For the first time since the theory of plate tectonics revolutionized our understanding of Earth's crust, revealing the forces that trigger earthquakes and volcanos and cause continents to drift, scientists have found some major new wrinkles in the system.
Researchers from Columbia University's Lamont-Doherty Earth Observatory have discovered that one of the 12 major plates that make up Earth's crust, like the shattered shell of a hard-boiled egg, is cracking in two. And where the break is happening, a process that began 8 million years ago, they have found a previously unknown type of boundary between plates.It's all happening because the huge plate that carries India and Australia, which had been moving irresistibly northward, collided with an immovable object - Asia.
That collision, which produced the Himalayan mountains and the Tibetan plateau - the largest bump on the planet's surface - began 50 million years ago. But after the crumpling, colliding landmasses created that vast mountain zone, the massive bulk of the uplifted region began to halt the very process that produced it. Stopped in its tracks, the Indo-Australian plate began to buckle and is now breaking into two distinct plates along a 500-mile-wide zone that is being pushed and pulled like taffy.
The two new plates, one carrying India, and the other Australia, are heading off in slightly different directions.
This crumpling zone is distinctly different from the three known types of plate boundaries. All of them, except for a few localized zones (such as the intricate web of fault lines in Southern California), are sharp, clearly-defined lines.
"There's nowhere else on the planet where this kind of phenomenon is taking place," said John Orcutt, director of the Institute of Geophysics and Planetary Physics at the Scripps Institute of Oceanography in California. "It's the only decent example of a boundary like this."
It had been accepted since the 1960s that Earth's surface is made up of 12 separate, rigid plates, all moving in different directions - a theory known as plate tectonics. The breakup of the Indo-Australian plate means the number of major plates has increased to 13.
Boundaries are defined by the way the plates are moving relative to one another - spreading apart, crunching together or slipping alongside each other. In this new boundary, the plates are rotating slightly, so that in the eastern part they are squeezing together, while on the western side they are pulling apart - two kinds of motion in one boundary. Rather than making a clean break, these motions are taking place across a wide zone of ancient fault lines - perhaps relics of a plate boundary in the distant past - that are becoming reactivated and moving at various rates and in different directions.
In all types of plate boundaries, earthquakes are common, produced by the jarring and jolting of the moving plates. In the colliding boundaries, volcanoes are also common, as molten crust from below bubbles to the surface. It was the observation that active earthquakes and volcanoes occur along distinct lines in various parts of the world that provided one of the first major clues that led to the theory of plate tectonics.
And it was a band of earthquakes below the Indian Ocean that provided the first tip-off that another boundary might be lurking there. But for years, scientists have debated whether this was indeed a boundary or simply activity within a single plate - a not-unusual occurrence.
That India and Australia really are breaking up was clinched by evidence published in a scientific paper by James Cochran and Jeffrey Weissel of Lamont-Doherty, James Van Orman, formerly of Lamont-Doherty and now at the Massachusetts Institute of Technology, and Florence Jestin of the Ecole Normale Superieure in Paris. Their data, gathered during cruises in the boundary zone, proves that the two parts of the plate are moving in different directions.
By sending sound waves into the sea floor and measuring their echoes, the researchers produced profiles showing how the layered sediments are being squeezed and displaced along hundreds of fault lines. Measuring the amount of displacement provides a direct gauge of the rate of squeezing.
The two lines of measurements, one from the French ship Marion Dufresne and one from Lamont-Doherty's now-retired research vessel Robert D. Conrad, showed that the plates are being squeezed together significantly faster at the eastern edge than they are farther west. That proves that the two pieces are moving differently and are now acting as two independent plates.
Looking at differences in compression, Cochran said, was "the crucial test." If a single plate were merely buckling, there would be no clear difference between the two. What they were trying to determine, he said, was "does the amount of shortening change in the systematic way it should if there's coherent motion of two plates?" And sure enough, after months of painstaking analysis, it turned out that it does.
"The eastern part of the Australian plate is rotating into the eastern part of the Indian plate. Further west, it is pulling apart," he said.
Not that they're going anywhere fast. While most plates are moving at rates measured in centimeters per year, the motion between India and Australia is only a few millimeters per year, Cochran said.
The breakup apparently began about 8 million years ago, Cochran said in an interview: "The Himalayan range had gotten about as high as it could get. It began collapsing under its own weight, spreading out." As that happened, the northward-moving Indo-Australian plate found itself facing "an immovable object there in Tibet. The resistance built up. . . . It became more than the plate could sustain, so the plate began to come apart."
The zone where the crumpling and stretching is taking place, he said, is "kind of a no-man'sland, not really part of either plate. This is a new type of plate boundary."
It may be just a transitional feature - albeit one likely to last for several million more years - that is a step toward the formation of one of the standard boundary zones. "It could become a more defined subduction zone," Cochran said, one where one plate is pushed underneath another. "That's one scenario.
"It's the only example we know of this," he added, "so we don't have anything in geological history to identify it with."
The Lamont-Doherty researchers' work, says Orcutt, has found immediate acceptance from specialists in the field. "It's unusual to find major new things like this," he said in a telephone interview. One reason it took so long to figure out what was going on there is that this part of the Indian Ocean is "about as remote a place as you can get. It's a difficult place to study."