Tiny Plankton Contribute to Continental Crackups

Nov. 17, 2008 -- The skeletons of microscopic plankton that flourished billions of years ago may be tearing continents apart.

Common throughout Earth's waters today, photosynthetic plankton have been around almost since the planet formed. Perhaps as far back as 3.8 billion years ago, their carbon husks began piling high on the ocean floor as they died, until over time they became black shales many kilometers (several miles) thick.

As hundreds of millions of years passed, the shales would get plastered to the edges of continents, then slowly kneaded into their interiors. Today they're prevalent in most major mountain ranges, including the Alps, Himalayas, Sierra Nevada and Appalachians.

Norm Sleep of Stanford University thinks these biologically-built rocks form huge weak areas in Earth's crust. And when plate tectonics begin stretching out a continent they are the first to break, encouraging the land mass to rip itself apart.

"Black shales sequester radioactive material, so you get areas of high heat and heat flow, which makes the rocks weak." Sleep said.

The is effect is just one of many ways Sleep sees living creatures affecting plate tectonics. He'll be presenting the controversial new theory next month at the Fall Meeting of the American Geophysical Meeting in San Francisco.

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Volcanic eruptions also owe a debt to biology. Limestone -- itself a rock born from the skeletons of ocean-going life -- has been subducted into Earth's mantle for hundreds of millions of years. There it heats up until it partially melts, mixes with other magmas and comes boiling up to the surface as lava.

"The composition of Mt. Pinatubo and Mount St. Helens is partly biologically controlled," Sleep said.

Kevin Hefferan of the University of Wisconsin-Stevens Point said parts of Sleep's theory, like radioactivity in black shales, maybe be a bit speculative.

"Some of these things he makes a pretty convincing case for -- others, like the radioactive material in shales, are a bit of a stretch," he said. "Granites also sequester lots of radioactivity, so why wouldn't they be weak?."

However, he added "black shales are notorious for landslides, and developing faults. They are inherently weak, so in a way it does make sense that rifting would start with them."

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American Geophysical Meeting