Long-Term Global Forecast? Fewer Continents

By WILLIAM J. BROAD
The New York Times
January 9, 2007

Kiss the Mediterranean goodbye. Ditto the Red Sea and its wonderland of coral reefs and exotic sea life. And prepare for the day when San Francisco has a gritty new suburb: Los Angeles. Indeed, much of Southern California, including the Baja Peninsula, will eventually migrate up the west coast to make Alaska even more gargantuan.

Geologists have long prided themselves on their ability to peer into the distant past and discern the slow movements of land and sea that have continuously revised the planet’s face over eons. Now, drawing on new insights, theories, measurements and technologies — and perhaps a bit of scientific bravado — they are forecasting the shape of terra firma in the distant future.

The maps and animations by these scientists are helping explain core principles of geology to increasingly wide audiences. Schools, textbooks, museums, Web sites and television shows now routinely feature images of what the forecasters say the planet will look like eons from now. And geologists are using the forecasts to deepen their own investigations of plate tectonics.

“It’s tremendous,” said Warren J. Nokleberg, a senior research geologist at the United States Geological Survey in Menlo Park, Calif. “It lets students and scientists better appreciate the mobile Earth, to see where it’s going. That’s very powerful.”

Practitioners acknowledge that their predictions, however intriguing, become more theoretical when pushed far into the future, as with advanced weather forecasts. Their most ambitious efforts peer 250 million years ahead. But their more short-term predictions, they note, draw on mountains of geophysical data and involve relatively small extrapolations of current trends, like the steady northward march of Southern California.

Despite uncertainties, the field of geopredictions is booming. One Web site has received almost 30 million hits since its debut in 1998, and the field’s admirers now include top scientists.

“It’s quite good pedagogically,” said Frank Press, a geologist and past president of the National Academy of Sciences. “It captures the attention.”

Dr. Press features one of the forecasts in his introductory college text, “Understanding Earth” (Freeman, 2006). He and three co-authors present a snapshot of how the planet’s surface might look 50 million years from now, calling it “a plausible scenario.”

Among other things, the snapshot shows that Africa has drifted to the north, plowing into Europe and fusing the two landmasses, eliminating the Mediterranean Sea and replacing it with the Mediterranean Mountains. The rugged range runs down the middle of a continent far bigger than current-day Eurasia, a giant new agglomeration that might be called Afrasia.

While peering 50 million years into the future may seem like a stretch, geologists consider such spans of time the blink of an eye. If one year represented Earth’s past, 50 million years would equal less than 4 days, or about the limit of accurate weather forecasts.

“Fifty million is fairly straightforward,” said Christopher R. Scotese, a geologist at the University of Texas, Arlington, who has pioneered the predictions in recent years. “It’s like you’re driving on the highway and you want to know where you’re going to be in 10 minutes. You check the speedometer, do a calculation, and project your present motion.

“But beyond 50 million years,” Dr. Scotese added, “like on the highway, unexpected things can happen.”

Forecasts of future continental motion developed slowly as offshoots of the theory of plate tectonics, which won acceptance in the 1960s and 1970s, shattering old dogmas of continental immobility. The theory of plate tectonics holds that the surface of Earth is composed of a dozen or so huge crustal slabs that float on a sea of partially molten rock. Over ages, hot convection currents in this sea, as well as gravitational forces, move the plates and their superimposed continents and ocean basins, tearing them apart and rearranging them like pieces of a giant jigsaw puzzle.

The theory, named for the Greek word “tekton,” or builder, is a study in slowness.
Colliding plates grind past one another about as fast as fingernails grow.

Today, geologists measure such changes with great precision thanks to the advent of global positioning satellites and small base stations that dot remote areas of the planet and operate unattended. Arrays of such instruments track the overall movement of plates.

From the start of the theory’s acceptance, geologists worked hard to discover what plate tectonics revealed about Earth’s past and to render it in credible reconstructions. A famous result was Pangea — “all the land” in Greek — a supercontinent that some 200 million years ago held the outlines of today’s continents in embryonic form.

In 1970, Robert S. Dietz, who uncovered major clues to plate movement in the deep sea, wrote a Scientific American article on the breakup of Pangea. He also took a first step toward predicting the future, sketched out plausible continental shifts for the next 50 million years, and projected their local repercussions.

Dr. Dietz zeroed in on the San Andreas fault, the deep gash beneath California that spawned the devastating San Francisco earthquake of 1906 — one among many — and today also threatens the Los Angeles area. The fault marks the seam where the Pacific plate slides relentlessly northward past the North American plate.

Ten million years from now, Dr. Dietz wrote, “Los Angeles will be abreast of San Francisco.” And in another 50 million years, he added, Los Angeles will have moved up the west coast into Alaskan waters.

Such visualizations caught the eye of Dr. Scotese in the late 1970s when he was a graduate student at the University of Chicago. He loved the challenge of the big picture, and of using his computer programs, new to such work, to speed the endless drawing and redrawing. With his graduate adviser, he made detailed maps showing the past evolution of the continents. And he toyed with their future shapes.

In 1982, Dr. Scotese got a call from Discover magazine asking if he would consider a bigger project: envisioning Earth not 50 million years from now but 200 million.

“I said, ‘Hold on, how could I possibly project that far?’ ” he recalled. “But being a rash graduate student in need of extra change, I decided to give it a try.”

The effort forced him to think beyond simple extrapolations and come up with rules that could govern the onset of major tectonic events, like the formation of subduction zones, deep ocean trenches that gobble up seafloor and tear continents apart.

Dr. Scotese drew a series of futuristic maps and, in the 1990s, found an ideal way to communicate his increasingly detailed visions of the terrestrial past and future: over the Internet. Today, Dr. Scotese’s Web site, http://www.scotese.com/, which he started in 1998, showcases his work, called the Paleomap Project. Teachers use his animations, and his site has won scientific awards. (He aids the publishing world, too, providing Dr. Press and his co-authors with their map of the future world.)

Today, some of his most ambitious efforts center on envisioning how Earth might look 250 million years from now. The easy part, Dr. Scotese said, is the continents.

Their masses might change shape but seldom disappear altogether because their bedrock weighs little compared with dense ocean crust. Continents literally float above the action. So do mountains. Once formed, they tend to persist, disappearing only after ages of erosion wear them down.

The difficult part, he said, is predicting the development of new subduction zones in the seabed, and in comprehending how aggressively they rearrange the land.
“It’s hard to understand all the forces down there,” he said. “There’s probably some input from the mantle,” the deep, hot, churning zone below the crust. “It probably has some say on which way the plates go.”

His long-term forecast, despite the uncertainties, portrays a distant time when the world’s continents come together again to form a new supercontinent, which he calls Pangea Ultima.

An animated depiction of Pangea Ultima demonstrates a bold exercise in futuristic thinking. First, the Mediterranean closes. Then — 25 million to 75 million years from now — Australia moves north, slamming into Indonesia and Malaysia before pirouetting counterclockwise to smash into the Philippines and then Asia, eventually merging with it.

Antarctica also moves north, shedding its icecap. Roughly 100 million years from now, it plows into the Indian Ocean, and 50 million years later wedges itself between Madagascar and Indonesia. The Indian Ocean becomes a virtual inland sea.

Meanwhile, Dr. Scotese said, the biggest tectonic change of all — driven by a giant subduction zone — has torn through the dark Atlantic and begun to eat up seabed, slowly closing the ocean. Some 200 million years from now, the closure forces Newfoundland to smash into Africa and, a bit later, Brazil to ram into South Africa.

The collisions gain force; 250 million years from now, the continents will have merged into a new supercontinent that encircles what remains of the Indian Ocean.

“It’s more like a big donut or bagel than Pangea,” Dr. Scotese noted. In looking for a name, “I tried Bagelea or Donutea but figured that would trivialize the whole experience. A friend suggested Pangea Ultima — classy, like a fancy car. It implies that it’s the last Pangea, which certainly isn’t true, but it’s the last one I’m going to come up with.”

Over billions of years, the continents have split and joined repeatedly in a slow dance. They do so, geologists say, in two very dissimilar ways. As Dr. Scotese envisioned, they can break apart and reverse course to merge again. Or they can move apart until their paths around the planet cause them to coalesce on the far side.

Some forecasters see the current expansion — with no course reversal — as dominating the remainder of this tectonic cycle. Instead of the Atlantic disappearing, they say, the Pacific might slowly vanish, sending North and South America careering into Asia.

Sergei A. Pisarevsky, a geologist at the Tectonics Special Research Center of the University of Western Australia, calls the resulting supercontinent “Amasia.” But he said his vision bore no special likelihood of coming true. “My guess,” he said, “is that the Pacific should disappear.”

J. Brendan Murphy, a specialist on tectonic cycles at St. Francis Xavier University in Nova Scotia, echoed that view. Geologists, he said, know too little about the causes of plate motion to predict the dance of continents into the far future.
But in the next few decades, he added, progress in geology is likely to lay bare Earth’s inner workings and enhance the art of plate forecasting.

Even with today’s efforts, Dr. Murphy said, “all the coughing stops in class when I show a projection. It’s a very interesting way to look at the world.”

“The most important thing to realize,” he said, “is that modern geology is just a snapshot of a continuous-action movie. When you zoom out and look over a long enough time, you realize everything is mobile.”