“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…”

“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

“Wait, Kareem, Eurasia and Africa acting like a nutcracker? I thought Africa is moving straight east, away from the Mid‑Atlantic Ridge.”

“Uh-uh, Sy, everything seems to be swinging around eastern Turkey, Africa going northeast—”

“Africa’s moving? How does anybody know that? How does this ‘continental drift‘ even work?”

“Eddie, those are the questions that messed up Alfred Wegener’s reputation.”

<Eddie settles back in his seat> “I can see this is gonna go on for a while.”

“A little bit. Don’t go throwing shade at my man Wegener, Sy. He wrote the standard textbook in meteorology. He was honored as a pioneer in polar weather studies. He even died saving other people during a polar expedition. His book Origins of The Continents And The Oceans went through four editions so it’s not like his proposals were ignored. The old-line geologists weren’t happy because he used evidence from outside their field and besides, they didn’t believe there was a power source big enough to move continents.”

“Evidence from outside the field?!! The nerve!”

“Nuts, huh? Like, <putting up fingers> Meteorology — glacier tracks from what are now tropical areas. Paleontology — fossils of animal and plant species found on multiple continents that are far apart today. Cartography — everyone who’s seen a world map has picked up on how South America almost fits into Africa’s west coast. Wegener showed that the fit’s much better when you work with the continental shelves. Better yet, he showed how all the major land masses could fit together that way into one big supercontinent.”

“Pangaea!”
 ”Pangaea!”

“Right, except being German he called it der Urkontinent, meaning the original continent. In his day there were well‑documented geological surveys that matched layers and faults between the North American Appalachians and the Caledonian formations in Scotland and Norway. Not even those forced the geologists to buy in until oceanographers in the 1950s came up with new kinds of evidence that sealed the case. Poor Wegener was 25 years dead by then.”

<Eddie sits up in his seat> “OK, everything moves, but how do you know what direction?”

“Magnetic measurements are a prime data source. When magma exits a volcano, its magnetic atoms like iron tend to align with Earth’s magnetic field and get locked that way when the magma freezes to become lava. Measure the magnetism of a good lava field, you know which way was north at the time of eruption. If it’s a continental lava field you can even grab rocks and assay their radioisotopes to date the field. Do that with two related fields and you can work out where the mass was going at the time and how fast it was getting there.”

<Eddie sits forward in his seat> “So what’s the answer to the geologists’ biggest gripe?”

“The power source? We didn’t have a clue until those 1950s oceanographics guys started mapping magnetic fields and comparing them with improved sonar maps of the ocean bottom. We’d long known about seamounts in the middle of the Atlantic, but sonar scans revealed they’re links in a continuous 10000‑mile chain centered on a broad ridge. It’s almost a single 10000‑mile‑long volcano. Meanwhile, magnetometer scans showed a strong signal right over the chain, just as you’d expect for a lava field. What researchers didn’t expect was two parallel sets of magnetic stripes on either side of the ridge. The stripes march all the way to the coasts on either side. That explained everything, almost.”

“Not to me, it doesn’t”

“Oh, I forgot to mention that we already had evidence from continental lava fields that Earth’s magnetic field flips every half‑million years about, and no, we don’t know why that happens, that’s the ‘almost.’ Anyway, each stripe echoes the field direction at the time it froze to make fresh seafloor. Each flip starts a new pair of stripes sliding away. It all fits a model assuming that below the seafloor there’s a 10000‑mile‑long roll of rising magma in the upper mantle. The rolling pushes up to create the ridge, which cracks open at the center to create the volcanoes. Meanwhile the magma diverges to either side and pushes the Americas apart from Africa and Europe.”

“But what about Italy and Greece?”

~~ Rich Olcott

Eddie gets impatient. “OK, I get why volcanoes don’t spit metal, but why do they line up like we got across Italy, Greece and Turkey?”

Kareem gets repetitive. “Like I said, tectonics.”

“Sounds like a brand name for fancy fizzy water. What’s that really?”

“Directly it’s a reference to mountain‑building. Really it’s about everything that happens when the continents move around. That starts with light things floating on top of dense things, like a planet’s rocky material floating on the core.”

“Wait, rocks are heavy. Why should they float on anything?”

“Depends on the rock. Pumice floats on water, but it cheats because it’s loaded with bubbles. Most rocks don’t have bubbles, though. I think of them as compact silicon dioxide structures with an optional sprinkling of metal ions. A silicon atom weighs twice as much as an oxygen, but single iron and nickel atoms weigh nearly as much as an entire SiO₂ unit. When everything’s all molten, like back when the proto‑planet was being pelted with millions of asteroids and stuff, atoms can move around and dense ones tend to move downward. Light atoms in the way get shoved towards the surface. Geologists call the process differentiation. Anyway, what you wind up with is a hot core of iron, nickel and other heavy atoms. The core’s surrounded by coats of lighter atoms, mostly silicon and oxygen because those were the most common atoms in the gas cloud we started with.”

“Not hydrogen?”

“Hydrogen was there originally, Sy, but many geologists think that the metal‑silicate mishmash was so hot that most hydrogen atoms shot from the mix beyond escape velocity and just sped off. Solar radiation drove them out to where the gas‑giant planets could capture them. The same geologists think the hydrogen we have now came later, as H₂O from incoming comets. There’s a lot of argument on the whole issue.”

“That’s all good, Kareem, but when does the tectonics happen?”

“About 4 billion years ago, Eddie, when the asteroid bombardment tapered off. That shut down a major heat energy source so things started to cool off. Each layer cooled off at a different rate. The silica‑rich slag that rose to the surface radiated heat directly to the Universe and formed a solid crust. Meanwhile the metal‑rich layers inside stayed fluid but contracted.”

“Wait, if the inside shrinks but the outside’s a solid it’d crinkle up like a grape going raisin.”

“Absolutely, and some of us think that’s what happened with Mars and maybe Pluto. That crinkle‑up kind of mountain building is called ‘thrust tectonics.’ There’s evidence that Mars now has a ‘tight cap’ structure with a continuous crust that completely envelops the planet. Along with volcanoes and meteor craters, thrust tectonism seems to have been a major landscape driver there.”

“If there’s a tight cap, there ought to be a loose cap.”

“There is, Sy, and we’re standing on it. About 30% of Earth’s surface is continental crust, high in silica and light metals like aluminum. The other 70% is oceanic crust, which is much thinner. It’s also denser because it’s richer in heavier metals like iron. Some people like the theory that Earth once had a tight‑cap crust of continental material, but a catastrophic collision tore off most of it and gave us the Moon. Anyhow, what continental crust we have is in pieces that are loose enough to wander across the surface.”

“This is starting to sound familiar. I bet they bump into each other, right?”

“On-target, Eddie. The big pieces are called plates. The study of ‘plate tectonics‘ is about the ways they collide.”

“Wait, they got different ways to collide?”

“Oh, yes. The simple case is an equal‑density collision, like north-bound India crashing into Asia. The edges of the plates crinkle up to make mountain chains like the Himalayas. More interesting things happen in a different‑density collision. The low‑density continental crust rides up over the high‑density oceanic crust, drives it down into the hot interior where it melts and rises up, burrowing through anything above it to make—”

“Volcanoes! And my Italy‑Greece‑Turkey line—”

“Is probably the leading edge of what may be the planet’s oldest ocean crust, squeezed in by the Eurasia‑Africa nutcracker.”

~~ Rich Olcott