Plate tectonics reshapes Earth — Science, 14–17 years
Earth's outer rocky layer is divided into moving plates whose interactions build mountains and cause earthquakes.
What plate tectonics means
Earth's rigid outer shell is broken into huge plates that move slowly over hotter, softer rock below. Most movement happens at plate boundaries. Plates can pull apart, collide or slide past one another, so the same process explains ocean ridges, deep trenches, volcanoes and many earthquakes.
Why the idea was needed
Continents seemed to fit together, and matching fossils and rocks appeared on shores now far apart. The puzzle was explaining how they could move. Evidence from the ocean floor showed new crust forming at ridges and older crust sinking at trenches, turning continental drift into the wider theory of moving plates.
Reading a plate boundary
Imagine an oceanic plate moving towards a continental plate. The oceanic rock is denser, so it bends and sinks beneath the continent at a subduction zone. As it descends, it can help generate magma; the result may be a deep trench, a chain of volcanoes and frequent earthquakes along the boundary.
The tempting mistake
A reasonable first picture is that continents plough through solid ocean floor like boats through water. They do not: continents are parts of thicker plates, and whole plates move over the softer layer below. Also, plate motion is usually only a few centimetres per year, but stress can build for decades before a sudden earthquake.
Where it is used
Geologists use plate boundaries to identify regions more likely to experience earthquakes, volcanoes or tsunamis. This helps with hazard maps, building standards and emergency planning, although it cannot predict the exact time of an earthquake. Plate tectonics also guides the search for some mineral deposits and explains why landscapes differ around the world.
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