Earth’s moving plates
Earth’s rigid outer layer, the lithosphere (the crust plus the top of the mantle), is broken into about 15 major tectonic plates and many smaller ones. They float on theasthenosphere, a hot, slowly flowing layer of the mantle, and move a few centimetres a year. There are two kinds of crust: thin, dense oceanic crust made of basalt, and thick, lightercontinental crust rich in granite. Most earthquakes and volcanoes happen where plates meet.
What moves the plates
Heat from Earth’s interior keeps the mantle churning in slow convection currents: hot rock rises, cools and sinks again over millions of years. The plates are also pulled along by their own cold, heavy edges sinking at subduction zones (slab pull) and pushed away from high mid-ocean ridges (ridge push). Slab pull is thought to be the largest force.
Divergent boundaries
Where plates move apart, hot mantle rock rises, partly melts and erupts to form new crust. Under the oceans this builds mid-ocean ridges, a 65,000 km mountain chain winding around the globe. As the new sea floor moves away from the ridge it cools, becomes denser and sinks deeper, so the ocean is shallow at the ridge and deep far from it. On land, divergence creates rift valleys such as the East African Rift, which may one day become a new ocean.
Convergent boundaries
Where plates collide, what happens depends on the crust involved. When ocean meets continent, the denser ocean plate is forced down in subduction, forming a deep trench. Water released from the sinking slab lowers the melting point of the mantle above, creating magma that feeds explosive volcanoes such as those of the Andes. Ocean–ocean subduction builds volcanic island arcs such as Japan. When two continents meet, neither sinks: the crust thickens into great fold mountains such as the Himalayas and the Alps.
Transform boundaries
Where plates slide past each other sideways, crust is neither made nor destroyed. Friction locks the fault, so the rocks on either side bend as the plates keep moving. When the stored strain overcomes friction, the fault slips suddenly in an earthquake and the rocks snap back: this is the elastic rebound theory, developed after the 1906 San Francisco earthquake. The San Andreas Fault moves about 3–5 cm a year on average, but in sudden jumps of several metres.
The evidence
- Fitting coastlines and fossils: Alfred Wegener noticed in 1912 that South America and Africa fit together and share fossils such as Mesosaurus.
- Magnetic stripes: symmetric bands of normal and reversed magnetism on either side of ridges (Vine and Matthews, 1963).
- Sea-floor age: the youngest rock is at the ridges; no ocean floor is older than about 200 million years.
- GPS: today satellites measure plate motions directly, to the millimetre.
Key terms
- Lithosphere / asthenosphere
- The rigid outer shell broken into plates / the soft, slowly flowing mantle layer beneath it.
- Subduction
- One plate sinking beneath another into the mantle.
- Fault
- A fracture where rocks on either side have moved.
Common misconceptions
- “Plates float on an ocean of liquid magma.” The mantle is almost entirely solid rock that flows extremely slowly.
- “Continents move but the ocean floor doesn’t.” Plates carry both; ocean floor is constantly made and destroyed.
- “Earthquakes only happen at plate boundaries.” Most do, but some occur inside plates on old faults.
Key takeaways
- Earth’s lithosphere is divided into moving plates driven by mantle convection and slab pull.
- Divergent boundaries make new crust; convergent boundaries destroy or crumple it; transform boundaries slide past.
- Plate boundaries are where most earthquakes, volcanoes and mountain ranges form.
- Magnetic stripes, matching fossils and GPS measurements all support plate tectonics.
Model note: cross-sections are schematic with exaggerated vertical scale. Sea-floor depth uses the age–depth relation d ≈ 2.5 km + 0.35 km × √age. Earthquake magnitudes on the transform fault are estimated from the slip using the Wells and Coppersmith relationship.