Tuesday, December 02, 2008

Plate Tectonics Earlier Than Thought

via Astrobiology, news of research into Earth's ancient past appears to indicate the presence of water and the geophysics of the plate tectonic system far earlier than previous estimates. New analysis by UCLA geochemists of extremely early forming zircon chrystals found in ancient magma fields show that the chrystals formed at a much lower temperature than can be explained without the presence of water. Furthermore, the evidence seems to indicate that the crystals could not have formed anywhere but in a subduction zone, where colliding tectonic seafloor plates move underneath the continental plates.

"We are proposing that there was plate-tectonic activity in the first 500 million years of Earth's history," said geochemistry professor Mark Harrison, director of UCLA's Institute of Geophysics and Planetary Physics and co-author of the Nature paper. "We are reporting the first evidence of this phenomenon."

"Unlike the longstanding myth of a hellish, dry, desolate early Earth with no continents, it looks like as soon as the Earth formed, it fell into the same dynamic regime that continues today," Harrison said. "Plate tectonics was inevitable, life was inevitable. In the early Earth, there appear to have been oceans; there could have been life — completely contradictory to the cartoonish story we had been telling ourselves."

The crystal analysis was done by an ion microprobe, which shoots beams of ions into a substance to determine its precise chemical composition by means of a mass spectrometer. The analysis of the zircon samples found in Western Australia showed ages exceeding 4 billion years. The accepted theories of plate tectonics had the system starting much more recently, no more than 3.5 billion years ago. The Earth is thought to have formed around 4.5 billion years from today.

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