Just Floating Along

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