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What Is Plate Tectonics?

Plate tectonics
Tectonic plates of the Earth.
(Image: © USGS)


Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earth's mantle. This strong outer layer is called the lithosphere, which is 100 km (60 miles) thick, according to Encyclopedia Britannica. The lithosphere includes the crust and outer part of the mantle. Below the lithosphere is the asthenosphere, which is malleable or partially malleable, allowing the lithosphere to move around. How it moves around is an evolving idea.


Developed from the 1950s through the 1970s, plate tectonics is the modern version of continental drift, a theory first proposed by scientist Alfred Wegener in 1912. Wegener didn't have an explanation for how continents could move around the planet, but researchers do now. Plate tectonics is the unifying theory of geology, said Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York.

"Before plate tectonics, people had to come up with explanations of the geologic features in their region that were unique to that particular region," Van der Elst said. "Plate tectonics unified all these descriptions and said that you should be able to describe all geologic features as though driven by the relative motion of these tectonic plates."

How many plates are there?

There are nine major plates, according to World Atlas. These plates are named after the landforms found on them. The nine major plates are North American, Pacific, Eurasian, African, Indo-Australian, Australian, Indian, South American and Antarctic. 

The largest plate is the Pacific Plate at 39,768,522 square miles (103,000,000 square kilometers). Most of it is located under the ocean. It is moving northwest at a speed of around 2.75 inches (7 cm) per year.

There are also many smaller plates throughout the world. 

How plate tectonics works


Mid-ocean ridges are gaps between tectonic plates that mantle the Earth like seams on a baseball. Hot magma wells up at the ridges, forming new ocean crust and shoving the plates apart. At subduction zones, two tectonic plates meet and one slides beneath the other back into the mantle, the layer underneath the crust. The cold, sinking plate pulls the crust behind it downward.


Plate boundaries

Subduction zones, or convergent margins, are one of the three types of plate boundaries. The others are divergent and transform margins.

At a divergent margin, two plates are spreading apart, as at seafloor-spreading ridges or continental rift zones such as the East Africa Rift.

Transform margins mark slip-sliding plates, such as California's San Andreas Fault, where the North America and Pacific plates grind past each other with a mostly horizontal motion. 

Reconstructing the past


With clues left behind in rocks and fossils, geoscientists can reconstruct the past history of Earth's continents. Most researchers think modern plate tectonics began about 3 billion years ago, based on ancient magmas and minerals preserved in rocks from that period. Some believe it could have started a billion years after Earth's birth, at around 3.5 billion years.




Plates bumping into each other can also cause mountain ranges. For example, India and Asia came together about 55 million years ago, which created the Himalaya Mountains, according to National Geographic.

In the 20th century, researchers realized that the Earth's crust is not one piece, but is made up of many huge tectonic plates upon which the continents ride. (Image credit: by Karl Tate, Infographics Artist)

Additional reporting by Alina Bradford, Live Science contributor

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