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Published online 26 November 2008 | Nature | doi:10.1038/news.2008.1256
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Minerals yield signs of early plate tectonics
Evidence of 4-billion-year-old subduction points to an early start for modern-Earth geology.
Minerals preserved within ancient grains of rock, like flies within amber, have provided the earliest hint of plate tectonics on Earth. The minerals suggest that subduction — the process by which one tectonic plate slides beneath another — may have been active more than 4 billion years ago.
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Subduction is not the only way that water can sink to low depths in the earth. Thermal stress from cold water can open up deep cracks also, into which water can flow. Indeed, this is probably the way ocean trenches formed in the recent past and would account for their low heat release, their surface depth, and the absence of sea mounts being scraped off. You may see a discussion of this in http://charles_w.tripod.com/trenches.html Sincerely, Charles Weber