Image: Tectonic plates
This map shows the edges of Earth's tectonic plates. The magnitude-9 quake that touched off the Dec. 26 tsunami occurred at the boundary between the India plate and the Burma plate.
updated 1/10/2005 5:32:33 PM ET 2005-01-10T22:32:33

Sumatra's catastrophic Dec. 26 earthquake flattened cities and shifted landscapes in seconds with the force of a million atom bombs. Within hours, ferocious waves had turned the Indian Ocean into a cauldron of death and debris.

But the geologic circumstances that set up the worst natural disaster in a century were much longer in the making. How long?

Try 300 million years. Maybe twice that long.

Once, scientists believe, all the earth's continents were combined in a single gigantic land mass they call Pangea (Greek for "all lands"). But geological forces caused it to break apart; and ever since then, the pieces, which scientists call plates, have been drifting across the planet at an average rate of a few inches a century.

As the plates move, grinding collisions between them trigger earthquakes and even build mountains. The Indian subcontinent, for example, has been moving inexorably northward for millions of years, colliding with Asia like a slow-motion car wreck, the land at the edge of the collision buckling to form the Himalayas. Mount Everest and other mountains in the chain are still growing at a rate of about a half-inch (1 centimeter) per year.

Lurching collision
Geologists say it was a lurching collision between the Indian plate and the Burma plate, which grind together along a 750-mile-long (1,200-kilometer-long), north-south fault in the Indian Ocean, that triggered the recent earthquake off Sumatra, and the resulting tsunami.

The quake is believed to have shifted north Sumatra and smaller nearby islands by as much as 60 feet (18.2 meters).

In human time, earthquakes that powerful are rare, but in the vastness of geologic time, they are commonplace. "An earthquake of this magnitude, in this part of the world, has probably occurred about a million times since the breakup of Pangea," said Chris Scotese, a geophysicist at the University of Texas-Arlington. "No exaggeration."

Geologists believe they understand, at least generally, what causes the plates to move and collide.

The earth, they explain, is made up of pressurized layers. At the center is a hot metal core about 2,160 miles (3,500 kilometers) thick, the center of it solid and the outer layer molten. Then comes the hot, rocky mantle, about 1,800 miles (2,900 kilometers) thick. On top of that is the part we live on, a thin, cooler crust, perhaps 30 miles (48 kilometers) thick.

The crust is not solid and unbroken like the coating on a gumball. Rather, it is fractured into more than a dozen overlapping, rigid plates of rocky armor. The plates move relative to one another as they slide atop the hotter layers below.

The overlapping points between plates are called subduction zones, and that is where the biggest earthquakes strike, changing Earth's map slightly each time. Volcanoes most often erupt in these boundaries between the plates.

Elusive details
Understanding these forces more precisely — in enough detail to predict earthquakes, for example — has proven elusive, however. For one thing, many of the boundaries between plates are covered by deep oceans, making them inaccessible to study. Even when plates come together on land, as they do in California, the real action occurs out of sight miles below the surface.

Only now, in central California, have scientists started to drill about 2 miles (3 kilometers) into a fault zone to learn more about these forces. But even that will offer only a blurry snapshot; the Dec. 26 earthquake occurred on a larger fault nearly 6 miles (10 kilometers) down.

Really big earthquakes — those like the Sumatra quake that noticeably rearrange the landscape — are so rare that there are few opportunities to study them. Until the Sumatra quake, it had been 40 years since a magnitude-9 temblor occurred — in Alaska. Like fishermen waiting for the big one, geologists can spend their entire careers waiting for the chance to study the huge temblor that never comes.

"A lot of what we see that is catastrophic occurs in the snap of a finger," says David Wann, a geophysicist for the U.S. Geological Survey. "And then nothing happens for hundreds of years."

Piecing together Pangea
Obviously, there were no human beings around when Pangea broke apart. So how do scientists know it ever existed?

"Look at the fit of the matching coasts of Africa and South America," Scotese said, the continents fitting together like jigsaw puzzle pieces.

More detailed evidence can be found in both the geologic and fossil record.

For example, researchers studying one of Antarctica's rare rocky exposures in 2003 reported finding metals with the same chemistry as those in China, suggesting the two were once joined together.

Meanwhile, fossils of extinct tropical plants have been found in Antarctica and Australia, indicating that those continents were once located near the equator.

Fossils of Lystrosaurus, a reptile that lived during the Triassic Period 200 million years ago, have been found in Africa, India and Antarctica — strong evidence that those continents were still connected then. Much later, during the Cretaceous Period 65 million years ago, the continents had separated into an arrangement roughly like today's map, making it nearly impossible for Tyrannosaurus rex to range beyond his isolated home in North America.

Not the first or the last time
Scotese and many other geologists believe that Pangea was not the first supercontinent on the planet, nor do they believe it will be the last. Rather, they say, supercontinents have formed and broken apart several times during the earth's 4.6 billion-year history.

Computer models of the earth's moving plates show that the continents are again drifting together. For the last 40 million years, Africa has been melding into Europe, its northern drift pushing up the Alps and the Pyrenees and causing earthquakes in Italy, Greece and Turkey. The Mediterranean is slowly shrinking, a small remnant now of what was once a large ancient ocean.

Australia, meanwhile, is on a collision course with Southeast Asia.

Beyond that, the computer models become increasingly speculative, but they suggest North America is inching toward the building Euro-African-Asian monolith.

In another 250 million years, Scotese believes, Earth will be one supercontinent again — Pangea Ultima.

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Data: Continental shifts through the ages


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