Pompeii vs. Herculaneum in 79 A.D.

Susie just loves to visit Italy, and suffers the twenty-plus hour one-way trip whenever she can find the time (and airfare).

I tagged along several years ago, and while the focaccia bread on the Cinque Terre was one of the tastiest (and most sinful) treats I’ve ever had the pleasure of wolfing down, the highlight of the trip, at least for me, was our day in the ruins of Pompeii.

Thanks to letters written by Pliny the Younger — whose uncle (Pliny the Elder) happened to be in the area and apparently succumbed to the effects of the blast — we have a partial contemporary account of what happened to Pompeii.

The short version is sadly predictable: The year was 79 A.D. and the magma chamber beneath nearby Mt. Vesuvius had again reached critical mass. This was long before the United States Geological Survey had begun learning how to predict that an intermediate volcano is getting ready to erupt, so the locals had no idea of what was coming for them. But the end result was the same.

As discussed in an earlier post, volcanoes come in two flavors: the kind that ooze and those that kaBoom. Vesuvius is of the latter variety, and kaBoom it did; sending out a pyroclastic cloud (a.k.a.stone wind) at nearly the speed of sound and at a temperature in excess of 550° C.

This is fast enough to pretty much catch up with anything (you WILL NOT outrun one of these), and then incinerate whatever it manages to engulf. Speed and temperature both decrease with distance, so it’s safe to assume that the closer you are, the more risky is your position.

Which brings us to an article I read a couple days ago on Physics.Org. The main thrust of this one was trying to understand the variations in damage to the surrounding communities. In particular, the devastation in Herculaneum as opposed to Pompeii.

It seems to be a recurring trend in human history (we did, after all, reestablish New Orleans after Katrina, and once again below sea level): a new version of Pompeii was rebuilt on top of the buried ruins of the original community. Fifteen hundred years later, the city fathers decided that they had an archeological gold mine hidden beneath the streets of the city.

So, in the late 1500s they started to dig, and since that time have managed to excavate much of the original Pompeii that was engulfed by the pyroclastic ejecta and preserved… just as it was at the time of eruption.

And what a window into history! Homes and stables. Markets and brothels. Paintings and murals and friezes. Artifacts beyond count. Enough to put together a snapshot of what life was like during the glory days of the Roman Empire in southern Italy, two thousand years ago.

Workers found (and are still finding) some pretty amazing stuff. Some of it was obvious from the beginning (wagons and statues and urns and such), but… they also started finding these weird voids in the ash — holes of indeterminate size and shape.

Well, it didn’t take them long to start wondering what they were, and some clever archeologist decided that the best way to add some pertinent data to the conundrum would be to mix up a bunch of Plaster of Paris, pour it into the next one they came across and let it harden, then dig it out and see what it looked like.

Oh my! It turns out that the voids were people (and dogs and cats and even horses). Casts of residents of Pompeii that had been buried in the ash and baked into nothingness. The process also managed to cool and harden the surrounding ash, which formed and then maintained the molds — good news for future archeologists, to be sure!

But this same process didn’t happen at Herculaneum! Like Pompeii, the town site was surely interred before the 79 A.D. eruption was over, and there are artifacts galore if one is willing to dig.

But… there is nothing remaining of the people. Supposedly, a single piece of human residue has been located: a skull, with a “vitrified” brain inside that was apparently turned to glass by the heat, followed by rapid cooling. But that’s it.

The obvious conclusion is that residents of Herculaneum, at barely five miles from the volcano as opposed to Pompeii (which was about twice as far away) were completely incinerated by the blast.

The article in Physics.org ends by issuing a warning to the people of Naples — they, too, live in a city close enough to Vesuvius to suffer a similar fate when the volcano lets loose again. All well and good, and this is a warning that the locals should probably take with quite a few grains of salt.

But Vesuvius isn’t the only explosive volcano capable of generating a “pyroclastic density current” (PDC). Explosive volcanoes occur all along the Pacific Ring of Fire — hence the name. Here in the United States we have the Cascade Range of Washington, Oregon, and northern California to give us pause.

Of the dozen or so candidates, my personal favorite is Mt. Rainier. It has erupted (with the accompanying PDCs and lahars) many times in the past, and will again. Certainly the city fathers (and mothers) of Tacoma and Seattle are finally waking up to the risk (not to mention the folks in Orting). But this is a story best left for a future post all its own…