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Young and Restless: Ancient Earth shows moving crust

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7:39am, March 21, 2007
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Tectonic movements of Earth's crust began at least 3.8 billion years ago, according to new evidence from the oldest known rock formation.

After Earth formed about 4.5 billion years ago, its cooling surface eventually developed continental plates and ocean basins that move as they do today. But scientists haven't known just when this transition to plate tectonics occurred.

Previously, the oldest rocks showing signs of tectonic activity came from the 2.5-billion-year-old Dongwanzi complex in northern China. The new evidence, from a much older rock formation in southwestern Greenland, pushes this date back by 1.3 billion years, nearly to the time when heavy bombardment by asteroids kept Earth's crust largely molten.

Over Earth's history, most of the original crust has recycled by sinking into the planet's hot interior, melting, and heading back toward the surface. However, a few pieces of the ancient crust never sank.

The oldest such area is the Greenland formation known as the Isua supracrustal belt, with rocks from 3.7 to 3.8 billion years old. It's "by far the oldest material on the planet that is structurally coherent, [but] people had never really found these telltale signs of oceanic spreading," says study coauthor Hubert Staudigel of the Scripps Institution of Oceanography in La Jolla, Calif.

Staudigel and his colleagues recently observed ancient outcrops of oceanic crust marked by long, parallel cracks filled with volcanic rock. Tension caused this ancient piece of seafloor to split, the researchers propose, allowing magma from the hot interior to rise and seal the cracks. Those physical characteristics, along with the chemical signature of oceanic magma, indicate that the rock was part of a spreading oceanic plate—a sure sign of tectonic activity.

"It's a marvelous case of solving a jigsaw puzzle, and a very difficult one because these rocks are all very old and have been badly mangled," comments Gustaf Arrhenius, also of Scripps.

Most geologists had argued that plate tectonics must have begun soon after asteroid bombardment ended, Staudigel says. Theoretical calculations show that in a static crust, lighter elements wouldn't have risen into the continents, leaving heavier elements in the oceanic crust, as geologists observe today.

The severe deformation of the Greenland rocks partly explains why scientists hadn't discovered these signs of tectonic activity sooner. Staudigel and his colleagues relied on multiple lines of chemical and structural evidence to prove the origin of the battered rocks, they report in the March 23 Science. Christopher Fedo, a geologist at the University of Tennessee at Knoxville, and other scientists agree that the Scripps team's analysis is sound.

"Just that the oldest rocks on Earth show evidence of tectonic processing, that in and of itself is a huge thing," Fedo says.

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