entry Oct. 2003 updates through 2023
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
Volcanoes
Vesuvius
Pozzuoli
The "Ring of
Fire"
Monte Nuovo
The
Islands in the Bay
On Predicting
Eruptions and Earthquakes
Iceland's
Fagradalsfjall eruptions change what we know about
volcanoes
Introduction
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.
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 helps to visualize this layer as
relatively thinner compared to the earth than the skin of
an orange is to the fruit. 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.
USGS stands for 'United States Geological
Survey'
The 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 are
only the size of a big county and may be 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 —i.e. 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° K;*
(2) Frictional heating, from 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° K*;
(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.
*K=degrees Kelvin. The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion. The Kelvin scaleis the base unit of temperature in the International System of Units (SI). Absolute zero Kelvin corresponds to -273 C and -460 F.Absolute zero Kelvin corresponds to -273 C and -460 F
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 (although we know that Mars
has two moons, at least one giant volcano. ("Planetary
differentiation" is what happens when you vote to join a
solar system. If you don't like it, go back were you came
from.)
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.
HMS Challenger
The mid-oceanic ridges were
discovered through primitive string soundings by the HMS
Challenger in 1858s, 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.)
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.
Volcanoes
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,” lava
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.)
[See this
entry on the Archiflegrean caldera.]
Mt. Etna
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…
Vesuvius
Mt. Vesuvius, 1944 eruption. Photo
courtesy of Herman Chanowitz. Photo
restoration by Tana A. Churan-Davis.
courtesy of Herman Chanowitz. Photo
restoration by Tana A. Churan-Davis.
The
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 was significant, occurring on 5 February AD 62. It had an estimated magnitude of between 5 and 6 and was IX or X on the Mercalli intensity scale (which indicates perceived damage to the environment). The towns of Pompeii and Herculaneum were severely damaged. Contemporary philosopher and dramatist Seneca the Younger wrote an account of the earthquake in the sixth book of his Naturales quaestiones, entitled "De Terrae Motu" (Concerning Earthquakes). The damage caused by the main shock and the subsequent series of tremors was at least partly repaired in both Pompeii and Herculaneum by the time of the AD 79 eruption, but one of the tremors 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.
Pozzuoli
(click here to
link to a separate item on the the town 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 Camp Flegrei in Naples
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.
The area I live in is
volcanically a bit “iffy.” From my balcony (where
the word "Chiaia" is in this image, lower right) I
see Mt. Vesuvius to the east. It (Vesuvius, not my
balcony), has been quiet these last decades. (But that’s
only the tip of the volcano. Vesuvius is a child (less
than 20,000 years old) compared to the roaring
land-forming engines that produced almost everything
else in this image 40,000 years ago: the Fuorigrotta
Plain and everything to the west of the Posillipo hill
until you get to Capo Miseno, Monte di Procida, and Cuma
at the western end of the Gulf of Naples. There are
remnants of the cataclysmic caldera collapse of the
so-called Archiflegrean volcano or Caldera (also known
as the Campanian Ignimbrite Eruption); it was a
super-volcano that tore the roof off itself, settling
back to sea-level and below. Bits of the ancient volcano
rim of Big Archie (my term of endearment) are very
visible on the surface. As you go through the area, you
go through Agnano, the Astroni, and other places, all
parts of the Campi
Flegrei, or Flegrean Fields. Flegrean means
"fiery."
They are remnant and extinct (probably) volcanoes from
secondary eruptions from the so-called Second Flegrean
Period
(c. 20,000 ago). That second, smaller area is named the
Flegrean Volcano (bounded by the black lines with
wedges,
centered on the town of Pozzuoli). One area, the
Solfatara, is still wheezing if not active, but it could
erupt, they say. The Flegrean Volcano produced the
Posillipo hill, the slopes of which attracted the
Greeks, then Romans and now a bunch of other optimists
who never studied geology.
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.
Stromboli (one of the islands north of Sicily
- left and right)
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.
When the land bridge of Gibraltar gave way about 4 million years ago, 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 man-made 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.)
Volcanism in in the Bay
the Solfara - part of the Campi
Flegrei
(left & right)
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. (But, then, elephants can swim. 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 Severe 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) "Captain, this volcano is about to erupt," and (2) "a quake of magnitude 7.2 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 new technology dedicated to the study of 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. Scientists from the U.S. Geological Survey accurately predicted the eruption of the Pinatubo volcano in the Philippines, allowing for the 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, again, Star Trek: ("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"). Predicting earthquakes requires, first, a good knowledge of the geologic history of an area. Then —more difficult— you have 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. Can we measure any or all of these predictors and come up with a yardstick, a profile, that tells 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
require a great number of sensors of extreme
sophistication and a knowledge of how predictors
correspond to the behavior of rock stressed by tectonic
movement. Such prediction mean modeling geological
phenomena across many orders of magnitude, 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
claim, 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 how 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 geologist will take a greater
predictive leap than: "Given what we know about the
history of this area, there is a 75% chance of a major
quake within 10-15 years 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 advances that make the Star Trek scenario less fantastic. There are now early warning systems in the world. You don't need much advance warning in order to save some lives: even a minute is enough to get out of a building, 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 now 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" 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, 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. These things 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.
[added Jul 2019]
Attempts to refine earthquake
prediction continue. Recent (June 2019) efforts show that the rupture process and
ground displacement in small to moderate earthquakes may
be indistinguishable, implying an identical early rupture
process for all quakes. But large quakes (more than
magnitude 7) may be distinguishable by the kinds of waves
they generate as they pass through the moderate stage and
then into the powerful stage. Importantly, powerful quakes
are not instantaneous. They can last minutes, which makes
them less like a single underground blast and more like a
series of explosions moving outward. If the outward
journey of these explosions differs depending on the power
of the quake, you should be able to determine the
final magnitude in as little as 10 to 15 seconds
after it begins, and well before it ends. Since a
single-digit leap in earthquake magnitude means that 32
times more energy is being released, you should be able to
warn people if they are in the path of "just" a
potentially damaging quake or a catastrophic one and tell
them how much time they have to stop whatever they're
doing and move to a safer location.
and this added 23 July 2023:
----------------------------
A pair of seismologists at Côte
d'Azur University in Nice, France, have found what might
turn out to be an accurate way to predict earthquakes.
In their study, reported in the journal Science, Quentin
Bletery and Jean-Mathieu Nocquet looked at high-rate GPS
time series data that was gathered in the time leading
up to the moment earthquakes of magnitude 7 or above
occurred. Roland Bürgmann with the University of
California, Berkeley, has published a Perspectives piece
in the same journal issue outlining the work done by the
team on this new effort.
Seismologists have long sought to predict
earthquakes so that people could react. In many cases,
several minutes
warning would be helpful—it would allow people to exit
buildings that might collapse. Finding a precursor is
difficult due to the lack of information regarding what
was happening in the vicinity of an epicenter before a
quake. In this new effort, Bletery and Nocquet have
found a way to go back in time to learn more about land
shifting before a big quake.
In looking for an earthquake precursor, the
researchers obtained and studied precise GPS data for
geographical areas
surrounding the epicenters of 90 quakes over magnitude 7
over the past several years. They found a pattern —a
slip between tectonic plates that caused the land above
them to move in a measurable, horizontal direction.
They also found that such slips could be observed
and measured using GPS, that they occurred up to two
hours before the earthquake struck and were too small to
show up on standard seismographs. Most important, they
saw the same slip in all the earthquakes they studied.
The work suggests that a reliable earthquake
system could be designed based on a precise GPS
listening system. On the
downside, Bürgmann notes that more work is required to
prove that such a precursor exists for all, or at least
most,
large earthquakes. Also, he adds, some upgrades to GPS
technology are required to allow for measuring
individual events
around the clock.
Describing
Earthquakes (added August 2023)
The
observatory —now, officially, the Vesuvius
Observatory, Naples Section of the National
Institute of Geophysics and Volcanology— is
visible on the western slopes of Vesuvius. It rests on
Colle del Salvatore,
a knoll, putting it out of the range of ejecta and in
a position where lava from an eruption is channeled
around the observatory and not through and over it. It
is the oldest such institution in Italy and is still
an active institution for important research in
geophysics and volcanology. The old observatory is
primarily a museum. The newer observatory monitors the
volcano it and also keeps tabs on other geological
happenings in the area, such as those at the nearby
Flegrean Fields and the island of Ischia.
(this section,below, was revised, August
2023)
In 1970 the
original building became a museum, exhibit hall, and
library. A new building, the white structure below it
met the needs of modern science. One directors was
the best-known Italian geologist, Giuseppe Mercali
(director from 1911-14). He devised the scale to
classify quakes by effect on the environment. There are two ways to describe a
quake: a magnitude scale or an intensity
scale.
The first tells you how much energy was released in
'magnitude" - say, 'Magnitude 4.8'. It is of
interest to scientists, who know the numbers. The
other scale is the one devised by Mercali for Italy, an
intensity scale. It is useful for the average person.
There is now (2023) a
European macroseismic scale (EMS) to evaluate seismic
intensity in European countries and is also used in a number of
countries outside Europe. Issued in 1998 the scale is
called EMS-98.
It runs as follows:
- I. Not felt Not felt by anyone.
- II. Scarcely felt Felt only by people at rest in houses, especially on upper floors of buildings.
- III. Weak The vibration is weak and is felt indoors by a few people. People at rest feel swaying or light trembling. Noticeable shaking of many objects.
- IV. Largely observed The earthquake is felt indoors by many people, outdoors by a few. A few people are awakened. The level of vibration is possibly frightening. Windows, doors and dishes rattle. Hanging objects swing. No damage to buildings.
- V. Strong The earthquake is felt indoors by most, outdoors by many. Many sleeping people awake. A few run outdoors. Entire sections of all buildings tremble. Most objects swing considerably. China and glasses clatter together. The vibration is strong. Top-heavy objects topple over. Doors and windows swing open or shut.
- VI. Slightly damaging Felt by everyone indoors and by many to most outdoors. Many people in buildings are frightened and run outdoors. Objects on walls fall. Slight damage to buildings; i.e, fine cracks in plaster and small pieces of plaster fall.
- VII. Damaging Most people are frightened and run outdoors. Furniture is shifted and many objects fall from shelves. Many buildings suffer slight to moderate damage. Cracks in walls; partial collapse of chimneys.
- VIII. Heavily damaging Furniture may be overturned. Many to most buildings suffer damage; chimneys fall; large cracks appear in walls and a few buildings may partially collapse. Can noticed by people driving cars.
- IX. Destructive Monuments and columns fall or are twisted. Many ordinary buildings partially collapse and a few collapse completely. Windows shatter.
- X. Very destructive Many buildings collapse. Cracks and landslides can be seen.
- XI. Devastating Most buildings collapse.
(this section was revised, August 2023)
- XII. Completely devastating.
- - - - - - - - - -
- - - - - - - - - - - - - - - - - below, added September 2022
On top of old Fagr... Fafrks... Smokey, Fiery,
and Icy
You may to wish review the
material on General
Geology near the top of this page.
Iceland is going in two directions at once
(image, left), spreading
apart of about 2.5 centimeters (1 in) per year.
Stand on the spread long enough with your beloved and
the earth will move for you both and yet you'll be
drifting farther apart. That's the way it goes. Iceland
sticks up from the Mid-Atlantic Ridge (image,
right, top center, next to large, red
Greenland). Someplace in Iceland erupts almost
constantly. That ridge (shaded light blue) fits within the
accepted theory of plate tectonics; that is, the Earth's
crust is made up of a number of large plates that have
been slowly moving for about 3.4 billion years. That model
builds on the idea of continental drift, an idea developed
during the first decades of the 20th century. Plate
tectonics was accepted after seafloor spreading was
validated in the 1960s. At 65,000 km
(40,000 mi) the Mid-Atlantic Ridge is the longest
mountain range on Earth. It is mostly all underwater, but
portions of it have enough elevation to extend above sea
level, for example in Iceland.
Iceland's Fagradalsfjall eruptions
change what we know about volcanoes
(the original study appeared in Nature
(2022). DOI: 10.1038/s41586-022-04981-x)
north
While sampling
magma from the Fagradalsfjall volcano in Iceland
researchers have uncovered a process far more dynamic than
anyone had assumed in the two centuries they have been
studying volcanoes.They were able to get close enough to
sample the lava continuously from the start, thanks to
winds that blew the
noxious gases away, and the lava's slow flow. They were
trying to find out (1)how deep in the mantle the magma
originated, (2)how far beneath the surface it was stored
before the eruption and (3) what was happening in the
reservoir both before and during the eruption. Such
questions are some of the biggest challenges for
volcanologists due to the unpredictability eruptions, and
the remoteness and inaccessibility of many active sites.
The assumption was that a magma chamber fills up slowly
over time, and the magma becomes well-mixed and then
drains over the course of the eruption. That is a
well-defined two-step process; as such, geologist do not
expect to see much change in the chemical composition of
the magma as it flows out of the earth. This is what you
see at Mount Kīlauea, in Hawaii. You have eruptions that
go on for years with only minor changes.
an earlier
eruption from Fagradsfjall, 9 May 2021,
seen from Rekjavik, 40 km (25
mi) to the NE
But in
Iceland there was more than a 1,000% higher rate of change
for key chemical indicators. In a month, the
Fagradalsfjall eruption showed more compositional
variability than the Kīlauea eruptions showed in decades.
The total range of chemical compositions that were sampled
at this eruption over the course of the first month span
the entire range that has erupted in southwest Iceland in
the last 10,000 years. Scientists say that is a result of
later batches of magma flowing into the chamber from
deeper in the mantle. For the first few weeks what erupted
was expected: "depleted" magma that had been accumulating
in the reservoir about 10 miles (16 km) below the surface.
But then the chamber was recharged by deeper, "enriched"
melts with a different composition that came from a
different part of the upwelling mantle plume beneath
Iceland. This new magma had a less modified chemical
composition, with a higher magnesium content and a higher
proportion of carbon dioxide gas, indicating that fewer
gases from this deeper
magma had escaped. Then the magma that dominated the flow
was the deeper, enriched type. These rapid, extreme
changes in magma composition at a plume-fed hotspot have
never before been seen in near real-time. Such changes may
not be rare, but opportunities to sample eruptions at such
an early stage are not common. For example, before the
2021 Fagradalsfjall eruption, the most recent eruptions on
Iceland's Reykjanes peninsula was eight centuries ago.
This new activity may signal the start of a new, possibly
centuries-long volcanic cycle in southwest Iceland.
Geologists don't often have a record of the first stages
of eruptions because these get buried by lava flows from
the later stages, but here researchers saw for the first
time a phenomenon they thought possible but had never seen
before. Now they have a new constraint: how to compare
volcanoes around the world. It is not yet clear how this
Icelandic model compares to other volcanoes, or what role
it plays in triggering an eruption. That, of course is
what everyone, geologist or not, wants to know.
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