The Gulf of Naples is quite different geologically from east to west. To the east, along the coast, a branch of the Apennines stretches to Capri which is not volcanic, but made of limestone and dolomite deposited by an ancient warm-water sea. In the west, the volcanoes of Lake Averno, Monte Nuovo, Astroni and the bradyseism (ground moving up and down) of Pozzuoli circle the bay arriving at Naples and Vesuvius.

To the northwest there are the volcanic islands of Ischia and Procida, and then there are the Campi Flegrei. From the Greek for “burning fields”, 24 volcanic craters both off and on shore bubble away just west of the city of Naples. Also known as the Phlegraean Fields, it is probably one of the most complex volcanic configurations in the world, due to repeated volcanic activity resulting in a bunch of calderas (from the Spanish, caldera and from the Latin caldaria, meaning cooking pot)volcanic craters that are in various states of collapse, nested one on top of the other.

Geologic cross section of the Campi Flegrei area showing the magma source and the various faults, fractures and veins through which the magma, gases, vapors and mud boil up to the surface.

The main caldera at the Campi Flegrei is 13 km across. It is thought to have been formed during an eruption which produced a deposit of volcanic rock, a tuff, consisting of pumice and rock fragments. It is gray and decomposes easily. If you’d like to impress your friends, the name of the common gray tuff that is seen in the area is the Campanian Ignimbrite.

Located 20 km west of Naples, this caldera lies half onshore and half offshore, centered on the town of Pozzuoli. The floor of the crater is pock marked with smaller calderas and cones from other later eruptions and these are clearly visible in space satellite images.

Since the eruptions during the last 50,000 years were Plinian and Strombolian in style and strength, they produced some 250,000 tons of material that was distributed over 30,000 sq. km around Naples. This area is also subject to hydromagmatic eruptions in which the magma interacts with water. The eruption of the adjacent Astroni Volcano produced a classic structure seen in Campi Flegrei, a tuff ring (a crater with a rim built of compacted volcanic deposits).

About 12,000 years ago a major eruption centered on Pozzuoli produced the Neapolitan yellow tuff. And though it may be hard to believe, this stone, that you see nearly everywhere, would become extremely important in the settlement and history of the city of Naples.

Today, there is considerable hydrothermal activity in the Campi Flegrei. In locations such as the Solfatara, mythical home of the Roman god of fire, Vulcan, the ground continues to emit steam and boiling, sulfurous mud.

The area is also subject to bradyseismic activity (slow upward and downward motion of the earth’s crust) which is readily seen in the Temple of Serapis in Pozzuoli and less evident but more fascinating, in the Roman villas which are now underwater at Baia.

What is so marvelous about bradyseism is that if, done right, things move up and down, many feet, without being damaged, just like a stage floor which rises and descends gradually without disrupting the scenery. Naples is rare in that not only does it have bradyseismic activity in many areas, but those areas are home to priceless antiquities which have not been been harmed, just moved around.

Next up –The First Destruction of Pompei 62 A.D.

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Ann Pizzorusso is a geologist and Italian Renaissance scholar. After many years of doing virtually everything in the world of geology (drilling for oil, hunting for gems, cleaning up pollution in soil and groundwater) she turned her geologic skills toward Leonardo da Vinci. See her work on Leonardo’s Geology. You can find Ann on Facebook at Leonardo da Vinci Virgin of the Rocks and on Twitter @VirginoftheRock.