Plate Tectonics

Earth is about 12,750 kilometers across. Nobody has ever drilled more than a tiny way into it, but scientists can work out what is inside by studying how earthquake waves travel through the planet. They have found layers, a bit like a peach: a thin skin, a thick middle, and a pit at the center.

Lithosphere and asthenosphere

For plate tectonics, the most important split is not crust versus mantle. It is stiff versus soft.

• The lithosphere ("rock sphere") is the stiff outer shell: the crust plus the very top of the mantle, at least about 80 kilometers thick on average. It is the lithosphere that is broken into plates.
• Below it is the asthenosphere ("weak sphere"). It is still solid rock, not a sea of lava, but it is so hot that it can slowly flow, like very thick putty. The plates ride on top of it.

Wegener's big idea: continental drift

Look at a world map, and you might notice that the east coast of South America and the west coast of Africa look like puzzle pieces. People had noticed that for centuries. A German scientist named Alfred Wegener took it seriously.

On January 6, 1912, Wegener gave a talk in Frankfurt and published two articles proposing that the continents had once been joined in one giant landmass. He first called it the Urkontinent ("original continent"); he later called it Pangaea ("all lands"). In 1915 he laid out the whole idea in a book, The Origin of Continents and Oceans, He kept revising it, and the 1922 edition was translated into English two years later, in 1924. Scientists today think Pangaea began to break apart about 200 million years ago.

Wegener's evidence

• The fit. The edges of South America and Africa fit together.
• Fossils. Fossils of Mesosaurus, a small reptile that lived in fresh water, turn up in both South America and southern Africa. It could not have swum across an ocean. Fossils of the seed fern Glossopteris are found on all the southern continents, including Antarctica.
• Climate clues. Coal, which forms from swampy plant life, is found in Antarctica. Scratches and rock deposits left by ancient glaciers, about 280 million years old, are found in places that are warm today, like southern Africa. If you put the continents back together, the ice deposits cluster around the South Pole.

Why most scientists said no

Wegener had lots of evidence that the continents had moved, but he could not explain how. What force could push whole continents across the planet? His guesses did not work. The British scientist Harold Jeffreys argued that solid continents could not possibly plow through the solid ocean floor. Most geologists, especially in North America, rejected the idea.

Wegener kept collecting evidence. He was also a polar explorer, and in 1930 he died on an expedition across the Greenland ice sheet, decades before his idea was accepted.

How do we know? Clues from the seafloor

Marie Tharp and Bruce Heezen map the ocean floor

After World War II, ships began using echo sounding: they sent pings of sound down to the seafloor and timed the echoes to measure the depth. At Columbia University's Lamont Geological Observatory in New York, the geologist Bruce Heezen collected these measurements, and Marie Tharp, who joined Lamont in 1948, turned thousands of them into maps.

In 1952, while plotting the Atlantic, Tharp noticed a deep notch running down the middle of an underwater mountain range: a rift valley, where the seafloor seemed to be pulling apart. Tharp later recalled that Heezen at first dismissed her idea as "girl talk." But when the team plotted where earthquakes happened, the earthquakes lined up along Tharp's rift. Heezen and Tharp published their map of the North Atlantic floor in 1957, and in 1977 a painted map of the whole world's ocean floor, made with the Austrian artist Heinrich Berann.

Their maps showed that the mid-ocean ridge is a chain of underwater mountains that winds around the globe for more than 50,000 kilometers, the longest mountain range on Earth.

Magnetic stripes

Earth's magnetic field has flipped many times: north becomes south and back again. When lava cools into the dark rock basalt, tiny magnetic minerals inside lock in the direction of the field at that moment, like a compass frozen in rock. Ships towing magnetometers found that the seafloor is striped with bands of rock that record "normal" and "reversed" magnetism.

Seafloor spreading

In 1962 the American geologist Harry Hess proposed an answer in a paper called "History of Ocean Basins." Hot material rises from the mantle at the mid-ocean ridge, makes new ocean floor, and the floor moves away from the ridge on both sides like a pair of conveyor belts. Far away, at deep ocean trenches, old seafloor sinks back into the mantle. Robert Dietz came up with a similar idea and gave it a name: seafloor spreading.

In 1963, Fred Vine and Drummond Matthews in England, and Lawrence Morley in Canada, each realized what the magnetic stripes meant. If new seafloor forms at the ridge while the magnetic field flips back and forth, the stripes should match on both sides, like a tape recording of Earth's magnetic history. Morley's paper was turned down by two journals, so the idea is now known as the Vine-Matthews-Morley hypothesis.

In 1968 the drilling ship Glomar Challenger drilled into the seafloor of the South Atlantic and found that the rock gets older the farther it is from the ridge, just as seafloor spreading predicted. That was strong evidence that the plates move, and the theory of plate tectonics came together in the late 1960s.

Measuring the motion today

Today, scientists can watch plates move using GPS stations fixed to solid rock. Most plates move a few centimeters each year. That is about as fast as your fingernails grow. It sounds slow, but over millions of years it adds up to thousands of kilometers. The Atlantic Ocean, for example, widens by about 2.5 centimeters a year, while the fastest spreading, along the East Pacific Rise, is more than 15 centimeters a year.

The plates

Earth's lithosphere is broken into a dozen or more large and small plates that fit together like a cracked eggshell. The biggest include the Pacific, North American, South American, Eurasian, African, Antarctic, and Indo-Australian plates. (Many scientists now treat the Indian and Australian plates as two separate plates.) Smaller ones include the Nazca, Cocos, Caribbean, Philippine, Arabian, and Juan de Fuca plates.

Notice that plates are not the same as continents. The North American plate, for example, carries North America and also the western half of the North Atlantic floor, all the way to the Mid-Atlantic Ridge. The Pacific plate is almost all ocean floor.

A plate can carry two kinds of crust. Oceanic crust is thin, dark, and dense (heavy for its size). Continental crust is thick and lighter. That difference decides what happens when plates collide.

1. Divergent: pulling apart

At a divergent boundary, two plates move away from each other. Magma rises into the gap and cools into new crust. Under the sea, this builds the mid-ocean ridge.

Real example: the Mid-Atlantic Ridge and Iceland. Iceland is a piece of the Mid-Atlantic Ridge that rises above sea level. It is being split between the North American and Eurasian plates. On land, a continent can start to split too: in the East African Rift, Africa is being stretched apart, and the Horn of Africa may one day become a separate landmass.

2. Convergent: ocean plate meets continent

At a convergent boundary, plates push toward each other. When dense ocean crust meets lighter continental crust, the ocean plate sinks underneath. This is called subduction. It makes a deep trench offshore. As the sinking plate goes down, rock above it melts, and the magma rises to feed volcanoes on the continent.

Real example: the Andes. The Nazca plate sinks under the South American plate at the Peru-Chile Trench, building the volcanic Andes mountains. The Cascade Range volcanoes in the U.S. Pacific Northwest form the same way.

3. Convergent: ocean plate meets ocean plate

When two ocean plates collide, one sinks under the other. Again there is a trench, and again melting feeds volcanoes, but this time they grow up from the seafloor into a curved chain of islands called an island arc.

Real example: the Mariana Trench and Mariana Islands. Here the Pacific plate sinks under the Philippine plate. The Mariana Trench is the deepest place in the ocean: it reaches nearly 11,000 meters down. If you dropped Mount Everest (about 8,849 meters tall) into it, the peak would still be more than 2 kilometers under water. The Aleutian Islands of Alaska are another island arc.

4. Convergent: continent meets continent

Continental crust is too light and thick to sink very far. So when two continents collide, neither one dives under. Instead the crust crumples, folds, and piles up into huge mountains.

Real example: the Himalaya. India used to be a separate landmass moving north. About 50 million years ago it began to crash into Asia. The collision built the Himalaya, including Mount Everest, and the Tibetan Plateau behind them, which averages about 4,600 meters high. India is still pushing north, and the Himalaya are still rising.

5. Transform: sliding past

At a transform boundary, two plates grind sideways past each other. Crust is not made or destroyed, so there are few volcanoes, but the plates often get stuck and then suddenly slip, causing earthquakes. The Canadian geologist J. Tuzo Wilson first described these boundaries.

Real example: the San Andreas Fault. In California, the Pacific plate moves northwest past the North American plate, about 5 centimeters a year, along the roughly 1,300-kilometer-long San Andreas Fault. Streams and fences that cross the fault have been shifted sideways.

Earthquakes, volcanoes, and hot spots

If you mark every big earthquake and active volcano on a world map, they are not scattered randomly. They trace the edges of the plates.

The Ring of Fire

Around the edges of the Pacific Ocean is a horseshoe of trenches, volcanoes, and earthquakes called the Ring of Fire. It is the most active zone for earthquakes and volcanoes in the world, because so many plates are sinking there. About 81 percent of the world's largest earthquakes happen along it. The biggest earthquake ever recorded, magnitude 9.5, struck Chile in 1960. The second biggest, magnitude 9.2, struck Alaska in 1964. Japan, the Andes, the Aleutians, and the Cascades are all on the Ring of Fire.

Hot spots: volcanoes in the middle of a plate

Hawaii is in the middle of the Pacific plate, thousands of kilometers from the nearest boundary. So why does it have volcanoes? In 1963 J. Tuzo Wilson suggested that there are a few hot spots where unusually hot rock rises from deep in the mantle and melts through the plate above. The hot spot stays put while the plate slides over it, like a sheet of paper moved over a candle.

The Pacific plate moves northwest, so the Hawaiian Islands get older toward the northwest. The volcanic rocks of Kauai are about 5.5 million years old, while the Big Island, at the southeast end, is less than 0.7 million years old and still erupting. A new underwater volcano, Kamaʻehuakanaloa (once called Lōʻihi), is growing on the seafloor off its south coast.

Plates and the rock cycle

Plate tectonics is the engine of the rock cycle. At mid-ocean ridges and over sinking plates, rock melts and cools into new igneous rock such as basalt. When continents collide, rock is buried, heated, and squeezed into metamorphic rock, and pushed up into mountains. Those mountains are weathered and eroded into sediment that becomes sedimentary rock. And at trenches, old seafloor is carried back down into the mantle, where the trip can start again.

Credits

Written by Mary, Jugami's Earth science teacher. The cross-sections, the layers diagram, the magnetic stripes, and the field-notebook illustration are original, hand-coded SVG made for Jugami. They are simplified sketches and are not drawn to scale, except where the caption says so.