Material in oceanic lava flows is "recycled" and billions of years old Credit: Kelvinsong, Wikipedia, Creative Commons SAN DIEGO, April 25 (UPI) -- U.S. researchers say evidence suggests material contained in oceanic lava flows is "recycled," having originated in Earth's ancient crust billions of years ago. Scientists at the Scripps Institution of Oceanography at the University of California, San Diego, say the findings support the theory that much of the Earth's original crust has been recycled by the process of subduction, helping to explain how the Earth has formed and changed over time. Subduction takes place when one of the Earth's tectonic plates moves under another plate and sinks into the mantle as the plates converge. "Our new results are important because they provide strong evidence not only to tie materials that were once on Earth's surface to an entire cycle of subduction, storage in the mantle, and return to the surface as lavas, but they also place a firm time constraint on when plate tectonics began; no later than 2.5 billion years ago," Scripps geochemist James Day said. The study, which focused on the presence of specific sulfur isotopes found in some oceanic lava flows, adds support to the theory that most of the crust from the Archean geologic eon -- Earth's second oldest, dating from 3.8 to 2.5 million year ago -- was subducted or folded back into the Earth's mantle before being return to the surface in volcanoes. The study looked at lava that erupted from volcanoes in the Cook Islands in the South Pacific, researchers said, providing direct evidence that oceanic crust was recycled in the mantle. "We have identified material that was actually at the surface 2.45 billion years ago," Rita Cabral, lead study author and a geochemistry graduate student at Boston University, said.