Quakes still shaping Earth's map
Sumatra's catastrophic December 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 per year.
Geologists say it was a lurching collision between the Indian plate and the Burma plate, which grind together along a 750-mile (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.