A DAY IN AD79 - POMPEII
Dr. Paul Olver – 10 November 2010
Dr Paul Olver on A Day in AD79 centred on Pompeii & Herculaneum
Paul used an OHP in the first half to illustrate the locations and processes involved in plate tectonics in general and in the Bay of Naples in particular.
After coffee he showed us slides of photographs taken around the Bay of Naples.
He first showed how vulcanicity in Italy had moved southwards from the area of Nice and Bologna in mid Tertiary times to its current active location around Vesuvius, Stromboli, Vulcano and Etna. On a world scale he described the distribution of plates contrasting the crustal expansion from mid ocean ridges with the destruction zones where subduction or collision of plates is occurring. Detailed explanations followed of:
How convection cells power the creation of mid ocean ridges.
A collision zone under Japan where the mid Pacific and Eurasian plates meet, subduction occurs and the melting plate fuels volcanoes like Honshu.
Sporadic plate movement generating earthquakes.
Turning to Italy
Paul explained that:
The African plate is subducting under the Eurasian one underneath Stromboli and Etna.
The Mediterranean is a dying ocean. On its demise, the African plate will be fractured and deformed directly against the Eurasian plate resulting in major mountain building on a Himalayan scale.
The Bay of Naples is a massive 8 mile deep syncline with volcanic infill, surrounded by Triassic limestone to the north, east and south. Volcanic activity has continued with Vesuvius in 1944 destroying 2 villages, Ischia in 1306 and Solfatara in early medieval time. The grey rocks in the cliffs around Sorrento are the remains of a massive Campania caldera from 30000 years ago covering the whole of the Bay area.
Prior to AD79 Vesuvius was twice as high and known as Monte Somma. This was a wooded mountain with a lake on top and not seen as any threat.
From his naval base at Misenum west of Naples, Pliny the Elder saw an [Plinian] eruption – described as a pino or pine tree shape with a white column exploding vertically before spreading out in the stratosphere.
Exceptionally a north west wind brought the falling ‘fluffy’ pumice onto Pompeii [and beyond as far as Placetum] but Herculaneum was not affected. In 2 days it is estimated that 3 cubic kilometres of material was ejected with 3 metres of pumice falling on Pompeii at a distance of 8 kilometres. Pumice fell as far as Peastum
Pliny took the navy on a rescue mission but was forced by darkness to abort southwards to Stabiae, where he died of a heart attack.
As the explosive power reduced, pumice gathered on the upper slopes of Vesuvius and then generated pyroclastic flows [since known as Vesuvian eruptions] These overran Pompeii at 60mph and with a temperature of some 1500 degrees welding a layer of ash onto any bodies Severe rainstorms next developed on the volcano and the new deposits were turned into mud slides. These were relatively cool and slow moving but overran Herculaneum filling up but not destroying buildings. Inhabitants fled to the nearby coast only to be overcome in turn. Bodies were entombed in the mud and eventually decayed leaving voids . By filling these with plaster of paris, researchers have recreated detailed impressions of the bodies.
Paul outlined the techniques now being used to monitor the mountain.
Seismic indicators – eg of the magma chamber fracturing or a harmonic beat of magma being forced towards the surface.
Tiltmeters – accurate enough to record the effect of putting a 10p coin under one end of a one and a half kilometre ruler!
In the 1990’s an evacuation was ordered, but no eruption occurred. The area around Vesuvius remains the most hazardous in the world i.e. potential activity relative to population.
These included;
A satellite photo of a mini tectonic plate around the Red Sea.
1200 degree magma flows on Etna in1971.
Mt Teidi in Teneriffe – steeper slopes so more dangerous.
Restored remains of buildings [including frescoes] and streets in Pompeii and Herculaneum – latter more complete as eg. carbonised wood survived.
Use of earlier volcanic material as building stone eg yellow tuff from Campania eruption [30000 years ago].
Fluctuating sea levels – evidence from boring pillots [bivalves] in limestone columns in Pozzuoli.
Solfatara – area of active gas emissions surrounded by tower blocks, whose windows have been etched by hydrogen choride.
An extremely well presented lecture. Clearly Paul has both expert knowledge and a clear and attractive way of putting it over for the maximum benefit of his audience.
Jim Handley