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What is Plate Tectonics?

Cross-section illustration of plate boundaries
This artist's cross-section illustrates the main types of plate boundaries.
Credit: USGS/José F. Vigil from This Dynamic Planet

From the deepest ocean trench to the tallest mountain, plate tectonics explains the features and movement of Earth's surface in the present and the past.

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.

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.

The driving force behind plate tectonics is a combination of pushing from mid-ocean ridges and pulling at subduction zones, researchers think. Scientists continue to study and debate the mechanisms that move the plates.

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.

Many spectacular volcanoes are found along subduction zones, such as the "Ring of Fire" that surrounds the Pacific Ocean.

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.

Plate tectonics
Tectonic plates of the Earth.
Credit: USGS

Reconstructing the past

Thanks to plate tectonics, with clues left behind in rocks and fossils, geoscientists can reconstruct the past history of Earth's continents. Most researchers think plate tectonics began about three billion years ago, based on ancient magmas and minerals preserved in rocks from that period.

As the continents jostle around the Earth, they occasionally come together to form giant supercontinents, a single landmass. One of the earliest big supercontinents, called Rodinia, assembled about 1 billion years ago. Its breakup is linked to a global glaciation called Snowball Earth.

A more recent supercontinent called Pangaea formed about 300 million years ago. Africa, South America, North America and Europe nestled closely together, leaving a characteristic pattern of fossils and rocks for geologists to decipher once Pangaea broke apart. The puzzle pieces left behind by Pangaea, from fossils to the matching shorelines along the Atlantic Ocean, provided the first hints that the Earth's continents move.

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