Earth’s moving plates — Earth science, 11–13 years
Earth’s hard outer shell is broken into huge plates that move slowly, changing the surface and causing many earthquakes and volcanoes.
A cracked, moving shell
Earth’s rocky outer shell is not one unbroken piece. It is divided into enormous plates, like pieces of a very slow jigsaw, floating on hotter, softer rock below. The plates move only a few centimetres each year, but over millions of years that is enough to build mountains, open oceans and shift continents.
Why plates matter
People once wondered why the coastlines of Africa and South America seem to fit, and why the same fossils occur on now-separated continents. Plate tectonics explains these clues: continents ride on moving plates. It also gives one connected reason for many earthquakes, volcanoes and mountain ranges instead of treating each as a separate mystery.
Reading a plate boundary
Imagine two plates moving towards each other beneath the ocean. First, the denser oceanic plate bends and sinks below the other. Next, the sinking rock heats and helps produce magma. Finally, magma rises, forming volcanoes, while pressure along the boundary can cause earthquakes. This is why a map may show both volcanoes and earthquakes in a long belt.
The slow-motion mistake
It is reasonable to think that movement of only a few centimetres per year cannot matter. We usually notice motion only when it is fast, and a plate is too large for its movement to be felt like a moving object. The mistake is forgetting the time scale: five centimetres each year becomes five kilometres in 100,000 years.
Using plate maps
Scientists use plate-boundary maps to study earthquake and volcano patterns. The maps cannot tell the exact day of a future earthquake, but they show where the hazard is more likely and help engineers plan buildings, bridges and emergency routes. The same idea helps explain why some islands grow from volcanic eruptions while others are worn down by waves.
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