Evolving Theories of Plate Tectonics

Егемен Қазақстан
21.12.2018 137

By Natalie Angier

Sulfuric gas in the Afar Triple Junction in Ethiopia, at the top of the Great Rift Valley. Three plates meet at this spot. (Massimo Rumi/Barcroft Media, via Getty Images)

The theory of plate tectonics is one of the great advances of our age, 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 puzzle pieces, or plates, gliding atop a 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 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 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 one 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.

Researchers also are exploring the link between plate tectonics and the evolution of complex life. Continental collisions and crashing mountains may well have supplied crucial nutrients at key moments of biological inventiveness.

Aubrey Zerkle, a geochemist at the University of St. Andrews in Scotland, said, “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.”

It wasn’t until the mid-20th century that the 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, devoured in the mantle’s hot belly.

Jun Korenaga, a geophysicist at Yale University, and others believe plate tectonics began right after Earth’s crust solidified. “That is when the conditions would have been easiest for plate tectonics to get started,” he said.

At that point, most of the water on Earth would still be on the surface, with little of it having found its way into the mantle. The heat convecting up through the mantle would exert a stronger force on dry rocks than on rocks that were lubricated. At the same time, the surface water would make it easier for the hot, twisting rocks beneath to crack the surface lid apart. The cracking open of the surface lid, Dr. Korenaga said, is key to getting the all-mighty subduction engine started. With subduction established, water, like oceanic crust, would cycle between Earth’s surface and mantle.

On the opposite end of the origins debate is Robert Stern, a geoscientist at the University of Texas at Dallas, who argues that plate tectonics is a mere billion years old or less, and that Earth spent its first 3.5 billion years with a simple “single lid” as its outer shell: a crust riddled with volcanoes and other means of heat ventilation, but no moving plates.

Most geologists opt for a middle ground. “The prevailing view is that Earth started to exhibit behaviors that look like plate tectonics 2.5 to 3 billion years ago,” said Michael Brown, a geologist at the University of Maryland.

That chronology decouples plate tectonics from the origin of life on Earth: evidence of the earliest single-celled organisms dates back more than 3.6 billion years. Nevertheless, scientists view plate tectonics as vital to the sustained evolution of that primordial life.

Plate tectonic activity kept a steady supply of water shuttling between mantle and crust, rather than gradually evaporating from the surface.

It blocked the dangerous buildup of greenhouse gases in the atmosphere by sucking excess carbon from the ocean and subducting it underground. It shook up mountains and pulverized rocks, freeing up essential minerals and nutrients like phosphorus, oxygen and nitrogen for use in the growing carnival of life.Plate tectonics also built the right playing fields for Darwinian games.

“Think about what drives evolution,” Dr. Stern said. “It’s isolation and competition. You need to break continents and continental shelves apart, and separate one ocean from another, for speciation to occur.”

© 2018 New York Times News Service

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