Ancient traces of plate tectonics?

Ancient traces of plate tectonics?

One of the best places for geologists to study ancient seaflooris the mountains. When continents collide and close up ocean basins, slivers of oceanic crust, called ophiolites, are sometimes thrust up into mountain belts. Because complete ophiolites have not been found in Archean-aged rocks (2.4 billion to 4 billion years ago), some scientists have speculated that the opening and closing of ocean basins, integral to plate tectonics today, did not operate during the earth's earliest geologic eon.

Now, in the July GEOLOGY, a group of geologists reports findingwhat they believe is a remnant of Archean oceanic crust that may provide the first evidence of seafloor spreading and basin closure during the Archean.

From top to bottom, modern seafloor consists of a layer ofsediment, "pillow" lavas that have split onto the seafloor, dikes through which magma has risen and a layer of magma that had crystallized in the magma chamber beneath a spreading ridge. In Archean rocks, scientists had previously found only sediments and pillow lavas, leading some to suggest that these rocks were created not by seafloor spreading in deep oceans but in the shallow seas that formed over continents as they were rifted apart.

Now Herwart Helmstaedt at Queen's University in Kingston,Ontario, and his colleagues have discovered a layer of dikes as well in 2.7-billion-year-old rocks in Yellowknife, Northwest Teritories. The structure of this dike complex, say the researchers, suggest that the Yellowknife rocks "evolved in a basin floored by oceanic [rather than continental] crust."

"This discovery extends the plate tectonics model backwardsin time to the Archean and suggests that something resembling modern seafloor was being incorporated into mountain belts then," comments Dugald Carmichael at Queen's University. "It's really an exciting finding."