A tectonic plate is a massive slab of solid rock that makes up part of Earth’s outer shell. Earth’s surface isn’t one continuous piece. It’s broken into a dozen or more of these plates, and they’re all slowly moving, floating on a layer of hotter, softer rock beneath them. That movement is responsible for earthquakes, volcanoes, and the shape of every continent and ocean basin on the planet.
What Plates Are Made Of
The outermost layer of the Earth is called the lithosphere, and tectonic plates are essentially fragments of it. This layer includes the crust (the very top) and a portion of the upper mantle beneath it. It’s rigid and brittle compared to what lies below.
Underneath the plates sits the asthenosphere, a zone of rock that’s so hot and under so much pressure that it behaves almost like a very thick fluid. Think of it like Silly Putty: technically solid, but capable of flowing extremely slowly over long periods of time. Because the plates are less dense than this layer, they float on top of it, similar to ice floating on water.
There are two types of material that make up plates: continental and oceanic. Continental crust is made of lighter minerals, roughly 40 km (25 miles) thick, with a density of about 2.7 grams per cubic centimeter. Oceanic crust is thinner, averaging just 6 km (4 miles), but denser at about 2.9 to 3.0 grams per cubic centimeter because it contains more iron and magnesium. Most plates carry a mix of both types, though some are entirely oceanic.
How Many Plates Are There
There are seven major plates and dozens of smaller ones. Six of the major plates are named after the continents sitting on them: the North American, South American, African, Eurasian, Antarctic, and Australian plates. The seventh major plate is the Pacific Plate, which is almost entirely oceanic and covers more area than any continent.
Smaller plates can still have a big impact. The tiny Juan de Fuca Plate, for instance, is largely responsible for the volcanoes that dot the Pacific Northwest of the United States, including Mount Rainier and Mount St. Helens. In East Africa, the continent is actually splitting along the boundary between the Nubian Plate to the west and the Somalian Plate to the east.
What Makes Plates Move
Gravity is the main engine behind plate movement. Two forces do most of the work: ridge push and slab pull.
Ridge push happens at the mid-ocean ridges where new crust forms. As fresh, hot rock rises and spreads outward, it cools and thickens. The lithosphere near the ridge essentially slides down the sloping boundary between itself and the softer asthenosphere below, pushing older crust ahead of it.
Slab pull is likely the stronger of the two. As a plate moves away from a ridge and cools over millions of years, it becomes denser than the hot mantle beneath it. When that dense edge sinks back into the mantle at a subduction zone, it drags the rest of the plate along with it, like a heavy tablecloth sliding off a table. Current thinking is that gravity drives the plates and the plates help stir the mantle, rather than mantle convection pushing the plates from below.
How Fast Plates Move
Tectonic plates move at roughly the same speed your fingernails grow. That’s typically a few centimeters per year. Scientists measure this using satellite-based GPS systems accurate to within a fraction of a millimeter per year, along with techniques like satellite laser ranging and very long baseline interferometry. These measurements confirm that plates are in constant, measurable motion, even though you’d never feel it.
Where Plates Meet: The Three Boundary Types
Almost all major geological activity, including earthquakes, volcanic eruptions, and mountain building, happens at the edges where plates interact. There are three types of boundaries.
Divergent Boundaries
At divergent boundaries, two plates pull apart and magma rises from below to fill the gap, creating new crust. The best-known example is the Mid-Atlantic Ridge, a massive underwater mountain chain running down the center of the Atlantic Ocean. In East Africa, a divergent boundary is actively tearing the continent apart along the East African Rift Zone. This same process already split Saudi Arabia away from Africa, forming the Red Sea.
Convergent Boundaries
Convergent boundaries form where plates collide. What happens next depends on what type of crust is involved. When an oceanic plate meets a continental plate, the denser oceanic plate dives beneath the lighter continental one in a process called subduction. This creates deep ocean trenches (the deepest points on the ocean floor, reaching 8 to 10 km down) and fuels volcanic chains on land. The Andes mountains formed this way, as the Nazca Plate pushed under the South American Plate.
When two oceanic plates collide, one subducts beneath the other, creating underwater volcanoes that can eventually build up into island chains called island arcs. When two continental plates collide, neither sinks because both are too light. Instead, the crust crumples and pushes upward. The collision of India into Asia about 50 million years ago created the Himalayas and the Tibetan Plateau, and the slow continued convergence of those plates is still pushing them higher today.
Transform Boundaries
At transform boundaries, two plates slide horizontally past each other. No crust is created or destroyed, but the grinding motion produces frequent earthquakes. The San Andreas Fault in California is the most famous example, where the Pacific Plate slides northwest past the North American Plate.
How Scientists Figured This Out
The idea that continents move was first proposed seriously by Alfred Wegener in the early 1900s. He noticed that the coastlines of South America and Africa fit together like puzzle pieces, something a mapmaker named Abraham Ortelius had pointed out three centuries earlier. But Wegener went further. He found identical fossils of plants and animals on coastlines now separated by thousands of miles of ocean. Tropical plant fossils turned up in Antarctica, suggesting it once sat near the equator. Glacial deposits appeared in places that are now arid, like the Vaal River valley in South Africa. Distinctive ferns called Glossopteris showed up in regions that are polar today.
Wegener proposed that all continents were once joined in a single supercontinent he called Pangaea, which broke apart and drifted to their current positions. His idea was rejected for decades because he couldn’t explain what force moved the continents. It wasn’t until the 1960s, when seafloor spreading was discovered and the mechanisms of subduction and ridge push were understood, that plate tectonics became the unifying theory of geology it is today.