The restless Earth
It feels solid, the ground. We build cities on it and call things "set in stone" when we mean they will never change. But the Earth's surface is not fixed at all. It is cracked into enormous slabs that are, at this very moment, sliding, colliding and grinding past one another — usually at about the speed your fingernails grow. That single idea, called plate tectonics, is the closest thing geology has to a grand unifying theory. It explains earthquakes, volcanoes, mountain ranges and the shape of the continents, all at once.
A cracked shell
The Earth's rigid outer shell — the crust and the topmost mantle, together called the lithosphere — is broken into about fifteen major tectonic plates, plus many smaller ones. These plates are not floating on liquid, exactly, but on the asthenosphere, a hotter layer of the mantle that is solid rock yet flows slowly over long timescales, like extremely stiff toffee. Heat rising from deep inside the planet keeps this layer churning, and the plates ride along on top. It is why the map of the world was completely different hundreds of millions of years ago, and will be different again in the deep future.
Where the action is
The crucial point is that almost nothing dramatic happens in the middle of a plate. The action is at the edges, where plates meet, and there are three ways they can interact. They can pull apart, letting molten rock well up to form new crust, as happens along the mid-ocean ridges. They can collide, with one plate diving beneath another or the two crumpling upward — this is how the Himalayas are still rising, as India ploughs into Asia. Or they can slide past each other, snagging and slipping, as along California's San Andreas Fault. Nearly every earthquake and volcano on Earth sits along one of these boundaries.
The Ring of Fire
Nowhere shows this more vividly than the Ring of Fire, the horseshoe of tectonic boundaries that encircles the Pacific Ocean. Around three-quarters of the world's active volcanoes and the great majority of its largest earthquakes occur along this belt, from the Andes up through the west coast of the Americas, across to Japan, the Philippines and down to New Zealand. When you notice that Chile, California, Japan and Indonesia all share the same hazards, you are really noticing the same plate boundaries running beneath them all.
Magma, and the making of a volcano
Where plates dive down into the hot interior, rock melts into magma — the name for molten rock while it is still underground; once it erupts, we call it lava. Magma is lighter than the surrounding rock, so it rises, collecting in chambers until pressure forces it to the surface. Depending on how thick and gas-rich the magma is, the result can be the gentle oozing of a Hawaiian shield volcano or the catastrophic explosion of a steep stratovolcano like Vesuvius, whose eruption in AD 79 buried the Roman city of Pompeii under metres of ash.
Measuring the shaking
Earthquakes happen when stress built up along a locked fault is suddenly released, and the ground lurches. The shaking radiates outward as seismic waves, recorded by an instrument called a seismograph. The size of a quake is given as a magnitude, and here a detail trips many people up: the scale is logarithmic. Each whole number up means about ten times more shaking and roughly thirty times more energy released, so a magnitude 7 is not slightly worse than a magnitude 6 but vastly so. If the quake happens under the sea, it can shove a wall of water upward and send a tsunami racing across the ocean at the speed of a jet plane.
Living on a living planet
It can all sound alarming, but the same restless forces that threaten us also made the world habitable. Plate tectonics recycles carbon, builds fertile land, raises the mountains that shape our weather and drives the slow chemistry that has kept the planet's climate liveable for billions of years. We do not live on a dead rock. We live on a machine — one that occasionally, and spectacularly, reminds us that it is still running.