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

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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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