Hawaii’s Kilauea volcano (lava flow shown) sits far from the nearest plate boundary. Kilauea and Hawaii’s other volcanoes are thought to be fueled by buoyant plumes of molten material originating hundreds to thousands of kilometers deep inside the planet.
CULTURA RM/ALAMY STOCK PHOTO.
By Carolyn Gramling.
In the early 1960s, two researchers working independently, geologist Harry Hess and geophysicist Robert S. Dietz, put the disparate clues together and added in Holmes’ old idea of an underlying layer of circulating currents within the hot rock.
The mid-ocean ridges, each asserted, might be where circulation pushes hot rock toward the surface. The powerful forces drive pieces of Earth’s lithosphere apart. Into the gap, lava burbles up — and new seafloor is born. As the pieces of lithosphere move apart, new seafloor continues to form between them, called “seafloor spreading.”
The momentum culminated in a two-day gathering of perhaps just 100 earth scientists in 1966, held at the Goddard Institute for Space Studies in New York. “It was quite clear, at this conference in New York, that everything was going to change,” University of Cambridge geophysicist Dan McKenzie told the Geological Society of London in 2017 in a reflection on the meeting.
“It was quite clear, at this conference in New York, that everything was going to change.”
But going in, “no one had any idea” that this meeting would become a pivotal moment for the earth sciences, says seismologist Lynn Sykes of Columbia University. Sykes, then a newly minted Ph.D., was one of the invitees; he had just discovered a distinct pattern in the earthquakes at mid-ocean ridges. This pattern showed that the seafloor on either side of the ridges was pulling apart, a pivotal piece of evidence for plate tectonics.
At the meeting, talk after talk piled data on top of data to support seafloor spreading, including Sykes’ earthquake data and those symmetrical patterns of zebra stripes. It soon became clear that these findings were building toward one unified narrative: Mid-ocean ridges were the birthplaces of new seafloor, and deep ocean trenches were graves where old lithosphere was reabsorbed into the interior. This cycle of birth and death had opened and closed the oceans over and over again, bringing the continents together and then splitting them apart.
The evidence was overwhelming, and it was during this conference “that the victory of mobilism was clearly established,” geophysicist Xavier Le Pichon, previously a skeptic of seafloor spreading, wrote in 2001 in his retrospective essay “My conversion to plate tectonics,” included in Oreskes’ book.
Plate tectonics emerges.
The whole earth science community became aware of these findings the following spring, at the American Geophysical Union’s annual meeting. Wilson laid out the various lines of evidence for this new view of the world to a much larger audience in Washington, D.C.
By then, there was remarkably little pushback from the community, Sykes says, “Right away, they accepted it, which was surprising.”
Scientists now knew that Earth’s seafloor and continents were in motion, and that ridges and trenches marked the edges of large blocks of lithosphere. But how were these blocks moving, all in concert, around the planet?
To plot out the choreography of this complex dance, two separate groups seized upon a theorem devised by mathematician Leonhard Euler way back in the 18th century. The theorem showed that a rigid body moves around a sphere as though it is rotating around an axis.
McKenzie and geophysicist Robert Parker used this theorem to calculate the dance of the lithospheric blocks — the plates. Unbeknownst to them, geophysicist W. Jason Morgan independently came up with a similar solution.
The Pacific Ocean’s Mariana Trench is the deepest known subduction zone, where a tectonic plate sinks back into Earth’s interior. Here, the Deep Discoverer explores the trench at a depth of 6,000 meters in 2016.
COURTESY OF THE OFFICE OF OCEAN EXPLORATION AND RESEARCH.