In other locations the movement sends continents careening into each other, creating massive mountain ranges. (Related: "Deadly Java Quake Highlights 'Ring of Fire' Dangers" [June 30, 2006].)
The phenomenon, known only to occur on Earth, is the reason the planet has continents and land instead of being one giant ocean.
But scientists have long debated when the process actually began, since it requires Earth to be cool enough for its surface to become solid and rigid, not liquidlike.
Many experts have argued for a late start, because until now the oldest known ophiolite was a 2.5-billion-year-old formation spotted in north China.
"There is considerable controversy about when plate tectonics started," said Jeffrey Karson, a professor of structural geology and tectonics at Syracuse University in New York state.
"The significance of this discovery is that it pushes the earliest evidence of seafloor spreading back very significantly in time."
But, he pointed out, this find shows only that seafloor spreading was occurring back then—not other features of plate tectonics.
"That is a very important clue as to the nature of the early Earth—but not definitive proof of plate tectonics," he said.
Study leader Furnes, however, suspects—but can't yet prove—that subduction was also going on 3.8 billion years ago.
Origin of Life?
The possibility that early plate tectonics paved the way for life to bloom on Earth brings study leader Furnes and his team full circle.
"The main reason for choosing these particular rocks in Greenland was to sample these oldest known pillow lavas [pillow-shaped formations created during underwater eruptions] in order to look for chemical traces of life," he said.
"At least one group of geoscientists suggests that life may have evolved in close association with oceanic spreading centers," he pointed out.
According to Karson, of Syracuse University, "hydrothermal systems that are focused at mid-ocean ridge spreading centers are places where water-rock chemical reactions and vigorous circulation systems concentrate potential nutrients, such as sulfur and volcanic gases.
"The fractures in oceanic crust," he added, "would be good places for early Earth organisms to survive, despite meteor impacts and other extreme environmental changes affecting the surface of the early Earth."
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