The Cataclysm: “Fully Down and Buried”

Geologists did a lot of talking to trees in the aftermath of Mount St. Helens’s eruption. They had a lot of questions, and the trees had a lot of answers*.

Few talked to the trees as extensively as Richard Waitt. He was investigating the blast deposits, and found the trees to be quite helpful. His work in the field led him to identify three primary layers: A1, the base, was pretty full of gravel. A2, the next level up, was a coarse sand, and the final, A3, a fine air-fall sand.** Throughout those layers were trees and bits thereof, and he queried them closely to figure out the progress of the lateral blast and how it had left its deposits.

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Near the volcano, at distances around 6-12 kilometers (3.7 – 6.2 miles) from the vent, he found downed trees either fully buried by the gravelly bottom layer, or protruding at an angle from it. Those trees retained some of their bark even on the sides that had taken the direct brunt of the blast before they fell, unlike the stumps and standing trees that had been stripped completely. Since there was no blast deposit beneath them, those fallen trees must have come to rest either just before or during those moments the gravel was rattling down. The fact it hadn’t scoured the bark away as it landed told him that layer A1 wasn’t A+ at abrading horizontal surfaces. Both layers A1 and A2 had trouble achieving more than a thin layer atop the logs, due to curvature, and the lateral nature of the blast.

Even those thinnish deposits were somewhat protective. The bits of tree that stuck out from them fared rather worse than the buried bits: they lost their bark, and got a good scorching in the bargain. Scorching didn’t happen on parts already coated by A1, which shows A1 wasn’t such hot stuff. The scorching showed that “a flux of very hot gas” had blasted out from the volcano just after A1′s arrival. After all the baking and abrading happened during A1 and A2′s deposition, layer A3 dusted the results, showing it got there last.

Nothing in that sector survived. Rick was talking to dead trees. But when he moved on 10-12 kilometers (6.2 – 7.5 miles) north, he discovered something remarkable: life. The trees that had remained standing throughout the blast were perished, stripped naked of needles and badly scorched. But some had flexed, bent, and been buried by the blast deposits, much like they might be covered by an avalanche. When water began cutting gullies in the deposits a few days after the eruptions, the trees were freed of their tomb and whipped back upright, all green and lovely, if a bit put-upon. Even the delicate mosses on resident rocks survived, so long as they were decently covered, then quickly disinterred. These green, growing things supported the testimony of their dead relatives: that blast of searing-hot gas had certainly arrived only after the erosive front and layer A1. And that layer had provided the insulation necessary for some trees to survive.

Young fir tree that had been buried by the blast deposit of May 18 eruption of Mount St. Helens. Skamania County, Washington. June 8, 1980. Image and caption courtesy USGS.

Young fir tree that had been buried by the blast deposit of May 18 eruption of Mount St. Helens. Skamania County, Washington. June 8, 1980. Image and caption courtesy USGS.

Not that survival was simple, or even likely.

Further from the volcano, at around 15-25 km (9.3 – 15.5 miles), the blast had lost enough energy to stop carrying A1, but the erosive edge was still powerful enough to knock down some trees and turn others into snags. The fallen trees, buried beneath a nice layer of A2, held on to their bark. The snags weren’t so lucky: they lost the bark on sides facing the volcano, though only a meter or two above ground and up. The hot flux had arrived with layer A2 and got busy stripping and singing, but lagged the erosive edge. On the far end of the downed timber zone, the standing trees lost their needles and had their twig ends scoured by the blast, but their lower branches, coated by A2, retained dead, scorched needles. A2 couldn’t protect them from the hot gas it arrived with and was outlasted by, but did provide some defense.

Stratigraphic section atop distal downed tree, 15-25 km from Mount St. Helens. Layers A2 and A3 overlie bark. Rule for scale. Skamania County, Washington. 1980. Figure 267, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Stratigraphic section atop distal downed tree, 15-25 km from Mount St. Helens. Layers A2 and A3 overlie bark. Rule for scale. Skamania County, Washington. 1980. Figure 267, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

At the scorch zone, the layer of A2 became thin indeed. But the flow hadn’t lost its heat, hence the scorching. A coat of silty A3 plastered some of the seared branches, which were just as toasted as the bare ones. The hot flux, then, got there and left before A3 arrived.

    Scorched needles beneath layer A3 plastered on tree, about 20 km from Mount St. Helens. Needles beneath layer A3 are just as scorched as those not covered. Skamania County, Washington. 1980. Figure 268, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Scorched needles beneath layer A3 plastered on tree, about 20 km from Mount St. Helens. Needles beneath layer A3 are just as scorched as those not covered. Skamania County, Washington. 1980. Figure 268, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Those were the tales of trees covered by the deposits. Next, we’ll interview those trees that actually became deposits themselves.

 

*No illicit substances were involved. The trees didn’t actually talk. Everyone’s fine.

**Not beach sand – sand refers here to the size of the rock particles. This is sand-sized ash.

Previous: The Cataclysm: “The Path of Maximum Abrasion”

Next: The Cataclysm: “Stripped from the Proximal Forest”

References:

Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.

 

Previously published at Rosetta Stones.

An Apt Analogy for Varieties of Creationist

I don’t know if any of you read Paul Braterman’s blog, Eat Your Brains Out. No, it’s not a blog about zombies, although occasionally Jesus is mentioned. It’s actually a blog about science and creationism, and I’ve now read it in its entirety. Great stuff within.

And, sometimes, a very funny and apt bit. Paul took on the arguments of mathematician and theologian John Lennox, who rejects this god-of-the-gaps nonsense, yet apparently associates with Douglas Axe, director of the Biologic Institute (part of the Discovery Institute; and Norman Nevin, a biblical literalist and Chairman of the Centre for Intelligent Design. Lennox took Lawrence Krauss to task for words about the Higgs boson being more important than God with a bit of a Ford analogy:

That is as wrong-headed as thinking that an explanation of a Ford car in terms of Henry Ford as inventor and designer competes with an explanation in terms of mechanism and law. God is not a “God of the gaps”,  he is God of the whole show.

And Paul took that analogy and ran with it to places where I’m sure Lennox would have preferred he not gone:

To pursue the Ford analogy further, Lennox believes that the car works because it is well designed, Axe believes that it works because there is a miracle-working mechanic inside the gearbox, and Nevin believes that it was sabotaged by the drivers’ grandparents.

Precisely. I don’t think anyone’s ever summed up the differences between old-school science-accepting theologian/scientists, intelligent design proponents, and Biblical literalists more succinctly. I laughed.

Image shows a blueprint for a Model T engine with God photoshopped in.

“The Engine of God” Original images courtesy Wikimedia Commons, photoshopped poorly by moi.

A Whole Lotta Shaking: Some Thoughts on Magnitude

So Sunday was a big day for earthquakes. In the wee hours of the ay-em, we had the West Napa Fault Zone (probably) cutting loose, and then, a bit later in the day, Peru got hit big-time. Thankfully, Peru’s quake was in a sparsely-populated area, and California’s was – well, California. They’ve been dealing with this stuff for half of forever. So while Sunday was dramatic for earthquake happenings, it wasn’t so bad as far as death and destruction.

But Cali got lucky – their quake was pretty small compared to Peru’s. Like, way smaller. The South Napa quake was a mere 6.0 – big, but not unimaginably huge. Peru’s was 6.9, same size as Loma Prieta (and we all know how awful that was).

Okay, you may think. 6.9. That’s not that bad.

Except that’s not how the scale works.

At magnitude 6.0, this quake is classed as a strong quake, but one of the unfortunate diagrams I’m seeing in media reports is that anything between a 6.0 and a 6.9 like the Loma Prieta quake are being lumped together on a bar graph as “strong”. The difference between a 6.0 and 6.9 is profound, and is a reason that we are not hearing about dozens or hundreds of people killed in the event. On the magnitude scale, the amount of energy released increases by about 30 times with each whole number. In other words, a magnitude 7.0 quake is just over 30 times more powerful than a magnitude 6.0, and a magnitude 8.0 is just over 30 times more powerful than a 7.0 (this make an 8.0 around a 1,000 times more powerful than a 6.0).

Yeow.

Those numbers can be hard to picture. So I came up with a bit of an analogy that may help. Picture yourself in a car, headed toward a solid wall (in this scenario, you’re a crash test dummy. Sorry). For the first run, you’re going Magnitude 6. We’ll say that’s 25 miles per hour.

Image shows a gray car with it's slightly-crumpled nose against a wall. The poor dummy has its face planted in the airbag.

Still from the video 2013 Dodge Dart / Fiat Viaggio | 25mph/40kph Frontal Crash Test by NHTSA | CrashNet1.

Okay, not so bad.

Now, Wikipedia tells me that a 7 is roughly 32 times larger, so we’ll go with Garry’s 30 figure and see where we end up. Hmmm, math… 25 x 32 … carry the ZOMG it’s 750 mph. We’re headed for a wall at 750 mph! We don’t have a crash test at 750 mph! Here’s the Mythbuster’s doing a 100 mph test and being appalled by the result.

Image shows a yellow car with its front half pretty much gone and its back half off the ground.

Mythbusters 100 MPH test. ZOMG WTF etc.

I love how their marker dealios look like earthquake focal mechanism symbols. Very apropos. And if this is what 100 mph can do to a car, you can image 750 mph would leave it, the wall, and half the neighborhood beyond in fragments. No wonder the Bay area was in such bad shape after the Loma Preita quake.

Okay. I hate to look, but we’ve gotta do it. Our next victim car is going to hit the wall at 1,000 times the speed of our first test. So we’ve leapt from 25 mph and a little mild damage to something that could wipe out the entire metro area. It’s certainly a much larger impact than the fastest crash test ever, which was only a paltry 120 mph.

Image shows a cloud of debris and one sadly intact wheel.

Crash test of a Ford Focus. Well, former Ford Focus.

Maybe we should’ve switched to planes, but even then…

So those are some pretty intense differences. It’s why we go from this:

Image shows a kitchen, with open cabinets and a lot of wine bottles scattered on the floor.

South Napa earthquake, 6.0. Photo courtesy Eiko’s Restaurant in Napa, used with permission.

To this:

Image shows a collapsed double-decker freeway.

Loma Prieta, 6.9. Image courtesy USGS.

To this:

Images shows downtown San Francisco, high-rises in ruins. It looks like Dresdan, Germany after the Allied bombing.

San Francisco Earthquake, est. 7.8. Image courtesy National Archives.

Now that we’ve had this little visualization exercise, I’m going to go crawl into bed and whimper, because I live in a place that expecting a 9.

Image shows a gray and white kitten on a pink blanket, on its back and looking terrified. Caption says, "Iz scared. mommy."

New at Rosetta Stones: Earthquake Safety Tips

Funny thing is, I’d been looking up real safety tips for surviving earthquakes when I was fact-checking our Christianists texts on the subject. And I learned that I had a lot of wrong-headed ideas. In light of the Napa earthquake that went on today, I figured I’d share those tips so that folks in seismically active areas can polish up on their earthquake survival.

Here’s the takeaway lesson, although you should read the whole thing so you know what to do before, during, and after:

Image shows the three steps essential to staying safe in an earthquake: drop, get under a sturdy piece of furniture, hold on until the shaking's over.

Excellent advice from the Great California ShakeOut. Click the image to visit their page and sign up for the drill.

The Cataclysm: “Stripped from the Proximal Forest”

A rather extensive forest became part of a directed blast deposit: that’s the summary. One moment, you’re a green and pleasant home for much of the local wildlife; the next, you’ve been rudely ripped apart and incorporated within a bunch of rock and ash by a volcano having a bad turn. So it goes.

When Rick Waitt traced the fate of Mount St. Helens’s magnificent forests, he found they’d had quite the adventure (aside from being knocked flat, bruised, battered, buried, and burnt).

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250.

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Within the down-timber zone, it was clear some rather spectacular force had been applied. It wasn’t piddly little wood fragments and needles that became deposits, but entire tree trunks. Whole limbs had been ripped off, splintered, and subsequently dumped. The heavier bits, as heavy bits tend to do, remained close to the ground as the blast carried them along. As the flow lost energy, the heavy bits of layer A1, including its compliment of ex-trees, settled out first, fining upward as the deposit accumulated. Mind you, when I say “fined upward,” I don’t mean they got all demure and small, even close to the volcano. No, the ex-tree bits in subsequent layers within layer A2 and the pieces that landed atop layer A3 were as mind-blowingly large as 75 centimeters (29.5 inches). Not only that, but the way they landed show they were first torn loose by that erosive front of the blast, then heaved high in the air by the following phase, held airborne by convection, then unceremoniously dumped moments later.

Warner Bros., I think, could have animated that sequence in the tradition of Wiley E. Coyote to fine effect.

Stratigraphic section atop distal downed tree, 15-25 km from Mount St. Helens. Layers A2 and A3 overlie bark. Rule for scale. Skamania County, Washington. 1980. Figure 267, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Stratigraphic section atop distal downed tree, 15-25 km from Mount St. Helens. Layers A2 and A3 overlie bark. Rule for scale. Skamania County, Washington. 1980. Figure 267, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

Other branches, pine cones, and bits ripped from the unfortunate forest were light enough to continue traveling. They sailed the volcanic winds even beyond the boundary of layer A2, past the devastated area, and came to rest in a bed of silty layer A3, then were covered with a blanket of the following air-fall deposits left by the central eruption column. Some of those fragments were as long as 15 centimeters (6 inches). Imagine how much force it requires to take pieces of wood half the length of a school ruler and keep them in the air for twenty minutes or more.

Yeah.

Mixed up in all that were smaller remains, a mulch of fir needles, splinters, and twigs. In most areas, they can be found in all three layers, but to the north the energy of the blast was so ferocious it wouldn’t let them settle out until layer A3 did. Almost everything was burnt black, no matter where it landed, showing it all got seared before coming to rest. Only the needles and branches flying through the southern edge of the east side of the blast managed to come out without a thorough scorching, showing the blast cloud wasn’t so hot there. Still fast and furious enough to rip trees apart and turn them from biology into geology, though.

Thus ends the story of The Forest that Was. From here on, our relationship with the blast deposits will get decidedly rocky.

Scorched needles beneath layer A3 plastered on tree, about 20 km from Mount St. Helens. Needles beneath layer A3 are just as scorched as those not covered. Skamania County, Washington. 1980. Figure 268, U.S. Geological Survey Professional paper 1250.

Scorched needles beneath layer A3 plastered on tree, about 20 km from Mount St. Helens. Needles beneath layer A3 are just as scorched as those not covered. Skamania County, Washington. 1980. Figure 268, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

 

Previous: The Cataclysm:”Fully Down and Buried”

References:

Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.

 Previously published at Rosetta Stones.

 

Ha Ha Ha Whoops. Also: Help Me Keep an Eye on Creationists

I’ve had myself so buried in Christianist textbooks, frantically trying to get this talk pulled together, when I wasn’t compulsively reading about the awful things police in Ferguson are up to now, I haven’t thought to keep an eye on my email… and it turns out that due to unforeseen circumstances, FtBCon’s postponed anyway. We’ll be trying again in a few months. So what does this mean? It means you’ll still get a talk on Why Geology Matters – To Creationists, only it will be a much better talk, because I’ll actually have gotten through these books. Well, at least through all the geology bits of the books. Ye gods, it takes ages to fact-check and debunk this stuff now that we’re in to the portions of Earth science Christianists love to hate.

Image shows an orange kitten sitting in a terra cotta pot, with one paw over an eye. Caption says, "Whew! Close Call!"

Thank you, all of you who helped me calculate mammoth populations! You’re amazing. You’ll also love the resulting post, although it’ll take a while before it comes up in the queue – trying to do this stuff in order.

Now I’m going to ask you all now to do me another favor: over the next few months, would you keep an eye out for any news about creationists and geology? It can be things like creationists infiltrating the American Geophysical Union or Geological Society of America meetings (again), creationists trying to sneak “Flood geology” in or fighting earth science standards, creationists trying to pull the wool over journal editors’ eyes and attempting to slip religion in to science publications, anything like that. You can send tips to dhunterauthor at gmail.

Some of you who are interested may want to join me for a private dry-run of said talk when it’s finished – if you’d like to help me not suck in public, and be one of the elite, exclusive ETEVers who gets to hear it first, let me know. We’ll set up a Google Hangout and do the thing when I’ve got it all written. And, if there’s room on the schedule and you’d be interested in joining me for a panel on Women in the Geosciences, also let me know that.

But wait! There’s More!

For the next few days, I’m preparing the Adventures in Christianist Earth Science Education series relaunch, and also hating my uterus, and going to go photograph All The Sea Mammals for your squees and enjoyment. I’ll also have a social justice post up on our fucked-up police state and ways you can help soon. And there’ll be a little something over at Rosetta Stones eventually this week. I have a super-awesome geology comic book I was sent that I’m going to review for you, probably early next week. People, you have no idea how excited I am about it! But you will know. I’ll also be reviewing a book I read the other morning that will give you a whole other look at the Christianist homeschool life. It’s called Homeschool Sex Machine: Babes, Bible Quiz, and the Clinton Years. And yes, it’s as whacked as it sounds.

Also, YES I AM WRITING A BOOK ON MOUNT ST. HELENS I PROMISE. I know you won’t stop asking, and it makes me happy you don’t, but I figured I’d better reassure you. I’ll be jumping back into that series shortly as well. And yes, finishing the Seattle Seahawks rings. SO MANY THINGS TO DO.

There will also be a post coming soon on that awesome bird at Mount Rainier, and muchmuch more. Good times ahead! Now if you’ll excuse me for just a bit, I need to lose the last of my hair to the geology chapters in ES4 and continue arguing with my uterus over paying attention to the pain relievers I’m feeding it…

How Pompeii Perished

(Yes, I will be watching and reviewing that bloody film on Pompeii for you. Perhaps even within a month! Meanwhile, enjoy this repeat while I despair of my country, deal with the war zone that is my uterus, and try to wrestle a bunch of creationist crap into a presentable form for you. Also, for my mental health, we are going to the zoo later this week to look at cute fuzzy animals. Even if it means I’m up 48 hours straight before this talk I haven’t actually scheduled yet… I’m afraid my brain is going to actually divorce me and take the cat when it leaves if I don’t give it a day off. And you yourselves will need cute fuzzy animals when I’m done. Creationism hurts, folks. Sigh.)

 

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.

The ruins of Pompeii, with Vesuvius brooding in the background. Image courtesy Perrimoon.

The ruins of Pompeii, with Vesuvius brooding in the background. Image courtesy Perrimoon.

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.

Pompeii. This area has not been fully excavated and restored. Image and caption courtesy Margaret Napier.

Pompeii. This area has not been fully excavated and restored. Image and caption courtesy Margaret Napier.

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.

Human cast and remains of a shop. Image courtesy Rome Cabs.

Human cast and remains of a shop. Image courtesy Rome Cabs.

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.”

Vesuvius, with an umbrella pine in the foreground. Image courtesy Paull Young.

Vesuvius, with an umbrella pine in the foreground. Image courtesy Paull Young.

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).

he bottom stories were buried by pumice fall. Image courtesy Elliott Brown.

he bottom stories were buried by pumice fall. Image courtesy Elliott Brown.

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.

Not even horses could run from the pyroclastic flow. Humans had no chance. Image courtesy Craig Morey.

Not even horses could run from the pyroclastic flow. Humans had no chance. Image courtesy Craig Morey.

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're still excavating Pompeii from its volcanic tomb. About a quarter of the city remains buried: more victims doubtless lie within. Image courtesy Irene.

We’re still excavating Pompeii from its volcanic tomb. About a quarter of the city remains buried: more victims doubtless lie within. Image courtesy Irene.

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.

Volcanic landscapes are beautiful - and dangerous. Image courtesy Lyn Gateley.

Volcanic landscapes are beautiful – and dangerous. Image courtesy Lyn Gateley.

 

References:

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!

 

Previously published at Scientific American/Rosetta Stones.

The Cataclysm: “From Unbaked Fragments to Vitreous Charcoal”

There’s a fundamental fact one learns about trees when growing up in dry country forests: they’re flammable. Folks in Flagstaff, Arizona can tell what part of summer it is by the smell. If it’s all piney-fresh, it’s May or early June, and everything’s still safely damp from the spring snowmelt; if it smells like warm turpentine and dust, it’s mid-June; and if it smells like winter with all of the fireplaces cozily burning logs, its the late-June-early-July dry-lightning season, and you’re hoping the monsoon rains come before the whole county burns. I’ve seen smoke that looks like a volcanic eruption billowing from fierce fires. I’ve felt like someone caught in the middle of the apocalypse. I’ve choked on wood ash on hot summer nights. Our forests gets so dry you find yourself avoiding heated language in them. Our trees ceased being lovely green oxygen-producers with sweetly-scented wooden trunks and become tiki torches, just waiting for one stray spark to light the place up.

This view of the Schultz Fire, one of Flagstaff's worst, shows just how intense the smoke can get - it looks like the mountains have erupted. You can just barely see the San Francisco Peaks peeking out at the left. "By 1:30 p.m. on June 20, 2010, the Schultz Fire had exploded and was in full-force." No kidding, right? Image courtesy Mike Elson and Coconino National Forest.

This view of the Schultz Fire, one of Flagstaff’s worst, shows just how intense the smoke can get – it looks like the mountains have erupted. You can just barely see the San Francisco Peaks peeking out at the left. “By 1:30 p.m. on June 20, 2010, the Schultz Fire had exploded and was in full-force.” No kidding, right? Image courtesy Mike Elson and Coconino National Forest.

You can imagine my relief when I moved to the Pacific Northwest and discovered that the trees on the western side of the Cascades are usually too wet to burn. But they’re still made of wood. Apply adequate heat, and they’ll at least char. Raise the temperature enough, and you can even persuade them to burst into flames.

The geologists who studied the cataclysmic May 18th eruption can tell you precisely how much heat you need to barbecue a west-side PNW tree in May: they experimented. Their adventures in pyromania sober science revealed you need temperatures of around 350°C (662°F), give or take 50°C (122°F), to achieve a nice, deep char. If you want just a thin crust of char with a nice unburnt center, turn the heat down to about 300°C (572°F), plus or minus 50°C (122°F). And if you just want a nice seared tree that’s perfectly raw beneath, yet still very dead, you’ll probably want to keep it between 50-200°C (122-392°F).

 Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250. Image courtesy USGS.

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250. Image courtesy USGS.

And yes, Mount St. Helens was perfectly capable of those temperatures. It could also serve up trees en flambé in certain sectors.

You might expect due north to be the hottest part of the blast, but it was actually the west-northwest and northeast sectors that endured the worst. Also, it was very not good to be the side of the tree facing Mount St. Helens. The closest trees, of course, were pulverized and incorporated into the blast deposits. Where the blast cloud left the trunks standing, it stripped the bark from the near sides, then charred the wood black to thicknesses between .1 mm up to .5 mm, which may not sound like much until you consider the fact these trees were several kilometers away, and all the bits of ash and rock riding the blast sandblasted off some of the char. Out to as far as 15 km (9 miles) north, the trees got scorched on their near sides – and that wasn’t even the hottest part of the blast.

In areas where trees were afforded a bit of protection by a ridge or a goodly amount of distance, the bark stayed on, but suffered if it faced volcano: it was “dried, cracked, blistered, and partly detached from the underlying wood.” But if you walked round to the far side, the bark was intact and not cooked. Sap even continued running under it for several months, until, like a headless chicken whose nervous system finally gets the memo that the brain is now in a bag, the remains of the trees finally finished dying.

 Tree blowdown on the South Fork Toutle River showing small stand of trees protected from the force of the blast but not from the heat. Photo by Lyn Topinka. Cowlitz County, Washington. August 14, 1981. Image courtesy USGS.

Tree blowdown on the South Fork Toutle River showing small stand of trees protected from the force of the blast but not from the heat. Photo by Lyn Topinka. Cowlitz County, Washington. August 14, 1981. Image courtesy USGS.

So what are these severely singed trees saying about the blast? Well, the fallen ones testified that the burst of searing-hot gas that scorched and bent their roots either outlasted the portion of the blast that knocked them flat, or followed that unhappy event. Other evidence, which we’ll explore further when we talk about the blast deposits, indicate that the hot gas wave followed the leading edge of the blast cloud.

The burst of burning-hot gas didn’t distribute its temperature evenly. The hottest gas from the cryptodome escaped to the northeast, with another, extra-hot lobe roared off to the northwest. We know this because the surge of gas left trees smoldering and burning throughout the area. Eleven days after the eruption, geologists had a look through night-vision goggles, and discovered fires still burning out to 15 km (9 miles). The west-northwest and northeast sectors were orders of magnitude more fiery than the north: two orders to the west-northwest and a whopping three to the northeast. Wood fragments in the deposits in those sectors were baked into vitreous charcoal up to a centimeter thick; some of the smaller trees are very well done, deeply charred on the outside and brown and brittle on the inside. Some of these trees kept burning for weeks. Weeks. In the PNW. In May. The eruption was powerful enough to defeat a soggy west side spring, which is a pretty amazing feat.

 Aerial view of Mount St. Helens looking northeast. Catastrophic eruption began at 0832 PDT. Photo taken at approximately noon. The day had dawned clear, and clouds in this scene may be eruption related. Vapor rises from vent and from lakes, rivers, melted snow and ice, and from hot deposits of debris avalanche and pyroclastic flows. Ash billows from vent and from pulverized material collapsing into crater. Smoke originates from forest fires ignited by initial eruptive blasts and from later pyroclastic flows. Lightning was occurring every few seconds. No air turbulence was felt on windward side of mountain. Photo by R.M. Krimmel. Skamania County, Washington. May 18, 1980. Portion of Figure 23, U.S. Geological Survey Professional paper 1250. Image courtesy USGS.

Aerial view of Mount St. Helens looking northeast. Catastrophic eruption began at 0832 PDT. Photo taken at approximately noon. The day had dawned clear, and clouds in this scene may be eruption related. Vapor rises from vent and from lakes, rivers, melted snow and ice, and from hot deposits of debris avalanche and pyroclastic flows. Ash billows from vent and from pulverized material collapsing into crater. Smoke originates from forest fires ignited by initial eruptive blasts and from later pyroclastic flows. Lightning was occurring every few seconds. No air turbulence was felt on windward side of mountain. Photo by R.M. Krimmel. Skamania County, Washington. May 18, 1980. Portion of Figure 23, U.S. Geological Survey Professional paper 1250. Image courtesy USGS.

Trees and tree fragments and other sectors fared relatively better. The bits weren’t as thoroughly baked in the northwest-northeast sector, so that seems to have been somewhat cooler. (Not that you would’ve wanted to dabble your toes in the resulting blast deposit – it was certainly too toasty for toeses.) The southern part of the devastated area on the volcanoes east flank contained completely unburnt twigs, and the heat only ever got intense enough to yellow the incorporated needles, so that part of the blast wasn’t very hot at all. The west flank is a different story – the fragments and trees there are scorched and rather wide zone, showing that part of the blast was searing-hot. If you wanted your trees done medium-rare, this sector was probably your best bet.

Now, here’s the really wild part: all that searing, scorching, en flambéing hot blast madness doesn’t gradually fade out: it ends abruptly. The standing trees in the scorch zone show a sharp demarcation between singed and unsinged. Geologists saw the base of the scorch pattern climb “through the trees from the inner edge to the outer edge of the scorch zone.” Those pattern showed that the blast cloud rapidly lost energy and density, and when it did, it lifted like the lid being yanked off the pièce de résistance by a particularly theatrical chef.

 View along Smith Creek showing tree blowdown, singed trees, and green trees which missed the Mount St. Helens blast. Photo by Lyn Topinka. Skamania County, Washington. April 26, 1982. Image courtesy USGS.

View along Smith Creek showing tree blowdown, singed trees, and green trees which missed the Mount St. Helens blast. Photo by Lyn Topinka. Skamania County, Washington. April 26, 1982. Image courtesy USGS.

So there you are: if you want to cook a whole forest of soggy PNW trees in the spring, all you need is a volcano with a hot cryptodome bulging out its side, an earthquake, and a nice toasty lateral blast. Shame about the uneven temperatures, but that’s one of the hazards of cooking volcanically.

 Sand-blasted and lightly charred piece of a tree limb which fell on Mount Adams from the May 18 eruption cloud of ID. Mount St. Helens. Ruler for scale. Yakima County, Washington. May 18, 1980. Image courtesy USGS.

Sand-blasted and lightly charred piece of a tree limb which fell on Mount Adams from the May 18 eruption cloud of ID. Mount St. Helens. Ruler for scale. Yakima County, Washington. May 18, 1980. Image courtesy USGS.

 

Previous: The Cataclysm: “All of the Trees Seemed to Come Down at Once”

Next: The Cataclysm: “The Path of Maximum Abrasion”

Originally published at Rosetta Stones.

References:

Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.

Help Me Calculate Wooly Mammoth Populations

Ya’ll, I’m sorry, but I need you to put on your calculating hats and help a woman defeat creationists. I have numbers, but no higher math skills to work ‘em out*. Any of you care to calculate?

Here’s what I need to know: how many wooly mammoths can we expect 900 years after the Food?

Let’s give creationists the benefit of the doubt, and pretend Noah kept two wooly mammoths aboard. Let’s further say they were of breeding age when they got off the boat, and there was lots of forage, and they got it on right away. Here are the relevant stats, pulled from their closest living relatives, the Asian elephant.

Breeding age: 10-15 years until around 50-55

Gestation: around 18-22 months

Weaning: around 3 years

Which gives us a birth interval of about 4-5 years.

Life expectancy: roughly 60-70 years.

So, if our wooly mammoths pump out bebbies on the regular, and all is ideal, and we even let ‘em all live to ripe old ages, how many mammoths will we have after 900 years?

Herd of wooly mammoths. Painting by Charles R. Knight, image courtesy Wikimedia Commons.

Herd of wooly mammoths. Painting by Charles R. Knight, image courtesy Wikimedia Commons.

I’ve got plenty of other ways to show that the creationist crap being spouted about wooly mammoths in this textbook is utter bunk, but it would be nice to hoist them by their own petard, while we’re at it. Thanks for your help, my more numerate darlings!

*Gawds, I can’t math. Up until pre-algebra, I was actually pretty good at the stuff, but I got jumped ahead before I had the proper foundation, then had a string of truly awful math teachers and never recovered. I shoulda kept up on the tutorials I was doing back in the early aughts, but I let my skillz atrophy because hey writers don’t need math right?

Let this be a cautionary tale to all aspiring authors: keep your math skills polished. Otherwise, you’ll end up on the intertoobz at three in the ay-em begging your readers to do the math for you and feeling a right nitwit.