“Kareem, will you ever actually tell me what’s going on with the volcanoes in Italy and Greece and Turkey? And do it quick, I gotta start getting ready for the lunch trade.”

“Eddie, you’re the one who keeps asking the side questions. Sy, I see you’re carrying Old Reliable.”

“I always travel ready for action, Kareem.”

“You got the GPlates system loaded in there? It’s a go‑to tool in our Geophysics lab.”

“Matter of fact, I do, but I’ve not had time to start playing with it. Here, show us what it can do.”

“I’ve got a particular display in mind, give me a minute. <busy‑fingers pause> There. What you’re looking at is Planet Earth as we think it was 195 million years ago.”

“Is that Pangaea?”
 ”Is that Pangaea?”

“Sure is. Most of Earth’s high‑silica slag had sutured together in one big supercontinent that stretched from pole to pole.”

“What’s on Earth’s other side?”

“Mostly a huge ocean, which is why I colored it flat blue. There were probably seamounts and rifts and stuff scattered around the seabottom but all that high‑density low‑silica structure is long gone, shoved below by the continents that rode over it. This is a snapshot at the time when Pangaea was just beginning to come apart — you can see where South America is ripping away from Africa at their southern juncture, and North America’s just started to move off to the west.”

“What’s the difference between the light blue and the darker blue?”

“Good eyes, Eddie, and it’s important. The light blue is the continental shelf.”

“It’s not part of the continent?”

“Oh, it is. The shelf’s the flooded margin, partly ancient consolidated rock and partly sediments that have washed down over the ages. There’s usually a steepish drop‑off from the shelf down to the abyssal bottoms. My hero Wegener is the guy who realized that when you’re putting the jigsaw puzzle together, the shelf is the border you need to pay attention to.”

“What’s the yellow-line kite shape?”

“It ties together some points that’ll help answer Eddie’s Italy‑Greece‑Turkey question. Let me put the video in motion…”

Earth from 195 million years ago to the present
rendered using the GPlates system
and configuration data from Müller, et al., 2019, doi.org/10.1029/2018TC005462

“I see you’ve got Africa in the center instead of the usual New World axis.”

“Why not? Anyway it’s convenient for Eddie’s volcanoes. See that fragment at the kite’s eastern corner? I marked it with dark circle. Watch what happened to it about 55 million years ago, and where it went after that.”

“It banged into what’s gonna be Turkey!”

“Mm-hm, and the land crinkled up and that’s the origin of Turkey’s volcano belt that I’ve marked purple. GPlates calls that chunk the Kirsehir plate. No connection with the vulcanism further west.”

“Wait. That little thing is a plate?”

“The definition depends on who you’re talking to and what about. Officially we’ve got cratons, major plates, minor plates, microplates and terranes, but there’s fuzzy lines between them. GPlates ‘plate’ list contains about a thousand chunks that have moved around independently and are big enough to pay attention to.”

“I can see why you called it the Africa‑Eurasia nutcracker, Kareem. It crunches right down on that continental shelf north of Africa.”

“That’s the planet’s oldest bit of seafloor, Sy, maybe 300 million years old, half again older than anywhere else. Maybe the rock got brittle with age, but the collision region’s faults and folds are incredibly complex.”

“It’s a hot mess, HAW!”

“Can’t say you’re wrong, Eddie. Anyway, south and west of Turkey there’s a whole series of trenches where north‑bound seafloor crust dives under south‑bound structures. The sunken material melts, puffs up and pushes up against what’s above it. All of that leaves beaucoodles of weak spots for magma to leak upwards and you get volcanoes throughout the red‑marked area.”

“One thing I get from this, Eddie, is that it’s not one long arc from Italy through Turkey. Kareem’s pointed out two different formation periods, 50 million years apart.”

“I get that, too, Sy. It’s amazing what you can see when you look close.”

“And when hundreds of researchers gather data over two centuries.”

“Thanks, Kareem. Gotta go.”

~~ Rich Olcott

Where would you put it all?

Vinnie’s a big guy but he’s good at fading into the background. I hadn’t even noticed him standing in the back corner of Cathleen’s impromptu seminar room until he spoke up. “That’s a great theory, Professor, but I wanna see numbers for it.”

“Which part of it don’t you like, Vinnie?”

“You made it seem so easy for all those little sea thingies to scrub the carbon dioxide out of Earth’s early atmosphere and just leave the nitrogen and oxygen behind. I mean, that’d be a lot of CO2. Where’d they put it all?”

“That’s a reasonable question, Vinnie. Lenore, could you put your Chemistry background to work on it for us?”

“Oh, this’ll be fun, but I don’t want to do it in my head. Mr Moire, could you fire up Old Reliable for the calculations?”

“No problem. OK, what do you want to calculate?”

“Here’s my plan. Rather than work with the number of tons of carbon in the whole atmosphere, I’ll just look at the sky-high column of air sitting on a square meter of Earth’s surface. We’ll figure out how many moles of CO2 would have been in that column back then and then work on how thick a layer of carbon stuff it would make on the surface. Does that sound like a good attack, Professor?”

“Sure, but I see a couple of puzzled looks in the class. You’d better say something about moles first.”

“Hey, I know about moles. Sy and me talked about ’em when he was on that SI kick. They’re like a super dozen, right, Sy?”

“Right, Vinnie. A mole of anything is 6.02×1023 of that thing. Eggs, atoms, gas molecules, even stars if that’d be useful.”

“Back to my plan. First thing is the CO2 was in that column back when. Maria, your chart showed that Venus’ atmospheric pressure is 100 times ours and Mars’ is 1/100 ours and each of them is nearly pure CO2, right? So I’m going to assume that Earth’s atmosphere was what we have now plus a dose of CO2 that’s the geometric mean of Venus and Mars. OK, Professor?”

“That’d be a good starting point, Lenore.”

“Good. Now we need the mass of that CO2, which we can get from the weight of the column, which we can get from the air pressure, which is what?”

Every car buff in the room, in chorus — “14½ pounds per square inch.”

“I need that in kilograms per square meter.”

“Strictly speaking, pressure’s in newtons per square meter. There’s a difference between weight and force, but for this analysis we can ignore that. Keep going, Lenore.”

“Thanks, Professor. Sy?”

“Old Reliable says 10194 kg/m².”

“So we’ve got like ten-thousand kilograms of CO2 in that really tall meter-square column of ancient air. Now divide that by, um, 44 to get the number of moles of CO2. No, wait, then multiply by 1000 because we’ve got kilograms and it’s 44 grams per mole for CO2.”

“232 thousand moles. Still sounds like a lot.”

“I’m not done. Now we take that carbon and turn it into coal which is solid carbon mostly. One mole of carbon from each mole of CO2. Take the 232 thousand moles, multiply by 12 grams, no make that 0.012 kilogram per mole –“

“2786 kilograms”

“Right. Density of coal is about 2 grams per cc or … 2000 kilograms per cubic meter. So. Divide the kilograms by 2000 to get cubic meters.”

“1.39 meters stacked on that square-meter base.”

“About what I guessed it’d be. Vinnie, if Earth once had a carbon-heavy atmosphere log-halfway between Venus and Mars, and if the sea-plankton reduced all its CO2 down to coal, it’d make a layer all over the planet not quite as tall as I am. If it was chalk it’d be thicker because of the additional calcium and oxygen atoms. A petroleum layer would be thicker, too, with the hydrogens and all, but still.”

Jeremy’s nodding vigorously. “Yeah. We’ve dug up some of the coal and oil and put it back into the atmosphere, but there’s mountains of limestone all over the place.”

Cathleen’s gathering up her papers. “Add in the ocean-bottom carbonate ooze that plate tectonics has conveyor-belted down beneath the continents over the eons. Plenty of room, Vinnie, plenty of room.”

~~ Rich Olcott

The Moon And Chalk

Cathleen’s talking faster near the end of the class. “OK, we’ve seen how Venus, Earth and Mars all formed in the same region of the protosolar disk and have similar overall compositions. We’ve accounted for differences in their trace gasses. So how come Earth’s nitrogen-oxygen atmosphere is so different from the CO2-nitrogen environments on Venus and Mars? Let’s brainstorm — shout out non-atmospheric ways that Earth is unique. I’ll record your list on Al’s whiteboard.”


“Plate tectonics!”



“The Moon!”

“Wombats!” (That suggestion gets a glare from Cathleen. She doesn’t write it down.)

“Goldilocks zone!”

“Magnetic field!”


She registers the last one but puts parentheses around it. “This one’s literally a quickie — real-world proof that human activity affects the atmosphere. Since the 1900s gaseous halogen-carbon compounds have seen wide use as refrigerants and solvents. Lab-work shows that these halocarbons catalyze conversion of ozone to molecular oxygen. In the 1970s satellite data showed a steady decrease in the upper-atmosphere ozone that blocks dangerous solar UV light from reaching us on Earth’s surface. A 1987 international pact banned most halocarbon production. Since then we’ve seen upper-level ozone concentrations gradually recovering. That shows that things we do in quantity have an impact.”

“How about carbon dioxide and methane?”

“That’s a whole ‘nother topic we’ll get to some other day. Right now I want to stay on the Mars-Venus-Earth track. Every item on our list has been cited as a possible contributor to Earth’s atmospheric specialness. Which ones link together and how?”

Adopted from image by Immanuel Giel, CC BY-SA 3.0

Astronomer-in-training Jim volunteers. “The Moon has to come first. Moon-rock isotope data strongly implies it condensed from debris thrown out by a huge interplanetary collision that ripped away a lot of what was then Earth’s crust. Among other things that explains why the Moon’s density is in the range for silicates — only 60% of Earth’s density — and maybe even why Earth is more dense than Venus. Such a violent event would have boiled off whatever atmosphere we had at the time, so no surprise the atmosphere we have now doesn’t match our neighbors.”

Astrophysicist-in-training Newt Barnes takes it from there. “That could also account for why only Earth has plate tectonics. I ran the numbers once to see how the Moon’s volume matches up with the 70% of Earth’s surface that’s ocean. Assuming meteor impacts grew the Moon by 10% after it formed, I divided 90% of the Moon’s present volume by 70% of Earth’s surface area and got a depth of 28 miles. That’s nicely within the accepted 20-30 mile range for depth of Earth’s continental crust. It sure looks like our continental plates are what’s left of the Earth’s original crust, floating about on top of the metallic magma that Earth held onto.”

Jeremy gets excited. “And the oceans filled up what the continents couldn’t spread over.”

“That’s the general idea.”

Al’s not letting go. “But why does Earth have so much water and why is it the only one of the three with a substantial magnetic field?”

Cathleen breaks in. “The geologists are still arguing about whether Earth’s surface water was delivered by billions of incoming meteorites or was expelled from deep subterranean sources. Everyone agrees, though, that our water is liquid because we’re in the Goldilocks zone. The water didn’t steam away as it probably did on Venus, or freeze below the surface as it may have on Mars. Why the magnetic field? That’s another ‘we’re still arguing‘ issue, but we do know that magnetic fields protect Earth and only Earth from incoming solar wind.”

“So we’re down to photosynthesis and … limestone?”

“Photosynthesis was critical. Somewhere around two billion years ago, Earth’s sea-borne life-forms developed a metabolic pathway that converted CO2 to oxygen. They’ve been running that engine ever since. If Earth ever did have CO2 like Venus has, green things ate most of it. Some of the oxygen went to oxidizing iron but a lot was left over for animals to breathe.”

“But what happened to the carbon? Wouldn’t life’s molecules just become CO2 again?”

“Life captures carbon and buries it. Chalky limestone, for instance — it’s calcium carbonate formed from plankton shells.”

Jim grins. “We owe it all to the Moon.”

~~ Rich Olcott