How do tectonic plates move?
Earth's rigid outer layer, the lithosphere, is broken into plates that move a few centimeters a year over the hotter, softer asthenosphere. They are driven mostly by dense ocean plate sinking at subduction zones, helped by push from mid-ocean ridges, and their boundaries are where most earthquakes, volcanoes and mountains form.
What a plate is and what moves it
A plate is not just crust. It is the lithosphere: the crust plus the rigid top of the mantle, roughly 100 km thick on average. Beneath it, the asthenosphere is solid rock that is hot enough to flow very slowly, like putty over long times. Plates are not floating on liquid magma.
- Slab pull: old oceanic lithosphere is cold and dense, so where it sinks into the mantle at a subduction zone it drags the rest of the plate behind it. This is considered the largest force.
- Ridge push: new lithosphere at a mid-ocean ridge sits high and slides down and away from it.
- Mantle convection: heat from the interior keeps the mantle slowly circulating, and the plates are part of that circulation.
The three boundaries, with examples
- Divergent: plates pull apart, magma rises and new crust forms. The Mid-Atlantic Ridge is one; the East African Rift is one forming on a continent.
- Convergent, ocean-continent: the denser oceanic plate subducts, making a trench and a chain of volcanoes, as in the Andes.
- Convergent, ocean-ocean: one plate subducts and a volcanic island arc forms, as in Japan and the Mariana Islands.
- Convergent, continent-continent: neither sinks easily, so crust crumples into high mountains, as in the Himalaya.
- Transform: plates slide past each other with earthquakes but little volcanism, as along the San Andreas Fault.
Hot spots such as Hawaii are the exception: volcanoes in the middle of a plate, where the plate moves over a long-lived plume of hot mantle and leaves a line of islands that get older with distance.
The evidence and common mistakes
Wegener noticed matching coastlines, fossils and rock types across oceans, but could not explain the motion. The evidence that settled it came from the seafloor: rock gets older moving away from ridges, and magnetic stripes recording past reversals of Earth's magnetic field are mirrored on both sides. Earthquakes and volcanoes trace plate edges, and GPS now measures the motion directly.
- Saying continental crust subducts under oceanic crust. Oceanic lithosphere is denser, so it is the one that sinks.
- Treating the asthenosphere as liquid. It is solid but ductile.
- Confusing continental drift (Wegener's idea) with plate tectonics (the full theory with a mechanism).
Turning the planet around
Boundaries make more sense in three dimensions, where you can see one plate dive under another. Ask Isotoon "How do tectonic plates move?" for a short video, or on Pro open the 3D interactive to rotate a cross section and step through each boundary type. For another Earth system, see the greenhouse effect, and more topics are on the earth and environmental science page.
Questions
- How fast do tectonic plates move?
- A few centimeters a year, roughly as fast as fingernails grow. Some plates move faster and some slower, and over millions of years that adds up to whole oceans opening and closing.
- What is the difference between the lithosphere and the asthenosphere?
- The lithosphere is the rigid outer layer, made of the crust and the top of the mantle, and it is broken into plates. The asthenosphere lies below it and is hot enough to flow slowly, which lets the plates move.
- Why are there volcanoes at subduction zones?
- As the sinking plate goes deeper, water released from it lowers the melting point of the mantle above. That rock partly melts, and the magma rises to feed a line of volcanoes.