Meteor Pelting May Have Triggered Plate Tectonics

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.