Dec. 10, 2007 -- The continuous overturning, melting and re-casting of Earth's crust over the eons may have started with a massive asteroid impact in Earth's infancy, suggests one geologist.
The unusual and iconoclastic hypothesis, if true, could help point the way to how and why plate tectonics did or did not get started on other worlds in our solar system and beyond. That's important because one of the critical ingredients of life on Earth is a constantly recycling crust.
"Everyone argues about when (plate tectonics) starts, but never asks about outside processes," said geologist Vicki Hansen of the University of Minnesota at Duluth.
Hansen had spent years studying Venus when it occurred to her there was a blind spot in the thinking of Earth-focused geological research: The role of impacts in a once asteroid-thick early solar system.
"As geologists we don't even consider impacts," she said, beyond perhaps causing one or two mass extinction events much later in Earth's history. "We should at least ask the question."
Hansen asks that question in an article in the December issue of the journal Geology. She also puts forth a possible model for how an asteroid could set plates in motion.
It all starts with an early Earth, more than 3 billion years ago, where the densest materials have sunk to the core, and the lightest materials settle into an outermost crust -- what is known as a warm "felsic" crust.
"You can't start plate tectonics because this (felsic) crust is like Silly Putty," Hansen says. It's like the crust that forms on pea soup, she says. "That crust is just going to go everywhere."
In some places that crust would be hotter and weaker from the heat-driven upwelling and convection in the Earth's mantle, immediately below. But those weaknesses would not be enough to cause the denser mantle material to erupt onto the surface, she said.
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.
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