Magazine article Science News

Young and Restless: Ancient Earth Shows Moving Crust

Magazine article Science News

Young and Restless: Ancient Earth Shows Moving Crust

Article excerpt

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 Standigel 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. …

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