Why the Ground Moves: The Secret Life of Tectonic Plates

Right now, under the runners of this old rocking chair, the ground is creeping away. Steady on, Earl! Don't go sliding down the garden path just yet. I've only just set down me brew. I am dead serious, Bridget. Last time out, we looked at how that microscopic twisted ladder of DNA writes the whole recipe book of life. But today, the whole world beneath our boots is doing something even wilder. We are all passengers on giant, creeping rafts of solid rock. Ey up. Rafts? You mean the continents aren't bolted down to the bottom of the sea? Not even a little bit. Now, I know how that sounds.

But hang on. The ground feels solid as an iron rail, don't it? Truth is, the outer crust of our planet is cracked all to pieces, like an eggshell you tapped on the edge of a skillet. Geologists call those broken pieces tectonic plates. A cracked eggshell. Well, that's a cheerful thought for a Tuesday. So what's keeping the yolk inside then, love? A massive layer called the mantle, sitting right under the crust. Up top, the crust and the very tippy-top of the mantle make a brittle shell called the lithosphere. But down deeper, that mantle rock gets so scorching hot and under so much pressure that it turns into something like stiff, warm tar.

It can actually churn and flow. Ooh, like a pan of thick porridge bubbling away on the back burner! Exactly like porridge! That deep heat from the Earth's core pushes hot rock upward. As it cools near the top, it sinks back down. Those churning currents drag the stiff plates riding on top of them right along for the ride. Well, how fast are these great rafts actually sailing, Earl? Should I be grabbing hold of the cat? You can let the cat sleep, Bridget. Most plates crawl along between one and ten centimeters every single year. Centimeters? That's... well, that's nowt! That is about the exact speed your fingernails grow.

But give a fingernail two hundred million years of runway, and it will push an entire ocean basin wide open. Two hundred million years. That's hard to get your head around, isn't it? It truly is. But look at what happens when those giant slabs of rock meet. Because they aren't all drifting in the same direction. When two continental plates crash head-on, neither one wants to sink. So they've got nowhere to go but up. You hit the nail right on the head. They crumple and shove the crust straight into the sky. That is how the Himalayas were built. India slammed into Eurasia fifty million years ago, and Mount Everest is STILL growing taller by about four millimeters every year because of that shove.

Four millimeters! So the roof of the world is still stretching its legs. But what about when they pull apart from each other? Then the Earth just unzips. Magma wells up from the mantle to fill the gap, cools down against cold seawater, and makes brand new ocean floor. That Mid-Atlantic Ridge is running right down the middle of the sea, making new rock every single hour. And what about the rough stuff? The earthquakes? That's when they get stuck, aren't it? That's the real drama. Two plates grind side-by-side, like along the San Andreas Fault in California. The rock is jagged, so it snags.

The convection currents keep pushing from below, the elastic strain builds up year after year, and then... snap. It slips three feet in three seconds. Like a rusty porch latch suddenly giving way. All that pent-up muscle let loose at once. All that energy radiates out as seismic waves. Shakes the whole county. You know, until about sixty years ago, clever folk in university halls didn't even believe it. Alfred Wegener pointed out that South America and Africa fit together like jigsaw pieces, and they laughed him right out of the room! They surely did. They asked him what engine could possibly shove a continent.

It wasn't until the nineteen-sixties, when sailors mapped the ocean floor and found magnetic stripes frozen in spreading rock, that everyone had to tip their caps. The plates were moving after all. Eh, fancy that. The ground never was asleep. It's just taking its own sweet time. Well. That is about the size of it. We're riding a living engine, Bridget. Right then, love. I think I'll finish this brew before Whitby drifts any further north. Night night, sleep tight, chuck.