By Natalie Angier
The theory of plate tectonics is one of the great scientific advances of our age, right up there with Darwin’s theory of evolution and Einstein’s theory of relativity. The idea that Earth’s outer shell is broken up into giant puzzle pieces, or plates, all gliding atop a kind of conveyor belt of hot, weak rock — here rising up from the underlying mantle, there plunging back into it — explains much about the structure and behavior of our home planet: the mountains and ocean canyons, the earthquakes and volcanoes, the very composition of the air we breathe
Yet success is no guarantee against a midlife crisis, and so it is that half a century after the basic mechanisms of plate tectonics were first elucidated, geologists are confronting surprising gaps in their understanding of a concept that is truly the bedrock of their profession. They are sparring over when, exactly, the whole movable plate system began. Is it nearly as ancient as the planet itself — that is, roughly 4.5 billion years old — or a youthful 1 billion years, or somewhere in between?
They are asking what caused the shell to crack apart in the first place, and how the industrious recycling of Earth’s crust began. They are comparing Earth with its sister planet, Venus. The two worlds are roughly the same size and built of similar rocky material, yet Earth has plate tectonics and Venus does not. Scientists want to know why.
“In the 1960s and 70s, when people came up with the notion of plate tectonics, they didn’t think about what it was like in the distant past,” said Jun Korenaga, a geophysicist at Yale University. “People were so busy trying to prove plate tectonics by looking at the present situation, or were caught up applying the concept to problems in their own field. The origin issue is a much more recent debate.”
Researchers also are exploring the link between plate tectonics and the evolution of complex life. Fortuitously timed continental collisions and mountain smackdowns may well have supplied crucial nutrients at key moments of biological inventiveness, like the legendary Cambrian explosion of 500 million years ago, when the ancestors of modern life-forms appeared.
“The connection between deep Earth processes and Earth surface biology hasn’t been thought about too clearly in the past, but that’s changing fast,” said Aubrey Zerkle, a geochemist at the University of St. Andrews in Scotland. It is increasingly obvious that “you need plate tectonics to sustain life,” Zerkle added. “If there wasn’t a way of recycling material between mantle and crust, all these elements that are crucial to life, like carbon, nitrogen, phosphorus and oxygen, would get tied up in rocks and stay there.”
The origin and implications of plate tectonics were the subject of a recent meeting and themed issue of Philosophical Transactions of the Royal Society. Researchers said that pinning down when and how Earth’s vivid geological machinations arose will do more than flesh out our understanding of our home base. The answers could well guide our search for life and habitable planets beyond the solar system.
Robert Stern, a geoscientist at the University of Texas at Dallas, argues that if we are looking for another planet to colonize, we want to avoid ones with signs of plate tectonic activity. Those are the places where life is likely to have evolved beyond the “single cell or worm stage, and we don’t want to fight another technological civilization for their planet.”
“A relatively benign way for the Earth to lose heat”
The idea that continents are not fixed but rather peregrinate around the globe dates back several centuries, when mapmakers began noticing the complementarity of various land masses — for example, the way the northeast bulge of South America looks as if it could fit snugly in the cupped palm of the southwest coast of Africa.
But it was not until the mid-twentieth century that the generic notion of “continental drift” was transformed into a full-bodied theory, complete with evidence of a subterranean engine driving these continental odysseys. Geologists determined that Earth’s outer layer is broken into eight or nine large segments and five or six smaller ones, a mix of relatively thin, dense oceanic plates riding low and thicker, lighter continental plates bobbing high.
At large fissures on the ocean floor, melting rock from the underlying mantle rises up, adding to the oceanic plates. At other fracture points in the crust, oceanic plates are diving back inside, or subducting, their mass devoured in the mantle’s hot belly. The high-riding continental plates are likewise jostled by the magmatic activity below, skating around at an average pace of 1 or 2 inches a year, sometimes crashing together to form, say, the Himalayan mountain chain, or pulling apart at Africa’s Great Rift Valley.
All this convective bubbling up and recycling between crust and mantle, this creative destruction and reconstruction of parts — “tectonic” comes from the Greek word for build — is Earth’s way of following the second law of thermodynamics. The movement shakes off into the frigidity of space the vast internal heat that the planet has stored since its violent formation.
And while shifting, crumbling plates may seem inherently unreliable, a poor foundation on which to raise a family, the end result is a surprising degree of stability. “Plate tectonics is a relatively benign way for Earth to lose heat,” said Peter Cawood, an Earth scientist at Monash University in Australia. “You get what are catastrophic events in localized areas, in earthquakes and tsunamis,” he added. “But the mechanism allows Earth to maintain a stabler and more benign environment overall.”