Tectonic Plates

The surface of the Earth is not one solid piece. It is broken into about 15 large slabs of rock called tectonic plates, plus several smaller ones, fitted together like a jigsaw puzzle around the planet. The plates float on a hot, partly molten layer beneath them (called the asthenosphere) and are constantly drifting at a few centimetres per year. When plates pull apart, push together, or scrape past each other, they cause earthquakes, build mountains, open oceans, and create most of the dramatic features of our planet.

  • Major platesApprox. 7Plus 8 minor plates and several microplates
  • Biggest platePacific PlateCovers approx. 20% of Earth's surface
  • Smallest major plateCaribbean Plate
  • Plate speed2 to 10 cm/yearAbout as fast as fingernails grow
  • Theory established1960sOne of the youngest big scientific ideas
  • Plates float onThe asthenosphereHot soft rock about 100 km down

The major plates

  • Pacific Plate: the largest. Covers most of the Pacific Ocean.
  • North American Plate: includes North America, Greenland and part of Siberia.
  • Eurasian Plate: most of Europe and Asia.
  • African Plate: most of Africa and part of the surrounding ocean floor.
  • South American Plate: South America and part of the South Atlantic.
  • Antarctic Plate: Antarctica and surrounding ocean.
  • Indo-Australian Plate: India, Australia and the surrounding ocean. Sometimes counted as two separate plates.

The smaller plates include the Nazca Plate (off South America), the Caribbean Plate, the Arabian Plate, the Philippine Sea Plate, the Cocos Plate and several others.

Three types of plate boundary

What happens when two plates meet depends on the direction they are moving.

  • Divergent (pulling apart): plates move away from each other. New rock rises up through the gap. The Mid-Atlantic Ridge (under the Atlantic Ocean) is the most famous example. Iceland sits right on it.
  • Convergent (pushing together): plates collide. If one is dense ocean crust and the other is lighter continental crust, the ocean plate slides under (called subduction) and the continent crumples up into mountains. The Andes and the Cascades formed this way. If two continents collide, both crumple up. The Himalayas formed this way when India crashed into Asia.
  • Transform (sliding past): plates grind against each other sideways without colliding or separating. The famous San Andreas Fault in California is a transform boundary.

What drives plate movement

The plates are pushed around by slow convection currents in the hot mantle beneath them, plus by gravity (heavier old plates sink into the mantle at subduction zones, dragging the rest of the plate along behind). Add it all up and the plates drift around the planet at a few centimetres per year: about as fast as your fingernails grow. Over millions of years that adds up to thousands of kilometres of movement.

Fact Roughly 250 million years ago, all the continents on Earth were joined together into a single huge landmass called Pangaea ("all earth"). Over the following hundreds of millions of years, Pangaea slowly broke apart as the tectonic plates drifted. The Atlantic Ocean has only been opening up since approximately 180 million years ago: even today it is still widening by about 2.5 cm per year, roughly the same speed as your fingernails grow.

How tectonic plates change the world

Almost every dramatic feature on Earth's surface is caused by plate tectonics.

  • Mountain ranges: formed where plates collide. Himalayas, Alps, Andes, Rockies.
  • Ocean basins: opened up where plates pulled apart. Atlantic, Indian, Red Sea.
  • Volcanoes: form where ocean plates dive under continents (Ring of Fire) or where hot magma rises through cracks.
  • Deep trenches: form where one ocean plate subducts under another. The Mariana Trench is the deepest point in the ocean (10,935 m).
  • Earthquakes: happen at and near plate boundaries.
  • Continental drift: continents have moved thousands of kilometres over millions of years. North America and Europe used to be joined.
Did you know? The idea that continents drift was first proposed by German scientist Alfred Wegener in 1912. He noticed that the coastlines of Africa and South America fit together like puzzle pieces and that matching fossils were found on both. Geologists laughed at him at the time. It took until the 1960s, after new evidence from the ocean floor proved he was right, for plate tectonics to become accepted. Wegener died in 1930 still considered wrong by most of science.
Deeper dive: how scientists figured out plates were moving

The idea that the continents move was a long battle in 20th-century science. For decades, most geologists dismissed it as nonsense because Wegener could not explain how the continents moved through solid rock.

The breakthrough came from the seafloor in the 1950s and 60s. New technology let scientists map the deep ocean for the first time. They discovered:

  • Mid-ocean ridges: long volcanic mountain chains running down the middle of every ocean, where new seafloor was being created.
  • Magnetic striping: rocks on either side of a ridge had stripes of normal and reversed magnetism, perfectly symmetrical, showing that the seafloor was spreading outward from the ridge over millions of years.
  • Deep ocean trenches: areas where old ocean floor was being destroyed as it dived under other plates.
  • Earthquake patterns: most earthquakes happen along narrow lines around the edges of plates.

Together this evidence forced a complete rethink of how Earth works. By the late 1960s, almost every geologist had accepted the new theory of plate tectonics: a complete framework explaining earthquakes, volcanoes, mountains, ocean basins and the slow movement of continents. It is sometimes called the unifying theory of geology, the equivalent of evolution in biology or the periodic table in chemistry.

For more, see continental drift, fault lines and what is an earthquake.