Tectonic Plates
Understand that tectonic plates are pieces of Earth's rigid outer shell, including crust and uppermost mantle, that move over hotter, slowly deforming mantle rock, usually a few centimetres per year.
What a learner can do afterwards
- Describe tectonic plates as pieces of crust and rigid uppermost mantle.
- Explain that solid plates move over hotter, mostly solid mantle rock that can flow slowly.
- State that plates usually move a few centimetres per year; use a given constant yearly movement to calculate total distance with the same unit.
1 · Read
You know Earth's crust is its thin, rocky outside layer. Beneath it is mantle rock. The crust and the stiff uppermost mantle together form Earth's rigid outer shell, called the lithosphere. This shell is split into huge pieces called tectonic plates. A plate can carry land, ocean floor, or both. The Ring of Fire's earthquake and volcano clusters lie near plate edges.
Plates rest on hotter, softer mantle rock underneath. This supporting rock is mostly solid. It can slowly change shape and flow over long periods, so the solid plates above it can move. A mountain moves with its plate: being made of solid rock does not mean it is fixed in place. The supporting mantle is not an ocean of melted rock.
Both the plate and the supporting mantle are mainly solid, but they behave differently over long periods.
Plates usually move a few centimetres each year, roughly like fingernails growing. This steady creep is too slow to feel. Sudden slipping at a plate edge can still cause an earthquake. Small yearly moves add up: plates moving apart carry places farther apart. On a movement number line, count from the hop's start to its end. If the same distance is added each year, multiply distance per year by the number of years. Keep the same distance unit.
Tectonic plates include crust and rigid mantle rock. They move slowly over softer mantle. Small yearly moves add up over time.
The rewritten readings were written for this dataset.
More to read
Plates on the Move
Earth's rigid outer shell is split into tectonic plates. Each plate includes crust and the stiff uppermost mantle. Plates can carry continents, ocean floor, or both.
Below the plates is hotter, softer mantle rock. It is mostly solid, but it can slowly change shape over long periods. Plates move over this supporting layer. They do not sit on a planet-wide ocean of magma.
Plates usually move a few centimetres per year, roughly like fingernails growing. Small yearly moves add up to large distances over millions of years.
Where plates meet, collisions can build mountains. A sinking plate can release water that helps some hot mantle rock above it melt. This magma can feed volcanoes. Sudden slipping where plates meet can also release earthquake waves.
Plate Tectonics: Continents in Motion
South America and Africa look as though they could fit together. Their matching coast shapes are one clue to a shared past. Fossils and matching rocks on the two continents give more evidence.
Long ago, the continents were joined in a large landmass called Pangaea. Over millions of years it broke apart. The continents moved with their tectonic plates, and they are still moving.
A plate includes crust and the stiff mantle rock directly beneath it. Below the plate is softer mantle rock that can slowly change shape. Both are mainly solid. Plate motion carries continents and ocean floor across Earth's surface.
Many of the changes are slow: oceans widen and mountain ranges grow over long periods. Sudden earthquake slip is a much faster part of this moving planet.
Volcanoes 101
Some mountains formed when rock was squeezed upward. Others grew as volcanoes added lava and ash. Both kinds contain solid rock.
A volcano is an opening in Earth's crust through which melted rock, ash, and gases can escape. Melted rock underground is called magma. At the surface it is called lava. Cooled lava and ash can build a volcanic mountain over many eruptions. Not every volcano has a steep cone shape.
Many volcanoes lie near tectonic plate boundaries. Plates include crust and the stiff uppermost mantle. Where plates pull apart, mantle rock can rise and partly melt. Where one plate sinks under another, it releases water that helps rock above it melt. Magma can rise in both settings.
Some volcanoes form within plates, away from their edges. So volcanoes give clues about Earth's interior, but they do not all mark plate boundaries.
How moving plates change Earth
Under continents and oceans, Earth's thin, solid surface layer is the crust. Below it lies the mantle. Each tectonic plate includes crust and the rigid top of the mantle.
Beneath the plates, mantle rock is hotter and softer, though mostly solid. Given enough time, it bends and flows. Plates usually travel just a few centimetres a year.
As plates separate, rising mantle rock loses pressure and some melts. The resulting magma may cool into fresh crust. When a plate sinks beneath another, it releases water. This helps melt mantle rock above the sinking plate, producing magma that may supply volcanoes. Continents pushing together can crumple crust into mountains.
Over millions of years, these movements change Earth's surface. An earthquake can happen within seconds when rock suddenly slips along a fault.
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