May 14, 2009 -- Earth's violent tremors and fiery volcanic eruptions are commonplace today, a natural outgrowth of plate tectonics. But it wasn't always so.
According to a new study, the planet's geologic engine mysteriously shut off 2.4 billion years ago. Volcanoes went on a 250-million year hiatus, and the atmosphere cooled until the world's first glaciers sprouted on the ancient continents.
"There was a widespread slowdown, maybe even a shutdown of plate tectonics," said Kent Condie of the New Mexico Institute of Mining and Technology in Socorro. "Earth may have reverted into a stagnant lid state of cooling."
Stagnant lid tectonics is an alien style of geology, thought to have occurred on Mars and still be active on Venus. Venus is so hot that the mantle is too soupy to force rocky plates into one another, or tear them apart. Instead the crust sits like a single, solid skin on the planet.
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Earth was a similar place billions of years ago, still fresh from the crucible of our young solar system. It may have undergone several periods of stagnant lid tectonics before the mantle cooled enough for plate tectonics to kick in, around 3.0 billion years ago.
Condie believes the lull in activity he and his colleagues have discovered is evidence that the stagnant lid came back one last time. Their work is published in the June issue of the journal Earth and Planetary Science Letters.
The quiet period may have had major consequences for global climate. Oxygen first emerged in the air in large quantities around 2.4 billion years ago, supplanting what had been a methane-dominated atmosphere.
With volcanoes nowhere to be found, carbon dioxide was in short supply, and global temperatures plummeted. There is evidence from rocks in China, North America and Australia that the first glaciers appeared around this time, too.
Kevin Burke of the University of Houston is not convinced. Most of the rocks formed billions of years ago have since been destroyed, so a lack of evidence of volcanic activity doesn't prove anything, he argues.
"They seem to think that what you see at the surface of the Earth is what happened," he said. "I think it's a record of what's left -- what didn't get destroyed."
Plate tectonics is a fundamental process on the planet, he added, not something that can switch on and off like a light.
"Heat gets out of the Earth by running plate tectonics, and the occasional mantle plume. I don't think you can turn it off," he said.
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