Plate Tectonics: the Evidence and the Engine

The ground under your feet feels perfectly still, but Earth's outer shell is moving all the time. That thin shell is broken into huge pieces called tectonic plates. They fit together like a puzzle, except the puzzle pieces drift. The plates rest on the mantle, a very thick layer of hot rock that flows slowly, and that flow carries the plates in different directions. Plates can pull apart, crash together, slide past each other, or dive one under the other. Each kind of meeting builds something different: new sea floor, mountain ranges, deep ocean trenches, or earthquake faults. So the direction a plate moves decides what you will find at its boundary. When you look at a volcano or a mountain chain on a map, you are usually looking at the edge of a moving plate.

Nobody can drill down and watch Earth's deep interior, yet scientists are sure the mantle is in constant slow motion. The reason is convection. Convection is what happens when warmer, less dense material rises and cooler, denser material sinks. You can see it in a pot of soup on the stove. In the mantle the same thing happens with rock, only it creeps along over millions of years. Those differences in density create buoyant forces that push material up in some places and pull it down in others. This is the engine that was missing when people first proposed drifting continents. Wegener could show the continents had moved, but he could not say what pushed them, so most geologists rejected the idea. Once the sea floor was shown to be spreading and the pattern of magnetic reversals in the rocks confirmed it, mantle convection supplied the driving force the theory needed.

At the mid-Atlantic ridge, volcanoes push out fresh rock that fills the gap where two plates are moving apart. When scientists measured the magnetism of that rock, they found something strange: the direction of magnetization flips back and forth in stripes as you move away from the ridge, and the stripes match on both sides like a mirror image. What does that tell us? As each batch of hot rock cools, it records the direction of Earth's magnetic field at that moment, like a compass frozen in stone. Earth's field has flipped many times over its history, so the sea floor preserves those flips as stripes. If you know how fast the plates are spreading, you can match stripe widths to times and read off the history of the magnetic field.

Matching coastlines and fossils show that continents moved long ago, but GPS can watch them move right now. GPS stations bolted into solid bedrock report their position so precisely that a shift of a few millimetres a year is easy to measure. In this unit, students read real GPS data from California, where the Pacific Plate slides past the North American Plate. The stations show the ground moving in different directions on each side of the boundary, slowly bending the rock like a spring. When that stored-up bend finally snaps, the result is an earthquake. This is a different kind of evidence from fossils or rock layers: it measures plate motion directly, in the present day, and it helps scientists judge where future earthquakes are most likely. It turns plate tectonics from a story about the past into something you can measure this year.