November 27, 2012 | 11
I see you jumping up-and-down with your hand in the air, saying “Ooo! I know this one!” I see you, too, over there groaning, “Doesn’t everybody know?” And I see you, glowering, wanting your Mount St. Helens and annoyed I’m spending time on Pompeii instead.
Look, I’ve got reasons. And Mount St. Helens has a little something to do with it. We’ll get to that. But first, let me tell you why I’m on about Pompeii. It’s because there’s one sure way to make a geologist howl:
For me, though, these plaster victims prompt other thoughts too – about the city of Pompeii as a whole, and what it stands for. Partly that’s because they are so eloquently trapped in that no man’s land between the living and the dead, captured at the very moment when they lost their struggle against the fumes and lava.
When my friend George read this, he tweeted, “Historian Mary Beard on the emotional power of Pompeii body castings… and how history is presented… But I can’t help wondering if Beard is confusing lava and ash. Can @Dhunterauthor help?”
Of course! Pompeii was one of the first volcanic stories I ever heard. I’d known since the tender age of six* how the people of Pompeii perished, and that it had nothing to do with lava. Absolutely pyroclastic flows. I’d never forgotten it, not after seeing the casts of those agonized bodies left in the hardened ash. I knew you could outrun lava, but not these fantastically fast flows of ash, gas and rock.
So how could Cambridge Professor Mary Beard, who had actually written books about Pompeii, get that important geological detail so very wrong? I figured I’d better ask. We had a brief conversation on Twitter, which brought to light the fact that she uses the word “lava” as a way of saying she’s not a volcanologist, and her book isn’t about the eruption but about life in Pompeii (not just the last few minutes of it). Fair enough. I asked her if she could at least use ash instead, to spare the feelings of geologists everywhere, and we ended up deciding that the Italian word “fango,” which means “mud,” must be popularized. It wasn’t mud that destroyed Pompeii, but the pyroclastic flow deposits did get reworked into lahars by water after deposition, so I’ll take it.**
I’m glad Professor Beard wrote this article, and I’m even glad she made geologists the world over grind their teeth, because it’s a thought-provoking look at how we react to the people of Pompeii. It also points out that the city we see today is a lot more put together than Vesuvius left it. And her intentional use of the word “lava” makes us look harder at what really happened to Pompeii. I think a lot of us see the restored ruins and think of ash raining down, almost gently. Sure, it suffocated people and buried them, but it also lovingly preserved the buildings. Look! Even crockery is intact!
Well, it’s true that some breakable items in protected cupboards and closets survived without breaking, but Pompeii’s death wasn’t gentle. A town doesn’t have to be buried in burning hot lava to suffer dramatically. The people in and around Pompeii spent a horrible last nineteen hours, and they didn’t have much of a chance.
Vesuvius erupted around one in the afternoon on August 24th, 79 AD. Magma moving up into the mountain had been shaking Pompeii and surrounding cities for some time, but no one was much worried – earthquakes happened here frequently, and they didn’t know the connection between earthquakes and eruptions then. So when Vesuvius exploded, it came as something of a surprise. Pliny the Younger witnessed the eruption from Misenum, about 21 km (13 miles) away. Later, he would describe the eruption for Tacitus: “It was not clear at that distance from which mountain the cloud was rising (it was afterwards known to be Vesuvius); its general appearance can best be expressed as being like an umbrella pine, for it rose to a great height on a sort of trunk and then split off into branches, I imagine because it was thrust upwards by the first blast and then left unsupported as the pressure subsided, or else it was borne down by its own weight so that it spread out and gradually dispersed. In places it looked white, elsewhere blotched and dirty, according to the amount of soil and ashes it carried with it.”
That was the phreatomagmatic phase, which lasted for hours. We know people close to the volcano were terrified: one of Pliny the Elder’s friends, Rectina, who lived right at its base, sent a message to him begging rescue: there was no escape for her except by boat. Pliny sailed off with warships to his death. In the towns, people who hadn’t fled tried to take shelter indoors as pumice rained down, first in a layer of white, then as the volcano tapped a different part of its magma chamber, gray. It hurled larger blocks of old lava and limestone at Pompeii along with the pumice. Some of the people who died outdoors had their skulls fractured by ballistic rocks. The pumice fall made it terribly difficult for people to flee Pompeii. What other choice did many have but to take shelter?
I can only imagine what it must have been like inside, listening to those rocks hit the roof: the quiet roar of thick pumice falls, the sharper thuds of denser stones. Pitched roofs shed their loads, filling the courtyards and streets with deeper drifts of the bubbly stone. It was falling at a rate of 15 centimeters (6 inches) per hour. Flat and less steeply pitched roofs, which couldn’t shed the load, collapsed within hours. People taking shelter within those rooms were crushed and killed. The rooms, now open to the sky, filled with pumice: some rooms with 1 meter (3 feet), some up to 5 meters (16 feet).
It’s an incomprehensible amount of pumice. It buried the first floors of buildings. Trying to flee through the stuff must have been nearly impossible; being trapped inside a house with a roof that survived the onslaught, only to see it pile up past the first floor, must have been horrifying. But some people survived. Only 394 bodies have been found in that deposit. The worst was yet to come for those who made it through this phase.
The first pyroclastic flow reached the city toward morning. We don’t know exactly when it was: we know it was after the pumice stopped falling, after roofs all over the city had collapsed. People may have begun to venture out, looking for escape routes, assessing the damage, wondering if Vesuvius was done. That first flow was probably just the distal end of a somewhat small pyroclastic flow: we know it didn’t do much more than deposit a layer of ash over the pumice. We know from studies of pyroclastic flows at Mount St. Helens and other volcanoes that the further away from the volcano a flow gets, the less dense it is – so a small flow wouldn’t have been powerful enough to do much damage by the time it reached Pompeii, 8 km (5 miles) away. It was certainly more than enough to traumatize already traumatized survivors. Breathing through it would have been agonizing. But it was survivable. So was the next explosion that deposited a blanket of ash over the city, but produced no pyroclastic flows.
There was a pause. Then the big one hit.
A pyroclastic flow is no joke. It can be incredibly hot, although the ones that buried Pompeii were relatively cool. But low temperature doesn’t equal survivability. People who wish to take their chances with a cool flow of ash, gas and rock as opposed to burning hot lava have made the wrong choice. You can run away from lava. Depending on the viscosity, you can outwalk it. You can’t run away from a current of pulverized rock and volcanic gasses flowing at speeds of up to 240 kilometers (150 miles) per hour. The people of Pompeii had no chance when that flow hit them full-force. They barely would have had time to see it coming.
This flow was huge. Its leading edge filled the air with ash, dust and gas. People who tried to flee it fell in the streets, unable to breathe. Then the main body arrived, powerful enough to tear through walls still standing after the roof collapses. Lower floors in Pompeii were protected by their pumice tomb, but above them, walls athwart the flow were bulldozed, surviving roofs ripped off. The flow poured in through those gaping wounds in the buildings; where roofs had managed to survive, ash and rock still found its way in through courtyards and other openings. Many people lived long enough to try to shelter their faces from the onslaught, but it buried them where they lay, some of them propped half-upright, fighting to breathe. Indoors or out, it buried them. When it was over, it had left a hard, dense, layer of pyroclastic material up to 3 meters (10 feet) thick.
Vesuvius finished its cataclysmic eruption with a few more phreatomagmatic explosions, blanketing the remains of Pompeii with more layers of ash. By the end of the eruption, around 8 in the morning on August 25th, only a few of the tallest buildings remained visible, like tombstones on a grave. The deposits, heavy and rich with fine ash and rock fragments, settled, hardened over ages. The city and the citizens who had died with it would remain buried for almost two thousand years.
We’ve found 650 of the people who died in that final pyroclastic flow. We’ve found their bones, and we’ve found the voids their bodies left in that hard deposit. We pour plaster in and an afterimage of a person emerges. Some of them look peaceful, some desperate and distraught. Some are huddled together, some alone. The adults are tragic to look at. The children are devastating. You can almost persuade yourself that the adults had a choice, that they decided to stay, tried their luck and lost, but you can’t say that about the kids. The adults didn’t have any good choices: the children had none at all.
Those voids in the ash, now filled, are so much more than bones could ever be. They don’t allow much of a distance. They look eerily like us. They make Pompeii a uniquely human tragedy; they make two thousand years seem like yesterday.
And they remind us of the tremendous power of pyroclastic flows. Lava is easy. We battle it off with seawater and hoses. We stand beside it as it runs by in molten rivers. We can’t always save our possessions from it, but we can generally outrun it. But a pyroclastic flow isn’t something we can run from. It destroys in an instant. This is why, when these subduction zone volcanoes wake up, it’s best for those nearby to get well out of the way, well in advance.
The people of Pompeii didn’t know what was coming. But in the years since, we’ve learned. Mount St. Helens, among others, taught us what to watch for and what to expect. We’ve successfully predicted eruptions. We’ve evacuated cities before they could become modern Pompeiis. We’re learning to live with Vulcan’s forges.
Pompeii reminds us never to forget what those mountains can do.
Giacomelli, L. et al, 2003: The eruption of Vesuvius of 79 AD and its impact on human environment in Pompei. Episodes, 23.
Luongo, G. et al (2002): Impact of the AD 79 explosive eruption on Pompeii, I. Relations amongst the depositional mechanisms of the pyroclastic products, the framework of the buildings and the associated destructive events. Journal of Volcanology and Geothermal Research.
Luongo, G. et al (2002): Impact of the AD 79 explosive eruption on Pompeii, II. Causes of death of the inhabitants inferred by stratigraphic analysis and areal distribution of the human casualties. Journal of Volcanology and Geothermal Research.
*Ripley’s Believe It or Not! Great Disasters. Every child should own a book like this.
**She also assures me that “the book is technically accurate,” so geologists needn’t fear apoplexy if they pick it up. I certainly intend to!
Secrets of the Universe: Past, Present, FutureX