On this day in science history: Mount Vesuvius erupted
In 79, the long-dormant Mount Vesuvius erupted in Italy, burying the Roman cities of Pompeii and Herculaneum in volcanic ash. An estimated 20,000 people died. When discovered, the sites became astonishing archaeological time capsules. Official excavations began on 6 Apr 1748 of behalf of the Italian king’s interest in collecting antiquities.
|Pompeii, with Vesuvius towering above. Qfl247 CC BY-SA 3.0 (http://ift.tt/HKkdTz) or GFDL (http://ift.tt/KbUOlc), via Wikimedia Commons|
Scientific knowledge of the geologic history of Vesuvius comes from core samples taken from a 2,000 m (6,600 ft) plus bore hole on the flanks of the volcano, extending into Mesozoic rock. Cores were dated by potassium–argon and argon–argon dating. The mountain started forming 25,000 years ago. Although the area has been subject to volcanic activity for at least 400,000 years, the lowest layer of eruption material from the Somma mountain lies on top of the 40,000‑year‑old Campanian Ignimbrite produced by the Campi Flegrei complex, and was the product of the Codola Plinian eruption 25,000 years ago.
It was then built up by a series of lava flows, with some smaller explosive eruptions interspersed between them. However, the style of eruption changed around 19,000 years ago to a sequence of large explosive plinian eruptions, of which the 79 AD one was the most recent. The eruptions are named after the tephra deposits produced by them, which in turn are named after the location where the deposits were first identified:
- The Basal Pumice (Pomici di Base) eruption, 18,300 years ago, VEI 6, saw the original formation of the Somma caldera. The eruption was followed by a period of much less violent, lava producing eruptions.
- The Green Pumice (Pomici Verdoline) eruption, 16,000 years ago, VEI 5.
- The Mercato eruption (Pomici di Mercato) – also known as Pomici Gemelle or Pomici Ottaviano – 8000 years ago, VEI 6, followed a smaller explosive eruption around 11,000 years ago (called the Lagno Amendolare eruption).
- The Avellino eruption (Pomici di Avellino), 3800 years ago, VEI 5, followed two smaller explosive eruptions around 5,000 years ago. The Avellino eruption vent was apparently 2 km west of the current crater, and the eruption destroyed several Bronze Age settlements of the Apennine culture. Several carbon dates on wood and bone offer a range of possible dates of about 500 years in the mid-2nd millennium BC. In May 2001, near Nola, Italian archaeologists using the technique of filling every cavity with plaster or substitute compound recovered some remarkably well-preserved forms of perishable objects, such as fence rails, a bucket and especially in the vicinity thousands of human footprints pointing into the Apennines to the north. The settlement had huts, pots, and goats. The residents had hastily abandoned the village, leaving it to be buried under pumice and ash in much the same way that Pompeii was later preserved. Pyroclastic surge deposits were distributed to the northwest of the vent, travelling as far as 15 km (9.3 mi) from it, and lie up to 3 m (9.8 ft) deep in the area now occupied by Naples.
The volcano then entered a stage of more frequent, but less violent, eruptions until the most recent Plinian eruption, which destroyed Pompeii.
The last of these may have been in 217 BC. There were earthquakes in Italy during that year and the sun was reported as being dimmed by a haze or dry fog. Plutarch wrote of the sky being on fire near Naples and Silius Italicus mentioned in his epic poem Punica that Vesuvius had thundered and produced flames worthy of Mount Etna in that year, although both authors were writing around 250 years later. Greenland ice core samples of around that period show relatively high acidity, which is assumed to have been caused by atmospheric hydrogen sulfide.
The mountain was then quiet (for 295 years, if the 217 BC date for the last previous eruption is true) and was described by Roman writers as having been covered with gardens and vineyards, except at the top which was craggy. The mountain may have had only one summit at that time, judging by a wall painting, “Bacchus and Vesuvius”, found in a Pompeiian house, the House of the Centenary (Casa del Centenario).
Several surviving works written over the 200 years preceding the 79 AD eruption describe the mountain as having had a volcanic nature, although Pliny the Elder did not depict the mountain in this way in his Naturalis Historia:
- The Greek historian Strabo (ca 63 BC–AD 24) described the
mountain in Book V, Chapter 4 of his Geographica as having a predominantly
flat, barren summit covered with sooty, ash-coloured rocks and suggested that
it might once have had “craters of fire”. He also perceptively
suggested that the fertility of the surrounding slopes may be due to volcanic
activity, as at Mount Etna.
- In Book II of De architectura, the architect Vitruvius reported
that fires had once existed abundantly below the mountain and that it had
spouted fire onto the surrounding fields. He went on to describe Pompeiian
pumice as having been burnt from another species of stone.
- Diodorus Siculus (ca 90 BC–ca 30 BC), another Greek writer,
wrote in Book IV of his Bibliotheca Historica that the Campanian plain was
called fiery (Phlegrean) because of the mountain, Vesuvius, which had spouted
flame like Etna and showed signs of the fire that had burnt in ancient history.
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