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How the theory of plate tectonics that shakes the Earth was born

Some great ideas shake the world. For centuries, the outermost layer of the Earth was thought to be static, rigid, locked in place. But plate tectonics theory has shaken this image of the planet to its core. Plate tectonics reveals how the Earth’s surface is constantly moving and how its features – volcanoes, earthquakes, ocean basins and mountains – are intrinsically linked to its warm interior. The well-known landscapes of the planet, we now know, are products of an eon cycle in which the planet is constantly rebuilt.

When plate tectonics emerged in the 1960s it became a unifying theory, "the first global theory accepted in the entire history of earth science," writes Naomi Oreskes, a historian of science at Harvard University, in the introduction to Tectonics of plates: story of an insider. of Modern Earth Theory. In 1969, geophysicist J. Tuzo Wilson compared the impact of this intellectual revolution on earth science to Einstein's general theory of relativity, which produced a similar shift in thinking about the nature of the universe.

Plate tectonics describes how the outermost layer of the Earth, 100 kilometers thick, called the lithosphere, is divided into a plate puzzle – rock slabs that carry the two continents and the bottom of the sea – that slide over a warm inner layer and which rotates slowly. Moving at speeds between 2 and 10 centimeters each year, some plates collide, others diverge and others grind in front of each other. The new seabed is created in the center of the oceans and is lost as the plates sink back into the interior of the planet. This cycle gives rise to many of the Earth’s geological wonders, as well as its natural hazards.

“It’s amazing how the pieces came together: the seabed stretching, magnetic strips at the bottom of the sea … where earthquakes form, where mountain ranges form,” says Bradford Foley, a Penn State geodynamicist. "Almost everything falls into place."

With so many lines of evidence now known, the theory feels obvious, almost inevitable. But the conceptual journey from fixed lands to a turbulent, turbulent Earth has been long and winding, punctuated by moments of pure intuition and guided by decades of data chase.

Continents adrift

In 1912, the German meteorologist Alfred Wegener proposed at a meeting of the Frankfurt Geological Society that land masses could be moving. At the time, the predominant idea held that mountains formed like wrinkles on the planet as it slowly lost the heat of the formation and its surface contracted. Instead, Wegener suggested, mountains form when continents collide as they drift across the planet's surface. Although now distant, the continents came together as a Wegener supercontinent called Pangea or "the whole Earth." This would explain why rocks of the same type and age, as well as identical fossils, are found on both sides of the Atlantic Ocean, for example.

The San Andreas Fault (shown) is the boundary between the Pacific and North American plates.KEVIN SCHAFER / ALAMY

block of houses in a row away from earthquake damageIn 1989, a landslide in the San Andreas Fault triggered a magnitude 6.9 earthquake that shook the San Francisco Bay Area, causing 63 deaths and billions of dollars in damage (shown).David Weintraub / Science Source

This idea of ​​adrift continents intrigued some scientists. Many others, especially geologists, were not impressed, hostile, even horrified. Wegener's idea, detractors thought, was too speculative, not sufficiently based on prevailing geological principles such as uniformitarianism, which holds that the same slow-moving geological forces on Earth today must also have been working in the past. It is believed that the principle required continents to be fixed in place.

German geologist Max Semper wrote with contempt in 1917 that Wegener's idea was "established with a superficial use of scientific methods, ignoring the various fields of geology," adding that he hoped Wegener would turn his attention to other fields of science and leave geology. in peace ". Saint Saint Florian, protect this house, but burn the others!” he wrote sardonic.

The debate between "mobilists" and "fixists" lasted during the 1920s, catching steam as it traveled through English-speaking circles. In 1926, at a New York City meeting of the American Association of Petroleum Geologists, geologist Rollin T. Chamberlin rejected Wegener's hypothesis as a mestizo of unrelated observations. The idea, Chamberlin said, "is of the loose type, as it takes considerable liberty with our globe and is less bound by constraints or bound by awkward and ugly ugly ones than most of its rival theories."

One of the most persistent points for Wegener's idea, now called continental drift, was that he could not explain how continents moved. In 1928, the English geologist Arthur Holmes found a potential explanation for this movement. He proposed that continents could float like rafts on top of a layer of viscous rocks, partially melted at the bottom of the Earth. He suggested that the heat produced by the disintegration of radioactive materials fixes this layer to a slow boil, creating large circulating currents within the molten rock which in turn slowly displace the continents.

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Holmes admitted that he had no data to support the idea, and the geology community remained unconvinced of continental drift. Geologists have turned to other issues, such as developing a scale of magnitude for earthquake intensity and designing a method to accurately date organic materials using the radioactive form of carbon, carbon-14.

Data flood

Revived interest in continental drift arose in the 1950s from evidence from an unexpected source: the ocean floor. World War II brought the rapid development of submarines and sonar, and scientists soon put new technologies to work by studying the seabed. Using sonar, which bogs the seabed with sound waves and hears the pulse back, the researchers traced the extent of a continuous, branched underwater mountain range with a long fissure running through its center. This global rift system covers more than 72,000 miles worldwide, traversing the centers of the world’s oceans.

Armed with magnetometers to measure magnetic fields, the researchers also mapped the magnetic orientation of the rocks at the bottom of the sea: how their ferrous minerals orient themselves relative to the Earth’s field. The teams found that the rocks at the bottom of the sea have a peculiar "zebra stripe" pattern: bands of normal polarity, whose magnetic orientation corresponds to the Earth's current magnetic field, alternate with bands of inverted polarity. This finding suggests that each of the bands formed at different times.

a submarine explores an underwater structureThe Pacific Ocean Marianas Trench is the deepest known subduction zone, where a tectonic plate sinks back into the Earth's interior. Here, the Deep Discoverer explores the 6,000-meter-deep trench in 2016.NOAA OCEAN EXPLORATION AND RESEARCH OFFICE

Meanwhile, growing support for the detection and ban of underground nuclear testing has also created an opportunity for seismologists: the possibility of creating a global, standardized network of seismograph stations. In the late 1960s, about 120 different stations were installed in 60 different countries, from the Addis Ababa Mountains of Ethiopia to the halls of Georgetown University in Washington, D.C., to the ice cream South Pole. Thanks to the flooding resulting from high-quality seismic data, scientists have discovered and mapped rumors along the fissure system in the middle ocean, now called mid-ocean ridges, and beneath the trenches. Earthquakes near very deep ocean trenches were particularly curious: they originated underground much deeper than scientists thought possible. And the ridges were very hot compared to the surrounding seabed, the scientists learned by using thin steel probes inserted into perforated cores from the edge to the bottom of the sea.

In the early 1960s, two independently working researchers, geologist Harry Hess and geophysicist Robert S. Dietz, joined the disparate clues and added to Holmes ’old idea of ​​an underlying layer of circulating currents within the hot rock. The ridges of the middle ocean, each asserted, may be where the circulation pushes the hot rock toward the surface. Powerful forces separate pieces of the Earth's lithosphere. In space, lava explodes and a new seabed is born. As the pieces of lithosphere separate, new seabeds continue to form between them, called "spreading seabeds."

hot lava and orange by the seaResearch suggests that volcanic island chains form as the plates move over the waters of the magma. But the origin of the Hawaiian Islands (shown the Kilauea volcano) and other similar chains remains a geological enigma.IMAGES OF ART WOLFE / GETTY

The impetus culminated in a two-day meeting of perhaps just 100 Earth scientists in 1966, held at the Goddard Institute for Space Studies in New York. “It was pretty clear, at this conference in New York, that everything was going to change,” Cambridge University geophysicist Dan McKenzie told the Geological Society of London in 2017 in a reflection on the meeting.

But coming in, “no one had any idea” that this encounter would become a crucial moment for earth sciences, says seismologist Lynn Sykes of Columbia University. Sykes, then a newly minted doctor, was one of the guests; he had just discovered a different pattern in earthquakes on the ridges of the middle ocean. This pattern showed that the seabed on both sides of the ridges was separating, a fundamental test of plate tectonics.

At the meeting, talk after talk accumulated data on top of data to support the spread of the seabed, including data from the Sykes earthquake and those symmetrical patterns of zebra stripes. It soon became clear that these discoveries were building towards a unified narrative: the ridges of the middle ocean were the birthplace of the new seabed and the deep ocean trenches were tombs where the ancient lithosphere was reabsorbed inland. This cycle of birth and death had opened and closed the oceans over and over again, bringing the continents together and then dividing them.

The evidence was overwhelming and it was during this conference "that the victory of mobility was clearly established," geophysicist Xavier Le Pichon, who was previously a skeptic of seabed diffusion, wrote in 2001 in his retrospective essay "My conversion to plate tectonics, ”included in Oreskes’ book.

Plate tectonics arises

The entire scientific community of the earth became aware of these discoveries the following spring, at the annual meeting of the American Geophysical Union. Wilson exposed the various lines of evidence of this new worldview to a much larger audience in Washington, DC. By then, the community had a small setback, Sykes says, "They immediately accepted it, which was amazing."

Scientists now knew that the sea floor and the Earth's continents were moving and that ridges and trenches marked the edges of large blocks of lithosphere. But how did these blocks, all arranged, move around the planet? To trace the choreography of this complex dance, two separate groups took a theorem devised by the mathematician Leonhard Euler as early as the 18th century. The theorem showed that a rigid body moves around a sphere as if it were rotating around an axis. McKenzie and geophysicist Robert Parker used this theorem to calculate the dance of lithospheric blocks: plates. Unbeknownst to him, geophysicist W. Jason Morgan independently found a similar solution.

satellite photo of icebergs and earthTerrestrial mass changes, such as the opening of the Drake Passage between South America and Antarctica (icebergs are shown around Elephant Island), can alter currents and climates.NASA IMAGE BY JEFF SCHMALTZ, LANCE / EOSDIS QUICK RESPONSE

With this last piece is born the unifying theory of plate tectonics. The rude dispute over continental drift now seemed not only outdated, but also "a troubling antidote to self-confidence," physicist Egon Orowan told Science News in 1970.

People have benefited greatly from this clearer view of how the Earth works, including being able to better prepare for earthquakes, tsunamis, and volcanoes. Plate tectonics has also shaped new research in the sciences, providing crucial information about how climate changes and about the evolution of life on Earth.

And yet there is still so much that we do not understand, as when and how the restless displacement of the Earth’s surface began and when it may end. Equally baffling is the reason why plate tectonics does not appear to occur elsewhere in the solar system, says Lindy Elkins-Tanton, a planetary scientist at Arizona State University in Tempe. "How can something be complete and inexplicable intellectual revolution at the same time?"

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