Background
The theory of plate tectonics explains that the majority of Earth’s geological features—from mountain ranges to ocean trenches—result from interactions between slabs of rock that make up Earth’s outer shell.
The slabs—collectively called tectonic plates—range from the size of Oklahoma City to as large as 20% of the planet’s surface area (learn about them). The plates are dragged, on average, about as fast as fingernails grow by material moving beneath them in Earth’s interior.
Differences in plate composition, movement direction, and interactions when they meet have produced continuous large-scale changes to Earth’s surface for at least 3 billion years (watch visualization).
Early Evidence
While scientists saw that Africa and South America seemed to fit together since cartographers mapped the latter in the late 1500s (see history of world maps), it was not until 1912 that German meteorologist Alfred Wegener developed a scientific basis for landmass movement called continental drift.
He proposed that all continents existed in a supercontinent, Pangaea, over 200 million years ago. The theory explained why similar rock layers, plant and animal fossils, and glacial deposits were found on unconnected continents (see evidence).
However, Wegener could not explain what caused the drifting of material that was seen as solid rock and faced criticism from colleagues until he died in 1930 (learn more).
A year later, English geologist Arthur Holmes published a paper describing how continents could move via mantle convection—currents that transfer heat from radioactive decay and matter throughout Earth’s mantle (watch visualization).
Seafloor Spreading
Advances in reconnaissance technology designed to search for submarines during World War II, including sonar and fluxgate magnetometers, were used by scientists after the war to map the ocean floor.
Instead of the expected flat plains, scientists observed underwater mountain chains tens of thousands of kilometers long. Earthquakes and volcanoes were common in these mid-ocean ridges, and rock samples taken from the ocean floor showed that their ages increased with distance from the ridges.
The ocean floor at equally long deep-sea trenches was as much as six times deeper than the Grand Canyon. The oldest underwater rocks were found here, but were only about 180 million years old—far less than the oldest rocks that made up continents, which were over 4 billion years old (radiometric dating, explained).
Scientists knew Earth’s magnetic field had flipped multiple times after observing crystallized lava on the surface (watch explainer). Matching flips were observed in ocean floor rocks, but in symmetric patterns on either side of mid-ocean ridges.
In the early 1960s, American geophysicist Harry H. Hess proposed the theory of seafloor spreading to explain these observations.
As magma rises from Earth’s mantle, ridges are pushed apart and downhill, creating a gap that the magma fills and cools to become new, young rock. Like a conveyor belt, ridge push continuously displaces older rock toward trenches, where it sinks beneath less dense continental rock back into the mantle.
Alongside ridge push, the weight of the sinking rock can pull the rest of the trailing plate faster into the mantle—a mechanism known as slab pull. Both ridge push and slab pull vary based on the size and composition of plates, producing the different speeds at which tectonic plates move (see visualization).
Plate tectonics emerged as a unifying theory of continental drift, mantle convection, seafloor spreading, and earthquake and volcano locations outlining plate boundaries (see maps).
Geological Activity
Plate boundaries—convergent, divergent, and transform—are where plates meet and produce the majority of geological features we see on the planet’s surface.
At convergent boundaries, different combinations of colliding continental and oceanic plates create volcanoes, island systems, mountain ranges, and about 80% of the world’s earthquakes (explore examples).
Besides mid-ocean ridges, divergent boundaries can create rift valleys as plates move apart (see examples).
At transform boundaries, plates slide horizontally past one another, causing them to deform or fracture rather than destroy or form new rocks (learn more).
