Meanwhile, out in space, the solar system was still pretty full of bits and pieces of failed planets -- asteroids galore. If one of those of sufficient size slammed down on one of the warmer, weaker parts of young Earth's felsic crust, it could punch a hole and trigger melting in the mantle, and an eruption. This erupted lava from the mantle would have solidified and become Earth's first heavier, more brittle "mafic" crust. This is the sort of crust being created today at ocean spreading centers as well as pouring out of the Hawaiian Islands volcanoes. The crack formed by the asteroid would spread, says Hansen, and the more brittle, denser mafic crust would grow larger until it was forced to slide under felsic crust. That would be the first subduction zone -- like those formed by colliding crustal plates under places like the Andes, the Aleutian Islands or Sumatra today. What with the lengthening eruption crack making new crust and the subduction zone taking up the excess crust, plate tectonics would be born. "Once started it's an on-going process," said tectonics researcher Peter Cawood of the University of Western Australia. "But it's making that initial start that is crucial." Perhaps, he said, even today's subduction zones don't start anew, but propagate from pre-existing ones. "It certainly gives our ideas about subduction initiation a shock," Cawood told Discovery News. Related Links: |
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