What Are Tectonic Plates?

Earth's layers: where plates fit in

Earth is built in layers. At the center is the iron-rich core. Around it is the thick mantle, made of hot rock. On top is the thin crust, the layer we live on. Oceanic crust under the sea floor is thinner and denser than the continental crust that makes up the continents.

For understanding plates, a different division is more useful. The lithosphere is the cold, rigid outer layer: the crust plus the uppermost mantle. It is often around 100 kilometers thick, thinner under young ocean floor and thicker under old continents. Beneath it lies the asthenosphere, a hotter part of the mantle that is solid but weak enough to flow very slowly over long periods. The lithosphere is broken into plates that move over the asthenosphere.

The major plates

Geologists generally recognize seven or eight major plates, plus many medium and small ones. Plates do not match continents: most carry both continental and oceanic crust. The North American Plate, for example, includes most of North America, part of Siberia and the western half of the North Atlantic Ocean floor.

• Pacific Plate, the largest, mostly beneath the Pacific Ocean
• North American Plate
• South American Plate
• Eurasian Plate
• African Plate
• Antarctic Plate
• Indo-Australian Plate (often split into the Indian and Australian plates)
• Smaller plates include the Nazca, Cocos, Philippine Sea, Arabian, Caribbean and Juan de Fuca plates

How fast do plates move?

Plates move at a few centimeters per year, about the speed at which fingernails grow. Some move faster, up to roughly 10 centimeters or more per year in parts of the Pacific, while others move only a centimeter or two.

That sounds tiny, but it adds up. Over a million years, a plate moving 5 centimeters per year travels about 50 kilometers. Over hundreds of millions of years, plate motion has assembled and broken apart whole supercontinents, including Pangaea. Today, scientists measure plate motion directly using GPS and other satellite techniques.

Why plates move

Plates move because Earth is slowly losing heat from its interior. Heat keeps the mantle churning very slowly, and the plates are part of that system. A major driving force is slab pull: old, cold ocean plate is dense, and where it sinks into the mantle at a Subduction zone, it drags the rest of the plate behind it. At mid-ocean ridges, new hot plate forms and slides away from the higher ridge, which also helps push plates along.

Scientists still study exactly how much each force contributes, but the overall picture of plate tectonics is very well established.

Three kinds of plate boundaries

Most of the action happens at the edges of plates. There are three main types of boundaries, each with its own kinds of earthquakes and landforms.

What happens when plates collide

Convergent boundaries come in different forms. When an ocean plate meets a continental plate, the denser ocean plate sinks beneath it. This creates a deep ocean trench, a chain of volcanoes on the overriding plate, and the potential for very large megathrust earthquakes. The Cascadia Subduction Zone off Oregon and Washington is an example.

When two continents collide, neither sinks easily, so the crust crumples and thickens. The collision of India with Asia has built the Himalayas and the Tibetan Plateau and causes large earthquakes across the region, such as the 2015 Nepal earthquake.

Plates and earthquakes

If you look at a map of earthquakes, the plate boundaries stand out as bright lines of activity. The ring of boundaries around the Pacific Ocean, often called the Ring of Fire, is the most active. Earthquakes also happen inside plates, but less often.

You can see this pattern on the live earthquake map, and the 3D depth explorer shows how earthquakes get deeper as they follow sinking plates down into the mantle. For an interactive explanation, try the lesson on how plates move.

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