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Thursday, January 10, 2008

The motion, formation, and recycling of Earth’s crust

Earth's Moving Crust May Occasionally Stop
The motion, formation, and recycling of Earth’s crust—commonly known as plate tectonics—have long been thought to be continuous processes. But new research by geophysicists suggests that plate tectonic motions have occasionally stopped in Earth’s geologic history, and may do so again. The findings could reshape our understanding of the history and evolution of the Earth’s crust and continents.

Synthesizing a wide range of observations and constructing a new theoretical model, researchers Paul Silver of the Carnegie Institution of Washington and Mark Behn of the Woods Hole Oceanographic Institution (WHOI) have found evidence that the process of subduction has effectively stopped at least once in Earth’s past. Subduction occurs where two pieces of Earth’s crust (tectonic plates) collide, and one dives beneath the other back into the interior of the planet.

Most of the major geologic processes on Earth—the formation of continents, the birth of volcanic island arcs, the opening and closing of ocean basins—are driven by tectonic plate motions and intimately linked to subduction and to seafloor spreading. If those processes were shut down, there would likely be a global decrease in earthquakes and volcanism.

Today, the vast majority of subduction occurs around the edges of the Pacific Ocean, which is slowly closing as the Atlantic Ocean opens. In roughly 350 million years, researchers estimate that the Pacific basin will be effectively closed and a new supercontinent will be formed.

Closure of the Pacific basin could shut down most of the Earth’s capacity for subduction, unless the process begins somewhere else on the planet. However, there is no evidence that subduction is currently expanding or initiating anywhere else on the planet.

Though such a shutdown defies the prevailing wisdom about plate tectonics, Silver and Behn read the geologic evidence to suggest that just such a dramatic decrease in subduction happened about one billion years ago, after the formation of the supercontinent Rodinia.

Their findings—captured in a paper entitled “Intermittent Plate Tectonics?”—were published in the January 4 issue of the journal Science.

“The scientific community has typically assumed that plate tectonics is an active and continuous process, that new crust is constantly being formed while old crust is recycled,” said Behn, an assistant scientist in the WHOI Department of Geology and Geophysics. “But the evidence suggests that plate tectonics may not be continuous. Plates may move actively at times, then stop or slow down, and then start up again.”

Behn and Silver started their investigation by considering how the Earth releases heat from its interior over time, also known as “thermal evolution.” If you take the rate at which the Earth is releasing heat from its interior today and project that rate backwards in time, you arrive at impossibly high and unsustainable numbers for the heat and energy contained in the early Earth. Specifically, if the planet has been releasing heat at the modern rate for all of its history, then it would have been covered with a magma ocean as recently as one billion years ago.

But we know this is not true, Behn said, because there is geological evidence for past continents and supercontinents, not to mention rocks (ophiolites) on the edges of old plate boundaries that are more than one billion years old.

The Earth cools more quickly during periods of rapid plate motions, as warm material is pulled upward from deep in the Earth’s interior and cools beneath spreading ridges.

“If you stir a cup of coffee, it cools faster,” said Behn. “That’s why people blow on their coffee to get the surface moving.”

“It is a similar process within the Earth," Behn added. "If the tectonic plates are moving, the Earth releases more heat and cools down faster. If you don’t have those cracked and moving plates, then heat has to get out by diffusing through the solid rock, which is much slower.”

Periods of slow or no subduction would help explain how the Earth still has so much heat to release today, since some of it would have been capped beneath the crust.

Silver and Behn conclude their paper by suggesting that there is a cycle to plate tectonics, with periods when the shifting and sliding of the crust is more active and times when it is less so. Rather than being continuous, plate tectonics may work intermittently through Earth history, turning on and off as the planet remakes itself.

On the Net:

The Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean's role in the changing global environment.

The Carnegie Institution of Washington has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

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