Learn · Plate tectonics · 7 minute read
Moving plates
The ground is broken into a dozen or so plates. The Atlantic gets 2.5 cm wider every year. Here is how we know.
Look at a world map and South America seems to fit into Africa like a jigsaw piece. People noticed in the 1500s. In 1912 a German meteorologist, Alfred Wegener, argued that the continents had once been joined and had drifted apart. Most geologists rejected it: nobody could explain what would push a continent. The answer came from the sea floor, fifty years later.
A cracked shell
The rigid outer layer of the Earth, the crust and the top of the mantle together, is called the lithosphere. It is at least 80 km thick over much of the planet, and it is broken into a dozen or so large plates and several smaller ones. They ride on hotter mantle rock below that is solid but soft enough to flow, very slowly.
Plates move a few centimetres a year. The Mid-Atlantic Ridge opens at about 2.5 cm a year, or 25 km in a million years. The East Pacific Rise, west of Chile, spreads fastest, at more than 15 cm a year. At 2.5 cm a year, the Atlantic is about 13 metres wider than when Columbus crossed it in 1492 (our own sum).
Three ways to meet
Where two plates meet, they can do one of three things.
Pulling apart. Hot mantle rises, melts a little, and fills the gap with new rock. Most of this happens on the sea floor, along a mountain chain that runs down the middle of the Atlantic. Iceland is a piece of it that pokes above the sea.
Pushing together. Ocean plates are dense, so when one meets another plate it usually sinks underneath, back into the mantle. This is subduction. Water carried down with it helps the mantle above melt, which feeds lines of volcanoes such as the Andes and Japan. When two continents meet, neither sinks easily, and the crust crumples into high mountains. The Himalaya are India running into Asia.
Sliding past. Plates grind sideways along a fault. The San Andreas Fault in California is the famous one: the Pacific Plate has been moving past the North American Plate there at about 5 cm a year for about 10 million years.
The tape recorder on the sea floor
Every so often, over hundreds of thousands of years, the Earth's magnetic field flips: north becomes south. When lava cools at a ridge, tiny magnetic minerals in it line up with the field of the day and are frozen in place. In the 1950s, ships towing magnetometers found that the sea floor is striped, with bands that point one way, then the other.
In 1963 Frederick Vine and Drummond Matthews in Britain, and Lawrence Morley in Canada, saw what that meant. The stripes are a recording. New sea floor forms at the ridge and is carried away on both sides, so the pattern on one side of a ridge mirrors the other. That was the push Wegener had been missing: the sea floor itself is moving.
Where the ground is restless
Plate edges are where most of the action is. About 90% of the world's earthquakes happen around the edge of the Pacific, the so-called Ring of Fire, where plates sink under one another. Britain sits in the middle of a plate, far from any edge, which is why its earthquakes are small and its volcanoes long dead. That was not always true: walk through The Walk and you will find England and Scotland arriving from opposite sides of an ocean.