Moon Is Dragging Continents West, Scientist Says
Richard A. Lovett
for National Geographic News
|January 24, 2006|
Someday not so soon Washington, D.C., may find itself about where San Francisco is now. According to a recent study, Earth's surface may be slipping slowly westward, dragged by the same lunar forces that produce tides.
The Earth's crust is divided into vast plates that slowly shift, producing earthquakes, mountains, and rifts where they collide or separate. Most earth scientists believe that this movement is the result of rising and falling currents of magma deep below the surface.
In addition to being jostled every which way from below, the planet's plates are sedately sliding toward the sunset, says Carlo Doglioni of the earth science department at Rome's La Sapienza university.
In a study published in the January-February issue of the Geological Society of America's journal Bulletin, Doglioni and a team of Italian and U.S. scientists argue that the westward motion is due to the tidal attraction of the moon.
As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly holds the Earth's surface layer back. This "lunar drag" causes the crust to slip slowly backward, like a loose handgrip on a bicycle handlebar.
No Easy Task
Proving the theory is no easy task.
After all, if everything on Earth is spinning in the same direction at about the same rate, what do you use as a reference point? For Doglioni the answer is to use the volcanic hot spots beneath places such as the Hawaii Islands.
Most geophysicists, including Doglioni, believe that such chains result from heat plumes rising from beneath the Earth's surface, as if from volcanic blowtorches.
As the crust drifts over these hot spots like a conveyor belt, the heat punches through the surface, producing chains of volcanoes, the thinking goes. (Build a virtual volcano online.)
The locations of these islands indicate the crust's motion above them.
For example, Kauai is much farther from the Hawaiian Islands' hot spot than its younger siblings. Kauai has been drifting for millions more years than its neighbor, Oahu. The Big Island, farther east, is currently over the hot spot, as evidenced by its volcanic activity.
Since the hot spots don't move with the crust, they provide reference points against which the movement of the crust's plates can be measured, Doglioni says.
Using this method, he has calculated that the Earth's plates are drifting westward at about 50 to 90 milimeters (2 to 3.5 inches) a year.
Another line of evidence, he says, comes from a "profound asymmetry" in the distribution of the Earth's big mountains.
For example, "why are the Andes and the Rocky Mountains only on the eastern side of the Pacific Ocean?" Doglioni asks.
The answer, he says, is that there is more mountain-building power in the eastern Americas.
There, the underground forces of plate tectonics are driving the Pacific plates eastward, toward the continental block, which Doglioni says is being pulled westward by the moon's gravity.
The Pacific plates and the American plates collide head-on, in an impact whose forces are large enough to lift the land far upward, building the great North and South American mountain ranges.
On the other side of the Pacific Ocean the mountain-building force is weaker. That's because the Pacific plate there is also moving westward, essentially rear-ending the Asian continent.
This type of impact produces less crumpling than a head-on collision, and the resulting mountains are correspondingly smaller.
The scientist took his theory a step further by attempting to determine what conditions are needed for lunar drag to occur.
He concluded that there must be a thin tier near the top of Earth's mantle (the layer between the Earth's core and its crust) that is less rigid than most earth scientists think it is.
That might be the case if the mantle contains slippery rock layers that slide sideways with relative ease. A recent report in the journal Nature said just that.
The seismologists behind the Nature study appear to have found traces of a slippery layer beneath eastern North America.
"Many Uncertainties" in the Theory
Karen Fischer, a seismologist who co-authored the Nature report, is cautious about Doglioni's study.
"What I would like to see is a quantitative analysis of the westward forcing you see in the paper, compared to other forces acting on the plates," said Fischer, who teaches at Brown University in Providence, Rhode Island.
Other geophysicists are equally skeptical.
"It's creative," said Carolina Lithgow-Bertelloni, a professor of geophysics at the University of Michigan in Ann Arbor. "But there are many uncertainties, and it is extremely hard to test."
She's not so sure about the theory's presumption that the hot spots really are stationaryand neither is Thorsten Becker, an earth sciences professor at the University of Southern California in Los Angeles.
"The hot spots move with respect to each other," Becker said. That motion is slow, but it's enough to make it difficult to argue that hot spots are not truly stationary and therefore are not good reference points.
"Another problem is that most of the hot spots [cited by Doglioni] sit on the Pacific plate," he said.
In other words, they may be appear stationary because they are all on the same plate. Hot spots on other plates may be moving at different rates and in different directions.
Even if lunar drag does exist, Becker says, it is probably impossible for the mantle to contain a sufficiently slippery layer for the moon to be the cause of westward drift.
Still, Doglioni sees exciting, even extraterrestrial, applications for his research.
Lunar drag, he said, "might explain why plate tectonics as we know them on Earth do not occur on moonless planets such as Mars or Venus."
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