Alfred Wegener sparked a scientific revolution in 1912 by theorizing that great slabs of the Earth's rocky surface -- tectonic plates -- slide under, over or past each other, setting continents adrift. Hotly debated as recently as the late '60s, tectonic plate theory is now universally accepted. But one major question remains: What drives the movement of the great plates?
"The scientific community has developed in two different directions, with half thinking that mantle convection drives the plates and the other half thinking that gravitational forces such as subduction drive the plates and that mantle convection doesn't have any role," said Götz Bokelmann, visiting associate professor of geophysics at Stanford. "I'm really excited that I and several other people as well have data that may help to resolve some of that." Bokelmann and Eugene Humphreys of the University of Oregon led a session on tectonics Dec. 17 at the annual meeting of the American Geophysical Union in San Francisco.
That morning, more than 800 geophysicists convened at the Moscone Convention Center to revive the "great plate debate," which had lain dormant for decades. Inspired by new knowledge created by recent advances in seismology and modeling, they hoped to find common ground concerning one of geology's great unsolved mysteries: how tectonics happens.
If scientists can gain a better understanding of the planet's large-scale dynamics, they will be able to better model small-scale dynamics responsible for earthquakes and volcanic eruptions common where plates meet. "But there is much that we don't know -- how stresses are transmitted, if plate tectonics is steady or episodic," Bokelmann said. Plate motions may not be a direct expression of mantle convection, the movement of molten rock as it rises from the Earth's core, cools near the surface, then sinks because cooler material is denser.
Like the latest American presidential election, the mechanism of tectonics splits the scientific community into two evenly divided camps. One contends that continental drift is driven from below: Mantle convection drags a plate toward the side that subducts, or descends beneath an adjacent plate. The other posits a side-driven mechanism, first proposed in the '60s by scientists Egon Orowan and Walter Elsasser, that resembles a conveyor belt: Upwellings of light material at oceanic ridges compress plates in the direction of the heavier subducting side, and gravity continues to pull the heavy plate edge downward into the mantle. Scientists including Stanford geophysics Professor Norman Sleep study such recycling of continents.
So what's the true mechanism? Like the legendary blind men who gave different descriptions of an elephant because one had felt the trunk, another the ears, another the tail, geologists have to look at complex data from many locations worldwide to get a true picture of how plates move.
One source of data is seismic waves -- vibrations produced by earthquakes or explosions. They travel through the planet in different ways, as compressional or shearing waves, and those differences reveal important information. Compressional waves travel like sound: Squeezed pulses alternate with expanded pulses. In contrast, shear waves wobble side-to-side like shaken jelly. Compressional waves can travel through anything. But shear waves do not propagate in liquids, which cannot store the energy needed to generate side-to-side motions. Seismometers, motion sensors that detect earthquake waves, can distinguish compressional waves from shearing waves.
Seismic data are collected at thousands of instrument stations worldwide and arrival time measurements are sent to repositories such as the International Data Center in England and the National Earthquake Information Center in Colorado. Arrival times are triangulated to pinpoint earthquake locations.
By characterizing seismic waves according to their origins and time delays, scientists have been able to create a sort of X-ray image of the planet. Earth has a solid inner core of iron with a little nickel surrounded by a liquid outer core of mostly iron. That in turn is encircled by the mantle, made mostly of rocky materials dominated by silicon and oxygen. And encasing the mantle is the crust, which joins part of the underlying mantle to form the strong lithosphere, like the tough skin of an onion. While some mantle is fluid like lava, most is solid rock. Nonetheless, over the long term the mantle deforms, flowing throughout centuries like glass in the bottom-heavy panes of a medieval European church.
Tectonic plates are about 50 to 100 kilometers (31 to 62 miles) deep, thinner under oceans and thicker under continents. Under the oldest part of a continent, the plate often has a thick rocky root that extends 200 to 400 kilometers (124 to 248 miles) into the deeper mantle, Bokelmann said.
Using his hand as a prop, he demonstrated what scientists would expect to see if the bottom-driven mechanism moved tectonic plates. Holding his palm down, he dangled a finger below the plane of his hand so that it extended forward slightly. "This is the plate," he said, pointing to his hand. "And this is the root here, which sticks into the mantle," he said, wiggling the finger. "Now, I'm moving the plate [the hand], but the root [finger] has to move too -- they're connected." If the mantle moves faster than the plate (as it would in the mechanism that's driven from below), then the root precedes the continent (the finger points forward). Conversely, if forces act from the side, the root would point away from the direction of plate movement (the finger points backward).
To determine which way the root points, Bokelmann used a crystal compass of sorts. Movement of a continent can cause deformation of its root. During deformation, crystals of olivine, the most common mineral in the root, reorient and align in the direction the root moves. Scientists can determine this orientation seismologically because waves propagate faster in this direction than in others.
Looking at the deep root of the stable portion of North America, called the North American Craton, Bokelmann saw evidence to support a bottom-driven mechanism by which the mantle drags continents: Seismic waves from the southwest arrived earlier than expected. "It turns out that everywhere under the craton the fast directions are dipping into the plate motion direction, and that suggests that the root is leading the southwestern motion of the plate and that North America is moving because the deeper mantle pulls it along," he said.
Similarly, India, which broke off from Africa 150 million years ago, has been racing northward at a rate of 5 centimeters (2.5 inches) per year, colliding with Eurasia to form the Himalayas. Said Bokelmann: "It's not clear what causes this motion why India wants to move to the north so quickly. It's very hard to explain this with driving from the side. So perhaps there is driving from below there."
But others geophysicists presented support for lateral forces. Mary Lou Zoback of the U.S. Geological Survey and Mark Zoback of Stanford suggested that stress patterns in the western United States are more consistent with driving from the side.
"I'm not so sure that everywhere on Earth this [bottom-driven] mechanism is the only one," Bokelmann said. "My technique shows what is happening under the continental shields but is not the full answer, especially because we have little data for the oceans yet."
Taken together, scientific evidence will guide geodynamicists, such as UCLA's Paul Tackley, who attempt to model mantle convection and the motion of surface plates. "Several groups attempt to model the dynamics of the Earth, and there's a hope to make models more realistic now," Bokelmann said. "So there is more need to understand what the Earth does."
Seismology is a broad and rapidly developing field. Though the discipline has existed for at least a hundred years, recent dramatic improvement in data coverage and data quality likely will lead to new insights into what is happening inside the Earth, Bokelmann said.
"As geophysicists, most of us are driven by observations that tell us what is happening," he said. "And in the past we didn't have many observations to tell us what the dynamics in the Earth's interior are. We're just now starting to get this."
Meanwhile, the controversy continues. "Everybody has a very strong opinion," Bokelmann said. "Everybody knows the answer. You ask somebody, and he says, 'Oh, sure, the plates are driven from the side -- no question.' Somebody else says, 'Of course the plates are driven from below. Everybody knows that.' So everybody's sure, but of different things, very different concepts. And that makes it very interesting to me because obviously we live on the same Earth -- but maybe really both mechanisms act with different importance in different regions."
Can both mechanisms co-exist peaceably? "We probably have a mixture between the two," Bokelmann concluded. "Truth is often a compromise."
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