Earth’s outer shell is broken into about 15 major and minor tectonic plates, plus dozens of even smaller fragments called microplates. These rigid slabs of rock fit together like a jigsaw puzzle, constantly shifting at an average rate of about 1.5 centimeters (roughly half an inch) per year. The “types” of tectonic plates can refer to two different things: their size classification (major, minor, or micro) and their crustal composition (oceanic, continental, or mixed). Both distinctions matter for understanding how plates behave.
What a Tectonic Plate Actually Is
A tectonic plate isn’t just the crust you stand on. It includes the crust plus a layer of rigid upper mantle beneath it. Together, these form the lithosphere, a stiff shell typically 70 to 150 kilometers thick. This shell rides on top of a softer, slowly flowing layer called the asthenosphere. The asthenosphere behaves almost like a very thick fluid over geologic time, allowing the rigid plates above to slide, collide, and pull apart.
Lithospheric plates generally thicken with age. Young oceanic plates near mid-ocean ridges are thin and hot, while old plates far from ridges have cooled and grown thicker. Temperature is the primary factor controlling how thick a plate becomes, though pockets of partially melted rock in the asthenosphere also influence where the boundary between “rigid plate” and “flowing mantle” falls.
Oceanic vs. Continental Plates
Plates differ in what kind of crust sits on top of them. This distinction controls everything from how dense a plate is to whether it sinks or floats during a collision.
Oceanic crust is made mostly of basalt, a dark, iron- and magnesium-rich rock. It’s relatively thin (about 5 to 10 kilometers) and dense, roughly 3.0 g/cm³. Continental crust is composed largely of granite, which is lighter in both color and weight, with a density of about 2.7 g/cm³. Continental crust is also much thicker, averaging 30 to 50 kilometers.
Most of the large plates carry both types of crust. The North American Plate, for example, includes the entire continent plus a wide stretch of Atlantic Ocean floor to the east and Pacific Ocean floor in parts of the west. A few plates, like the Pacific Plate, are almost entirely oceanic. No major plate is purely continental. This mixed composition is why, when two plates collide, the outcome depends on which type of crust is at the leading edge.
The Seven Major Plates
Geologists recognize seven plates as “major” based on their sheer size. Six are named for the continents embedded within them:
- Pacific Plate: The largest plate, covering most of the Pacific Ocean floor. It is almost entirely oceanic crust and is one of the fastest-moving plates on Earth.
- North American Plate: Spans from the Mid-Atlantic Ridge westward across the continent and into parts of the Pacific basin, including a slice of Siberia.
- South American Plate: Covers the continent and extends east to the Mid-Atlantic Ridge.
- African Plate: Encompasses the entire African continent and large sections of surrounding ocean floor. It is currently splitting along the East African Rift into the Nubian Plate to the west and the Somalian Plate to the east.
- Eurasian Plate: Stretches from the Mid-Atlantic Ridge across Europe and most of Asia.
- Antarctic Plate: Surrounds the South Pole and underlies the Southern Ocean.
- Indo-Australian Plate: Often treated as one plate, though some geologists now split it into the Indian Plate and the Australian Plate because the two sections move somewhat independently.
Minor Plates and Microplates
Beyond the big seven, there are roughly eight to ten minor plates and dozens of microplates. Minor plates are smaller in area but have a huge influence on local geology. The Juan de Fuca Plate, a small oceanic plate off the coast of the Pacific Northwest, is responsible for the chain of volcanoes running from northern California through Washington State, including Mount St. Helens and Mount Rainier. Other notable minor plates include the Nazca Plate (off South America’s west coast), the Philippine Sea Plate, the Arabian Plate, the Caribbean Plate, the Cocos Plate, and the Scotia Plate near Antarctica.
Microplates are even smaller fragments, sometimes only a few hundred kilometers across. They tend to sit in geologically complex zones where larger plates interact. New microplates are still being identified as imaging technology improves.
How Plates Interact at Their Edges
The type of boundary where two plates meet determines what happens at the surface. There are three main types.
Divergent Boundaries
At divergent boundaries, plates pull apart and magma rises from below to create new crust. The best-known example is the Mid-Atlantic Ridge, an underwater mountain chain running down the center of the Atlantic Ocean where the North American and Eurasian plates separate. Divergent boundaries can also split continents. In East Africa, the spreading process has already torn Saudi Arabia away from the African continent, forming the Red Sea. A new rift may be developing along the East African Rift Zone, which could eventually split the African Plate in two.
Convergent Boundaries
At convergent boundaries, plates push into each other. What happens next depends on the type of crust involved. When oceanic crust meets continental crust, the denser oceanic plate dives underneath in a process called subduction. This creates deep ocean trenches (some 8 to 10 kilometers below sea level) and volcanic mountain ranges on the overriding plate. The Andes mountains formed this way, as the oceanic Nazca Plate subducts beneath the South American Plate.
When two oceanic plates converge, one still subducts, and the resulting volcanic eruptions build chains of islands called island arcs. When two continental plates collide, neither is dense enough to sink. Instead, the crust crumples and buckles upward. The Himalayas are the most dramatic example: India collided with Asia about 50 million years ago, and the ongoing compression continues to push the mountains higher.
Transform Boundaries
At transform boundaries, plates slide horizontally past each other. No crust is created or destroyed. These boundaries produce earthquakes but little volcanic activity. Coastal California sits on a transform boundary where the Pacific Plate grinds past the North American Plate at nearly 5 centimeters per year, making it one of the fastest-moving plate boundaries on the planet.
What Drives Plate Movement
Three forces work together to keep plates in motion, all powered by heat escaping from Earth’s interior.
Slab pull is considered the strongest force. When an old, cold edge of a plate sinks into the mantle at a subduction zone, gravity pulls the rest of the plate along behind it, like a heavy blanket sliding off a bed. Ridge push works at the opposite end: newly formed crust at a mid-ocean ridge sits higher than the surrounding seafloor, and gravity pushes it outward and away from the ridge. Mantle convection, the slow circulation of hot rock rising and cooler rock sinking deep within the Earth, also plays a role, though it alone can’t explain why some plates move faster than the convective currents beneath them.
Plates with large subducting edges, like the Pacific Plate, tend to move faster. Plates without much subducting crust, like the African Plate, move more slowly. This is consistent with slab pull being the dominant driver of plate speed.