Plate Tectonics and Earthquakes

Look at a map of the world and the east coast of South America fits into the west coast of Africa like two pieces of a jigsaw. People noticed this almost as soon as there were decent maps. In 1912 the German scientist Alfred Wegener proposed that the continents had once been joined and had drifted apart. He was ridiculed for decades. Then, in the 1960s, surveys of the ocean floor proved him right, and geology gained its unifying theory.

A cracked shell

Earth's outer layer, the crust together with the rigid top of the mantle, is broken into about a dozen large plates and several smaller ones. They fit together like the plates of a turtle's shell, but they are not fixed. Beneath them the mantle is hot enough to flow slowly, and the plates ride on it, moving a few centimetres a year, about the speed at which your fingernails grow. That sounds trivial. Over a hundred million years it adds up to thousands of kilometres.

The continents are passengers. They are made of lighter rock that floats high on the plates, so they are carried along as the plates move, colliding, splitting and reassembling over geological time. About 250 million years ago they were all joined in a single supercontinent, Pangaea. It began to break apart around 200 million years ago, opening the Atlantic. Before Pangaea there were earlier supercontinents, and in a few hundred million years the continents will gather again.

Where plates meet

Almost all the drama happens at the edges. Where two plates pull apart, hot mantle rises to fill the gap, cools and becomes new ocean floor. This is happening along the Mid-Atlantic Ridge, a submarine mountain chain running the length of the Atlantic, where Iceland is one of the few places it pokes above the sea. The Atlantic widens by about 2.5 centimetres a year.

Where plates push together, one usually slides beneath the other and sinks back into the mantle, a process called subduction. Ocean floor is consumed this way, which is why no seafloor is older than about 180 million years while continental rocks reach back four billion. Subduction zones generate the deepest earthquakes, the biggest volcanoes and the deepest ocean trenches. When two continents collide, neither will sink, so the crust crumples upward. India has been ploughing into Asia for 50 million years, and the result is the Himalaya and the Tibetan plateau. Everest is still rising about four millimetres a year.

Where plates slide sideways past each other, as along California's San Andreas Fault, they do not move smoothly. They stick, strain builds for decades or centuries, and then they lurch.

What an earthquake is

An earthquake is that lurch. Rock on either side of a fault is locked by friction while the plates keep moving, bending the rock like a stick under growing pressure. When the stress exceeds the friction, the fault slips, sometimes by metres in a few seconds, and the stored energy is released as vibrations that race through the ground. The point where slipping begins is the focus; the spot on the surface directly above it is the epicentre.

Earthquakes are measured on the moment magnitude scale, which replaced the older Richter scale. It is logarithmic: each whole number means about 32 times more energy released. A magnitude 5 rattles dishes. A magnitude 7 destroys buildings. The largest ever recorded, magnitude 9.5 in Chile in 1960, released energy equivalent to thousands of hydrogen bombs and sent a tsunami across the Pacific to Japan. The 2011 Japan earthquake, magnitude 9.1, shifted the main island 2.4 metres east and shortened the day by a tiny fraction of a second.

Around half a million earthquakes a year are large enough to detect, and about a hundred cause damage. Most occur along plate boundaries, which is why Japan, Chile, Indonesia, California and Turkey shake so often, while places in the middle of plates rarely do.

Living with a moving planet

Earthquakes cannot be predicted; nobody can say the day or year one will strike. But their likelihood can be mapped, and buildings can be designed to sway rather than collapse. Modern seismic networks detect the first, fastest waves of a quake and send warnings seconds before the destructive shaking arrives, enough time to stop trains and open fire station doors. Most earthquake deaths come not from the shaking itself but from falling buildings, which is why the same magnitude quake can kill thousands in one country and nobody in another.

Why it matters

Plate tectonics is not just the cause of disasters. It built every mountain range, shaped every coastline and ocean, and brought the metals we mine to the surface. By burying carbon-rich rock and releasing carbon dioxide through volcanoes, it has acted as a planetary thermostat for billions of years, keeping Earth's climate within the range where liquid water and life can exist. Earth is the only planet we know with active plate tectonics, and that may be one of the reasons it is the only one we know with life.