What is plate tectonics? How the ground under us moves

Plate tectonics is the scientific theory that Earth's rigid outer shell, the lithosphere, is broken into seven or eight major plates and many smaller ones that slowly move over the hotter, softer rock beneath. They move a few centimeters a year. Where plates pull apart, new ocean floor forms; where they collide, mountains rise or one plate sinks under another; where they slide past, the ground shears. Most earthquakes and volcanoes happen along those edges.

In short

• The plates are pieces of crust plus the top of the mantle, about 100 km thick, carrying continents and ocean floor alike.
• There are three kinds of boundary: plates pull apart, push together or slide past each other, and each builds something different.
• The plates are pulled mostly by their own cold, heavy edges sinking into the mantle, though how much each force matters is still studied.

What is a tectonic plate?

A plate is a slab of the lithosphere: the crust together with the cold, stiff uppermost part of the mantle, about 100 km thick on average. As the note on what the Earth is made of explains, beneath it lies the asthenosphere, mantle rock hot enough to flow very slowly while staying solid. The plates ride on that weaker layer.

A plate is not the same as a continent. The North American Plate carries North America and the western half of the Atlantic floor. The Pacific Plate is almost all ocean floor. Continents are thick, light crust that rides along on the plates and never sinks, which is why continental rock can be 4 billion years old while the oldest ocean floor is about 200 million.

Most of the action happens at the edges, where plates meet. The interiors of plates are usually quiet, which is why Hawaii, in the middle of the Pacific Plate, needs a separate explanation, covered in the note on how volcanoes form.

What happens where plates meet?

There are three kinds of plate boundary. The U.S. Geological Survey also counts a fourth, broad zones where the boundary is blurred.

Boundary	What the plates do	What it builds	Real example
Divergent	Pull apart; magma rises to fill the gap	New ocean floor, mid-ocean ridges, rift valleys	The Mid-Atlantic Ridge, which runs through Iceland
Convergent	Push together; the denser plate sinks into the mantle, or neither sinks	Deep trenches, volcanic arcs, high mountains	The Himalayas, where India meets Asia; the Andes; the Mariana Trench
Transform	Slide past each other sideways	Long faults, frequent earthquakes, little volcanism	The San Andreas Fault in California

At a divergent boundary, the ocean floor is made. Rock rising under a ridge partly melts, erupts and cools into new crust, which moves away on both sides like two conveyor belts.

At a convergent boundary, ocean floor is destroyed, which is why Earth doesn't grow. When an ocean plate meets a continent, the heavier ocean plate dives beneath it, a process called subduction, and digs a trench. Water carried down with it lowers the melting point of the rock above, and the melt rises to feed a chain of volcanoes. That is how the Andes formed, and the same process rings the Pacific in the Ring of Fire. When two continents meet, neither sinks, so the crust crumples and thickens, which is how Mount Everest was formed.

At a transform boundary, nothing is made or destroyed. Along the San Andreas, the Pacific Plate has been grinding northwest past the North American Plate for about 10 million years, at about 5 cm a year on average, according to the USGS. The fault locks, strain builds, and it slips, which is what causes earthquakes there.

How fast do the plates move?

The plates move a few centimeters a year, about as fast as fingernails grow. The rates range widely. The USGS gives the Mid-Atlantic Ridge an average spreading rate of about 2.5 cm a year, the Arctic Ridge less than that, and the East Pacific Rise near Easter Island more than 15 cm a year. Today the movement is measured directly with satellite positioning: GPS stations on different plates move apart or together by measurable millimeters each year.

A few centimeters sounds like nothing. Here is what it adds up to, using the Atlantic:

1. The Atlantic widens at about 2.5 cm a year, which is 25 km every million years.
2. In a lifetime of 80 years, it widens by 80 × 2.5 cm = 200 cm: about 2 m.
3. Since Columbus crossed it in 1492, 534 years ago, it has widened by 534 × 2.5 cm, about 13 m.
4. Over the roughly 180 million years since the Atlantic began to open between North America and Africa, 180 × 25 km = 4,500 km.
5. The ocean today is about 3,000 to 6,000 km across along most of its length. The rough match, from a rate measured over decades, is one sign that the plates have moved about this fast for a very long time, though rates do vary.

Slow and steady at plate edges, it isn't always steady at a single fault. A fault can stay locked for a century and then move several meters in a minute, which is why earthquakes are sudden even though plates are not.

What makes the plates move?

Heat from the deep Earth drives it: the mantle is hot below and cool on top, and over millions of years it overturns, a slow form of convection. But the plates aren't simply carried along like boxes on a belt. Three forces act on them:

• Slab pull. At a subduction zone, the old ocean plate is cold and denser than the mantle around it, so it sinks and drags the rest of the plate behind it, like a tablecloth sliding off a table.
• Ridge push. New ocean floor at a ridge sits high because it is hot. As it cools, it slides down and away from the ridge under gravity.
• Mantle drag. Flowing mantle underneath can help or resist a plate's motion along its base.

Most geologists now think slab pull is the strongest. The USGS describes it as "now considered to be the driving force of plate tectonics." A 1975 study by Donald Forsyth and Seiya Uyeda found the clue: plates with long subducting edges, such as the Pacific, move fastest, while plates with almost none, such as the Eurasian, crawl.

Still argued

The balance between the forces is still studied, and the answer differs from plate to plate. Slab pull explains why the fastest plates are the ones with sinking edges. But it doesn't explain everything: some plates with no slabs still move, and how strongly the mantle couples to a plate's base, helping or holding it back, is hard to measure. Computer models of the whole mantle now treat plates and mantle flow as one system, and they disagree on details. The same question, why Earth has plate tectonics at all when Venus and Mars don't, is open too.

How did scientists figure this out?

The idea began as continental drift. In 1912 the German meteorologist Alfred Wegener argued that the continents had once been joined as one landmass, Pangaea, and had drifted apart. He had strong evidence of fit, fossils and rocks, but no mechanism, and most geologists rejected it for half a century.

The mechanism came from the ocean floor, mapped in detail after World War II. In the early 1960s, Harry Hess and Robert Dietz proposed seafloor spreading: new crust forms at mid-ocean ridges and moves away. Stripes of magnetized rock either side of the ridges, recording flips of Earth's magnetic field, confirmed it in the mid-1960s, because the stripes are mirror images about the ridge. By 1967 and 1968, geophysicists had joined spreading, subduction and transform faults into one theory of moving rigid plates. Plate tectonics is to geology what evolution is to biology: the frame that explains why mountains, oceans, earthquakes and volcanoes are where they are.

Check yourself

• Without looking back: what are the three kinds of plate boundary, and what does each build?
• Why doesn't the Earth get bigger, if new ocean floor is made all the time?
• Why do geologists think slab pull matters more than the mantle carrying plates along?

Sources

• Understanding plate motions, in This Dynamic Earth — U.S. Geological Survey
• Some unanswered questions, in This Dynamic Earth — U.S. Geological Survey
• Developing the theory, in This Dynamic Earth — U.S. Geological Survey
• Historical perspective, in This Dynamic Earth — U.S. Geological Survey
• Plate tectonics — Wikipedia