Scientists Offer Big Picture of Earth's Driving "Engine"

D.L. Parsell
National Geographic News
May 2, 2001
Standing on firm ground, we're bound to think of Earth as a stable, rock-solid place. But the surface of Earth is constantly being altered, propelled by powerful forces deep inside the planet that give rise to mountains, volcanoes and earthquakes, and even move whole continents.

Exactly what's happening in Earth's interior to drive the whole process isn't fully clear, but a growing body of diverse data and more sophisticated analytical equipment is moving scientists closer to the answer. Now, two Canadians have have drawn on these findings in an effort to explain what's happening.

"This [model] allows us to move well beyond the drift of continents described by plate tectonics," said Jerry Mitrovica of the University of Toronto, co-author of a report on the work published April 26 in Nature. "Plate tectonics was a wildly successful theory, but it's limited in its explanation of what forces are giving rise to the motion we're seeing in Earth's surface."

Mitrovica and Alessandro Forte of the University of Western Ontario drew on recent findings from many corners of earth science to construct an integrated picture of Earth's internal structure and dynamics. They say the model should provide a useful framework for studying many aspects of long-term changes in sea levels and the surface features of Earth.

Cold Slabs and "Hot Plumes"

In particular, Forte and Mitrovica's work is a synthesis of many ideas about two fundamental aspects of earth science that have been the subject of considerable study over the past decade: the structure of matter inside the Earth, and the nature of the convective flow that carries heat to the surface, where it radiates into the atmosphere and drives geological events.

Their picture suggests Earth is being churned by an internal "heat engine," with two immense cold slabs of matter sinking deep below the margins of the Pacific Ocean while two equally large masses of hot, elongated material ("plumes") under Africa and the Pacific Ocean are rising toward the surface like hot-air balloons.

Many individual elements of the picture, such as the existence of cold slabs and "hot plumes" in Earth's mantle, have been known for some time. But the new model represents "a big advance over vague ideas we had 10 years ago" because it provides a more detailed scenario, said Richard Gordon, a professor of earth science at Rice University who specializes in plate tectonics.

"We've known it had to work something like this, but we didn't have any specifics," Gordon said. "Now we have more specific knowledge from images about Earth's interior, and we also know more about the properties of material under high pressures and temperatures in the Earth, which we've learned from experimental lab work by geophysicists over the last 20 years."

Rob van der Hilst, a geophysicist at Massachusetts Institute of Technology who has done many influential studies on mantle structure, agrees that the new model is important and interesting. "This is a good attempt to integrate diverse lines of evidence into a single model of mantle flow and composition," he said, noting that Forte and Mitrovica "bring new combinations of data to bear."

Nonetheless, Van der Hilst cautions that the work is not a new "unifying theory" that ties together everything that's known about the internal forces that drive changes on Earth's surface because there are still too many uncertainties in the data, conclusions and other models on which the study is based. Resolving such uncertainties will require much more research.

"CAT Scans" of Earth

Scientists can't directly observe the internal structure of Earth, but they have learned a great deal about it from seismic images of earthquake waves that travel deep inside the planet—a method similar to medical CAT scanning that probes the inside of the human body. Earthquake waves travel quickly through cold, solid material and more slowly through hot matter.

This and better understanding of the properties of material under high pressures and temperatures have made it possible to determine the composition and temperature of different matter in the Earth, and roughly where it is located.

Much of this matter comes from tectonic plates and is continuously "recycled"; it changes form as it travels (via convective flow) through the relatively fluid mantle lying between the planet's surface crust and its outer core of molten rock.

When the edges of the plates (giant masses of crust) shift and clash in response to gravity forces in the Earth, dense pieces break off and are submerged into the mantle. Near the relatively cold temperatures of surface oceanic plates, this matter consists of fairly solid chunks, which sink deep into Earth. As the material moves closer toward the fiery core, it becomes more highly viscous, like smoldering tar, and elongates into "hot plumes" that rise toward the surface.

A major question for scientists has been how deeply into the mantle this circulation of matter that drives events on Earth's surface is rooted. "We haven't known how mixed up this is in the mantle, although we knew the essential dynamics," Gordon said. Van der Hilst and others have produced evidence in recent years indicating that it goes on at much lower levels of the mantle than previously thought.

Apart from this steadily deforming matter, scientists have also detected large reservoirs of material that appear not to circulate as part of the overall mantle "stew."

Forte and Mitrovica used this knowledge and other scientific results to sketch their picture of dynamic events inside Earth, where continent-sized "megablobs" of unclear origin and gigantic "hot plumes" influence Earth's ebb and flow.

Despite its broad scope, the model is "remarkably simple and symmetric," said Forte. "It really ignites the imagination," he added, "to realize how things are changing hundreds of kilometers beneath your feet and how this change connects to majestic features on Earth's surface."

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