Definition · Plain-language
Tectonic plates
Tectonic plates are the giant, slowly moving slabs that make up the rigid outer shell of the Earth.
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Earth’s broken outer shell
The Earth’s rigid outer layer, the lithosphere, is not one continuous shell but is broken into a number of large and small pieces called tectonic plates. These plates carry both the continents and the ocean floor, and they rest on the asthenosphere, a hotter, partly soft layer of the mantle that allows them to move. Driven mainly by heat from deep within the Earth, the plates drift extremely slowly — typically a few centimetres a year, about as fast as fingernails grow. Over millions of years this steady motion has rearranged continents and reshaped the globe.
What happens at plate boundaries
Most of Earth’s dramatic geology occurs where plates meet, at their boundaries. At convergent boundaries plates push together: one may dive beneath another (subduction), building volcanoes and deep ocean trenches, or two continents may collide and crumple upward into mountain ranges. At divergent boundaries plates pull apart, and molten rock rises to form new crust, as along mid-ocean ridges. At transform boundaries plates grind past each other sideways, storing and releasing strain as earthquakes. This is why earthquakes and volcanoes are concentrated in belts that trace the plate edges.
The theory of plate tectonics
Plate tectonics is the unifying theory that explains how these plates move and interact, and it transformed geology in the twentieth century. It grew out of the earlier idea of continental drift, the observation that continents seem to fit together and share matching rocks and fossils, but which lacked a mechanism. Plate tectonics supplied that mechanism, showing that the plates move and that the sea floor spreads. Today the theory accounts for the distribution of earthquakes, volcanoes and mountain belts, the shapes and history of the continents, and the slow recycling of crust in the rock cycle.
Key facts
At a glance
- Definition: large rigid slabs of Earth’s outer shell (the lithosphere)
- They rest on: the hotter, softer asthenosphere below
- Speed: very slow — a few centimetres a year
- Three boundary types: convergent, divergent and transform
- They cause: most earthquakes, volcanoes and mountain ranges
- The theory: plate tectonics, building on continental drift
Common misconceptions
What people often get wrong
Often heard: Tectonic plates only carry the continents, not the oceans.
Actually: Plates include both continental and oceanic crust. Much of a plate is ocean floor, and the boundaries where plates meet often lie under the sea.
Often heard: Tectonic plates move fast enough to feel day to day.
Actually: Plates creep at only a few centimetres a year, roughly the speed your fingernails grow. The sudden jolts we feel as earthquakes are the release of strain that built up slowly over many years.
Often heard: Continental drift and plate tectonics are the same theory.
Actually: Continental drift was the early observation that continents have moved, but it lacked a mechanism. Plate tectonics is the later, fuller theory that explains how and why the plates move.
