index, here.]

The "Ring of Fire"

The extinct craters in Pozzuoli are part of an enormous chain, now undersea, which runs out to the south on the seabed in the direction of Sicily. The entire sea between the Bay of Naples and Sicily thus contains its own "Ring of Fire," so-called in analogy to the mammoth ring of active volcanoes that perch on the perimeter of the great Pacific tectonic plate.  Mt. Vesuvius has four undersea cousins to the south: Palinuro, Vavilev, Marsili, and Magnaghi. The last three were discovered in the 1950s; Palinuro was known earlier. At present, there is some concern about the state of "dormancy" of Marsili. It is 3,000 meters high with the cone reaching to 500 meters from the surface of the water. Satellite cones of recent origin have been detected on Marsili.

[2014 update on Marsili here.]     

The entire "Ring of Fire" includes, then, Vesuvius; the extinct volcanoes of the Pozzuoli area (mentioned above); the volcano Epomeo on the island of Ischia; the four above-mentioned undersea volcanoes to the south; the active island volcanoes of Ustica, Stromboli, and Vulcano off the north coast of Sicily; the largest active volcano in Europe, Mt. Etna, on Sicily; and, finally, the volcanic island of Pantelleria, to the south of Sicily. It last erupted in 1891.

[Also see these 3 short items on: Stromboli (1)  (2)  (3)

That area of the "Ring of Fire" has at least one comic-opera-type episode connected with it. During the night of June 27, 1831, a small island surfaced off the coast of Sciacca, near Agrigento in southern Sicily. English, French and Neapolitan vessels raced to the scene to claim the island. The Neapolitans won and hoisted the flag of the Kingdom of the Two Sicilies, naming the new acquisition "Ferdinandea" for their King Ferdinand. Unfortunately for the bureaucrats and would-be colonizers, the island disappeared a few months later. Fortunately for 21st-century scuba divers, however, the island didn't sink that far, and now a good-sized underwater nature reserve thrives about 30 feet below the surface. The "Ferdinandea" episode made the papers in the summer of 2002 due to recent rumblings and small "seismic events" in the area. Active fumaroles are venting from the slopes of the sunken island. Is the island about to resurface? Probably not, say local geologists — but these are the same people who call earth- and seaquakes "seismic events." Time will tell.

One still significant feature of the Pozzuoli area is the famous Lake Averno, where Virgil accompanies Dante into Hell in the Divina Commedia. Mythology held the lake to be the descent into the underworld. It stank then of sulfur, as it does today, though the real stench of rotten eggs is a mile or so away at the nearby Solfatara pit. It is a bubbling brew of sulfur, and the venting fumaroles in the area are believed to possess medicinal properties. Pozzuoli and the offshore island of Ischia are also the sites of numerous thermal spas.

To take a long view, much of this volcanic activity certainly took place when the Mediterranean dried up about seven million years ago. Continental drift had actually created a large enclosed sea, which then evaporated, leaving thick deposits of salt between Italy and Spain. Oceanologists have now seen how the rivers of France carved deeply into such deposits at the bottom of the great, empty pre-Mediterranean bathtub, proving that it was indeed empty. With only rivers flowing in, this inland sea (like the Black Sea which evaporated at the same time) did, indeed, disappear.

When the land bridge of Gibraltar gave way, it is estimated that a volume of water equal to three or four hundred Niagaras started flowing into the basin, refilling the Mediterranean in about a century. It must have been quite a show for our hominid ancestors grunting around for roots and berries on the slopes of the Atlas Mountains in North Africa —("Holy  Archaeopteryx! Look at all that wet stuff! I'd better evolve some brains, learn how to use tools and start building a raft!")

Since the time of the Romans, volcanic and geothermal disturbances in the area, as well as the general worldwide rise in sea level have changed the Bay of Naples. Remnants of Roman villas have been found offshore, and the docks of the Roman port facility, Portus Iulius (near Pozzuoli), are now underwater. At the height of the Empire, it was the main port for the Western Roman "Praetorian" fleet. This facility connected the sea, by manmade canal, with the two nearby lakes, Averno and Lucrino, as well as with the small Bay of Miseno at the western end of the gulf.


Monte Nuovo

The recent (1538) volcanic appearance of Monte Nuovo destroyed much of Lake Lucrino.  Some slipshod, recent maps of Roman Pozzuoli show the anachronism of Monte Nuovo, making it impossible today to realize that at the time of the Romans no such mountain existed. Instead, there was a much larger lake, now little more than a pleasant puddle, which was a training lake for Roman ships. Also, some Roman roads in the area can no longer be traced along their entire length. The via Domiziana, for example, a main artery from Rome to Naples, tunneled beneath the Posillipo hill on the west end of the Bay of Naples; then, it came out and ran along the sea again. That seaside stretch of road is now covered by water.

(See the main article on Monte Nuovo.) 

The Islands in the Bay

The island of Ischia, the first Greek settlement in the bay, is dominated by Mt. Epomeo, once believed to be a deeply eroded central volcanic crater. In 1930, however, the Swiss vulcanologist, Alfred Rittmann, established that the greenish tufa rocks of Epomeo are not the remains of a crater, but the products of a powerful eruption that were thrust up and broken into blocks (called "uplifted horst"). Other volcanic eruptions in the 1300s on Ischia, however, destroyed villages and forced inhabitants to flee to the mainland. Volcanic activity on Ischia is what presumably drove the original Greek settlers away in the first place, forcing them to move to the mainland, where they founded Cuma half a millennium before Christ.  Also volcanic in origin, the nearby, low-lying island of Procida has its own small satellite isle of Vivara, obviously part of the rim of a crater. (It was also the site of a Mycenean Greek trading post, founded centuries before the "original" Greeks got to the area!) (Also see Pithecusa)

Capri is not volcanic, but was formed by the same general tectonic plate upthrust that formed the Sorrentine peninsula. When times and sea levels were much different, Capri was an extension of the Sorrentine peninsula. The presumed land bridge may account for the remains of mammoth elephants on Capri. (The alternative is swimming elephants. Pay your money and take your choice.) Finally, have a look at the cliff face as you drive along the peninsula. Note how the strata of the rock face angle up out of the sea and make a mountain. That's all part of a tectonic plate.

[Also see The Great Ischia Earthquake of 1883 and Other Sever Earthquakes in the South]

On Predicting Eruptions and Earthquakes

It's simple. There are about 500 active volcanoes on the land surface of the Earth; also, the tectonic plates that fragment that surface are in constant movement. Thus, your crystal ball can be made of low-grade zirconium oxide and you will still be reasonably accurate if you "foresee" a major eruption or earthquake "sometime this year". What most people mean by prediction, however, is something different. They want: (1) "This particular volcano is about to erupt," and (2) "a 7.2 on the Richter scale is imminent at this particular spot."

First, the good news. The outlook for predicting volcanic eruptions is not all that bleak. The United States Geological Survey uses ground-based sensors and high-orbit satellites in its attempts to keep up with volcanic behavior. The sensors pick up underground noises and, thus, know that something is moving. The frequency of the rumblings tells what those materials are. The sensors are tuned to a Global Positioning Satellite (GPS), pinpointing the position of the sensors and allowing a computer to tell precisely where those materials are. This system provides a good picture of the anatomy of a volcano. As a volcano swells with magma, the deformation—as calculated by the GPS—can help determine whether or not an eruption is about to take place.

"Sniffing" volcanic gas is another way to keep tabs on high-risk volcanoes. As magma rises in a volcano, light molecules such as carbon dioxide bleed off more than do heavier gases such as sulfur dioxide. The higher the CO2 levels, the likelier an eruption. Currently being tested is a remote gas sensor that detects changes in the infrared energy caused by different gases in the volcanic plume. Being able to gather this information from, say, 20 miles away is much safer than having to climb down inside a crater. So, there is considerable new technology dedicated to the study of at least some well-known volcanoes near major centers of population. This makes it increasingly unlikely that unsuspecting people near these monitored volcanoes will be caught napping by a major unpredicted eruption. A good example of successful forecasting occurred in 1991. Volcanologists from the U.S. Geological Survey accurately predicted the eruption of the Pinatubo volcano in the Philippines, allowing for the timely evacuation of Clark Air Base and saving thousands of lives. Similarly, new satellite maps of the precise topography around volcanoes are helping in the prediction of “lahars” (a Javanese term), those disastrous monsoon-soaked slurries of mud and rock that can surge downhill weeks or months after a volcanic eruption, often causing more damage at lower elevations than the original eruption.

The bad news is that earthquake prediction is staggeringly more difficult—and that is an understatement. There is, of course, the Star Trek scenario: ("Captain, our long-range sensors indicate that this planet will undergo a major earthquake within the next 4-6 hours with the epicenter at 52 degrees north and 12 degrees east"). If that is the kind of predictive abilities you expect, you have a long, long wait—if ever.

Predicting earthquakes requires, first, a good knowledge of the geologic history of an area. Then—much more difficult—it takes the ability to isolate what potential "predictors" might precede an earthquake. For example, certain things do happen to rock under stress. Among other things, permeability to water changes, as does electrical conductivity. Also, certain gases might escape before an earthquake, and there might be a slight crustal uplift beforehand. Is it possible to measure any or all of these predictors and come up with some kind of a yardstick—a profile—that might tell us reliably when and where an earthquake is about to happen?

The history of science has certainly shown us that those who say that something "will never happen” often turn out to be wrong. In practice, however, such powers of earthquake prediction would require a great number of sensors of extreme sophistication and a knowledge of how predictors correspond to the behavior of rock as it is subjected to the force of tectonic movement. Such powers of prediction would require the ability to model geological phenomena across many orders of magnitude in size from meters to thousands of kilometers and in time from seconds to the speed at which mountains move—eons. In other words, you need a multidisciplinary approach using computational mathematics, computer programming and geology on, yes, an imaginable scale—but only if you have a very good imagination. So, while "never" is a long time, the gap between theory and practice is vast.

Is it possible, as some have claimed, that certain animals sense an impending earthquake? Well, dogs do hear and smell things we don't, so maybe we shouldn't discount the possibility that they are tipped off a few minutes before an earthquake. On the other hand, until we can figure out a way to ask Fido exactly why he is chasing his tail over there, it is best not to rely too heavily on animal behavior as portents of seismic events. Thus, at present, no reputable geologist will take a greater predictive leap than, perhaps, to say: "Given what we know about the history of this area, there is a 75% chance of a major quake within 10-15 years somewhere in this area." Right now, maybe that’s about the best you can expect.

[revised following paragraph, Oct 2013]

On the other hand, there are recent technological advances that make the Star Trek scenario ever less fantastic. There are now a few attempts at earthquake early warning systems in the world. Consider that you don't really need that much advance warning in order to be able to save some lives: even a minute would in many cases provide time to get out of buildings onto open ground, get people out of elevators, shut down some utilities, slow trains, give surgeons a moment to withdraw scalpels,etc. Places such as Mexico, Japan and California already have in place sensors (not nearly enough, however) along fault lines that will give you a bit of warning if you are some distance away from the epicenter. The warning system works when sensors in the ground detect the first signs of earth movement, known as P waves, which travel at the speed of sound. The more damaging shaking, called the S waves, lag behind at a slower speed. The P waves will thus give you one minute warning (sent to home computers and personal cellphones, electronically, (at the speed of light) if you are 12 miles (20 km) from the epicenter. The greater the distance from the epicenter, the more time centers of population would have to prepare. Obviously, if you are right at the epicenter, you will have no warning. Also, a group of NASA and university scientists at JPL released a study in April 2003 on the feasibility of forecasting earthquakes from space. Their report outlines a 20-year plan to deploy a network of satellites—the Global Earthquake Satellite System (GESS). The system would use Interferometric-Synthetic Aperture Radar (InSAR) to monitor fault zones around the world. InSAR combines two radar images of a given tectonic area in a process called "data fusion" in order to detect changes in ground motion at the surface. This technique is sensitive enough to detect slow ground motions as tiny as 1 mm per year, letting scientists see the tiny motions and contortions of land around a fault line in detail, figure out where points of high strain are building up, and infer when stresses in the Earth's crust have reached a dangerous level. 

Other potential uses of satellite technology involve looking for surges in infrared (IR) radiation. Such surges indicate thermal anomalies, changes in ground temperature, that have been detected before earthquakes. Also, there appear to be fluctuations in the earth's magnetic field in the area of earthquakes that are about to happen. Both of these phenomena are potentially detectable using satellite-based sensors of sufficient sophistication. Of the three—tectonic motion, IR surges, and magnetic fluctuation—the first seems to be the most reliable, at least so far.

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