Plate Tectonics: Earth's Lucky Geology

an. 11, 2008 -- Four decades after the rise of the great, unifying theory of plate tectonics, geologists are still scratching their heads over a lot of the details.

Unanswered, for instance, are basic questions like how the shifting and colliding of plates got started, what keeps plates moving, why other planets in our solar system lack plate tectonics, and how important all the geological turmoil might be to the evolution of life.

"We didn't get it all right the first time, so let's ask the questions," said geologist Vicki Hansen of the University of Minnesota at Duluth, referring to the fact that despite decades of work, many mysteries remain.

Hansen recently stirred the pot with a controversial hypothesis published in last month's issue of the journal Geology. Meteorite impacts early in Earth's history, she suggested, created the first rifts in the crust, jump-starting plate tectonics.

Prior to the 1960s, geologists were hard pressed to explain such basic things as how most mountain ranges formed and why volcanic regions and earthquakes were clustered in certain parts of the planet. Plate tectonics put these phenomena, and many others, into a single, unified framework.

That framework is an Earth with a rocky crust divided into plates that are moving, rifting, colliding and overrunning each other. It finally made sense of a previously nonsensical geography and is now recognized as one of the greatest scientific breakthroughs of the 20th century.

Energizer Bunny Tectonics?

Another iconoclastic hypothesis just out last week goes after the question of whether plate tectonics ever stops. Has it ever done so? Geologist Paul Silver of the Carnegie Institution of Washington thinks it's possible.

"It's an implicit assumption that plate tectonics never shuts down," Silver told Discovery News. "But it's nowhere stated in plate tectonics theory."

Silver and his colleague Mark Behn proposed in the Jan. 4 issue of Science that all it takes to stop plate tectonics is the devouring of the crustal plate under the Pacific Ocean. And that's not as far-fetched as it sounds.

The Pacific Plate is surrounded by most of Earth's overriding (subducting) crust collision zones, so it's getting smaller all the time. Eventually, roughly 350 million years from now, the surrounding adjacent continents will collide.

Meanwhile, the lost crust is being made up on the other side of the planet by the Mid-Atlantic Ridge, which has been efficiently churning out magma and expanding the Atlantic for millions of years.

The end result would be a supercontinent, no remaining subduction zones, and virtually no plate tectonics, at least for a while.

Accidental Earth

In recent years, plate tectonics has also become a matter of importance in the search for life on other planets. Is it, for instance, just a coincidence that Earth is the only planet in our solar system known to have both life and plate tectonics?

Probably not.

"Plate tectonics helps make a planet habitable," said astrobiology researcher Diane Valencia of Harvard University. It does so by regulating a planet's climate, she said.

Valencia and her colleagues recently published an article in the Astrophysical Journal outlining how very large, rocky planets in other solar systems -- which they call super-earths -- can have plate tectonics, and therefore be great candidates for life.

On Earth plate tectonics help regulate the planet's long-term temperature by recycling climate-warming carbon from the atmosphere, Valencia explains.

Plate tectonics allows captured carbon that is buried in the seas to find its way back into the atmosphere via subduction zones. Where one plate is pushed under another, carbon-rich, wet ocean sediments are pressed into the Earth's mantle, where they are heated. The water there helps melt the sediments, which then buoy upwards to create chains of volcanoes -- which release the carbon back into the atmosphere.

"If you don't have plate tectonics, you don't have this way of transporting materials out of (and back into) the atmosphere," said Valencia.

This sort of recycling -- which takes place over a over a million-year timescale -- doesn't eliminate some millennial-scale climate swings, she said. But it's a thermostat which keeps Earth's long-term climate within the range that allows water to remain liquid -- the habitable range for life.

What this means for other planets in other solar systems is that plate tectonics can expand the Goldilocks zone of habitability around a star -- where it's neither too hot nor too cold -- by allowing a planet to better regulate its own temperature and keep water wet.

Very large, rocky planets -- those super-earths -- would be the most likely places for life because their greater internal heat causes them to experience larger forces on thinner plates, Valencia asserts. As a result, they would be particularly good at regulating their climates and allowing life to evolve.

It's likely that the lack of plate tectonics is the reason that both Mars or Venus -- Earth's closest local sibling planets -- are dead, Valencia explained.

"If Mars were to have plate tectonics, it would have to be bigger early on," said Valencia. This is because plate tectonics require a planet to have a lot of interior heat to keep things moving. Smaller planets dissipate their heat faster, and so have a very short window of time for plate tectonics.

Venus, on the other hand, is about the same size as Earth, but it lacks water, said Hansen. Without water in the mantle to help melt rocks and trigger volcanic recycling of material, Venus' crust appears to have remained stiff and locked up forever. Had Venus held more water, or if it had been a super-sized rocky planet, it too would have had plate tectonics and perhaps life.

The implication of all this, of course, is that little old Earth lucked out. A little less water and the planet may not have had plate tectonics. Climate swings would have been harsher, and life might have foundered early on.

Earth, just barely large enough to have the internal heat; just wet enough to melt and recycle its crust -- may have barely made the cut for life.