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Central Italian Geology

It starts with Steno
(Slide 1)

Niels Steensen 1638 — 86 (Danish — born Copenhagen).

First fame as an anatomist

Dissected a giant shark in 1666. Noted similarity of shark teeth to "glossopetrae" — led him to theory of fossils (i.e., "dug up things") that didn’t grow inside of rocks.

Geological laws of stratification and of solid bodies within rocks.

His book: De Solido Ontra Solidum naruraliter Contentus Dissertationis Prodromus usually just called the Prodromus. ("Preliminary discourse to a dissertation on a solid body naturally contained within as solid")


1. Strata are naturally horizontal.

2.  Only outside causes can cause deviation (upward pressure, forlding, falls into voids, intrusions, etc.)

3.  Anything that forms inside of something else will take the shape of the void (and not vice-versa).

4.  Newer layers will take the shape of existing strata.

Without Steno, no science in geology.
Plate tectonics
Plates worldwide -- "Pangea" -- "Macro-geology"
(slide 2)

European seismicity
(Slide 3)

Earthquakes clustered along lines of plates and mini-plates: European and African plates
Italian, Apulean, and Iberian mini-plates
Western European and Italian plate tectonics
(Slide 4) (Slide 5) (Slide 6)
What happens when plates colide? Motivating force is convection Rifts and trenches (Slide 7)   Subduction (= leading under / channeling downward) and other terminology
(Slide 8)

Compression forces on the subducting asthenosphere cause it to melt (It is almost ready to melt — semi-plastic — even when not under compression.)

Hot stuff rises through lithospere faults.

Pooling at the bottom of continental crust — melts bottom of crust — forms crustal magma chamber above.

Eventually finds an exit route.

Magma chambers can get to be huge -- the one under the bay of Naples has recently been measured at 400 cubic kilometers.
(Slide 9 — Bay of Naples)

The local "supercaldera" = Campi Flegrei = Fiery Fields

[superplumes — research has basically said that there is no superplume under Italy. In the 10th unit, we’ll talk more about superplumes and supercalderas]

Types of volcano structures
(Slide 10) Vesuvius is a stratovolcano / somma volcano on the edge of the caldera Volcanic "products"
(Slide 11)
Tephra — what falls from the sky — ash, pumice clasts, stones, blocks, bombs

Pumice — exploded, expanded glass — can come down in tiny (ash) or large (block) pieces

Tufa — what comes down in flows — pretty much the same as pumice, but often is hot enough to be "fused" — if it’s a flow product it might be called ignimbrite

Lava — flowing or not, depends on temperature and density — none in our 79 AD eruption but it does occur in the historical Vesuvius record (the layers of pumice, lave, tufa, etc. are the strata in the stratovolcano.)

Lahars — water mixed with other volcanic products: mud flows, etc. — can be driven by explosive forces, collapse forces, etc.

"Lahars hit Herculaneum but not Pompeii."  -- false Contrary to what was first thought, lahars weren’t the killers in Herculaneum -- six or more very hot surges.
Most of central Italy has deep and old tufa deposits
(Slide 12) (Slide 13) (Slide 14) Central Italy has two huge "fields" — dividing line appears to be Rome

Northern field through which the Tiber flows south before turning west toward the coast.

Tiber’s turning point appears to be determied by the northern edge of the slightly harder southern tufas.

Tufa / Ignimbrite around the Bay of Naples
(Slide 15) Two major eruptions of the Campi Flegrei Caldera 37,000 years ago

25,000 years ago

Often together called the "Campanian Ignimbrite" (but sometimes divided into the "Campanian" and "Yellow" ignimbrites) Depth of ignimbrite from the eruption 1500 meters beep in the center of the caldera Bradyseism has lowered the southern part below water.

Sea has eroded the western edge.

450 meters in circling area and to east Note the quick drop-off around northern fringe

It’s due to a pre-existing caldera wall blocking flows northward.

Terain determined spread of ignimbrite — note above But there are "Campanian ignimbrites on the southern slopes of the Amalfi Peninsula.
(Slide 9 again)

The Peninsula is topped by the Lattari Mountains a pre-existing limestone ridge that is over 1000 meters high.

The pyroclastic flow crossed the bay and overtopped ridge.

(Slide 16)
  12,000 years ago — Vesuvius forms on the edge of the big caldera

Eruptions before AD 79: (All dates approximate) 6,000 BC, 3500 BC, 1750 BC, pre-800 BC, pree 600 BC (and many more after, including 1631, a sub-phreatic explosion that killed more than 4000.

Area Volcanoes

Vesuvius (1500 crater diameter 1,281 meters high)
(Slide 16)

Monte Nuovo (1000 meter diameter, 134 meters high)
(Slide 17)

Sept 29-Oct 6, 1538 Lake Avernus (1300 mters diameter)
(Slide 17)  
Connected to the sea by the Grotto (water tunnel) of Cocceius — more than a kilometer long.
(Slide 18)
  Built by Cocceius fr Agrippa about 35 BC

Built to make Lacus Avernus a naval base, but soon obsolete and Misenum took over.

(Same time as Naples-Pozzuoli (land) tunnel, also built by Cocceius for Agrippa.)

Grotto of Cocceius was severely damaged by allied bombing in WWII because it was being used by the Germans — closed to visitors since then (but I got in years ago with a US/Italian military scavenging expedition.)

Thoroughly excavated.

Two additional lakes — semi-craters that are eroded and then coastal debris washed up to cut them in half.

Solfatara (500 meter crater diameter, semi-collapsed walls)(Now also a generic term for places and fumaroles like the ones here)
(Slide 19) A local phreatic "hot spot" — about 200 degrees C.
(Slide 20)

Sulfuric-Arsenic gasses and crystals.

Boiling mud, 200 degree hot spring.

Occasional hot sulphorous eruptions.

Carefully controlled pathways — doomed if you step off.
(Slide 20)

Mostly German and Scandanavian campers.
(Slide 21)

(Slide 22) (Slide 23) (Slide 24) A long-continued, extremely slow vertical instability of the crust, as in the volcanic district west of Naples, Italy, where the Flegreian bradyseism has involved up-and-down movements between 6 m below sea level and 6 m above over a period of more than 2.000 years.

We’ll see more about this in the unit in which we cover the Baia Bay area.

Types of eruptions Phreatic
(Slide 25) Explosive -- Gasses and water disloved in the magma suddenly released.

Caused by landslides or plug-popping.

Produces ash, but mostly "blocks", which are chunks of what forermly was the top of the volcano, and "bombs", which are molten blobs of lava that are shot up into the air.

Blocks can be huge!
(Slide 26)

Plinian (named after Pliny the Younger who described the AD 79 Vesuvius eruption
(Slide 27) Often preceeded by one or more phreatic explosions — this was the case with the AD 79 eruption.

Produces a tall mushroom cloud — described by Pliny as looking like an Umbrella Pine — which is supported by the continuing ejection of highly gasseous magma through a constricted volcano throat.

Fallout in the form of ash, pumice clasts, and lapili.
(Slide 28)

In the AD 79 eruption the fallout over Pompeii was 1.4 to 2 meters of which 90% was pumice clasts, mostly under 4 centimeters in diameter. None on Herculaneum

Eventually the gasses that generate the force supporting the the column is expended and/or the throat of the volcano erodes and widens. The column then colapses and a new Peleean phase begins.

Peleean (named after Mt. Pelee on Martinique)
(Slide 29) St. Pierre on Martinique was destroyed (29,000 deaths) by a Peleean eruption, May 7 and 8, 1902 (For more info,

Peleean eruptions generate pyroclastic surges and flows.
(Slide 30) (Slide 31)

An artist’s conception of the Plinian phase of the AD79 Vesuvius eruption
(Slide 32)

Discovery Channel reconstruction of the Plinian phase of the AD 79 Eruption.
Fallout is beginning over Pompeii -- from the TV film Pompeii, the Last Day