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A tectonic plate is dying under Oregon. Here’s why that matters.

Maya Wei-Haas

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Earth 101 Earth is the only planet known to maintain life. Find out the origins of our home planet and some of the key ingredients that help make this blue speck in space a unique global ecosystem.

What’s more, the study is giving scientists a peek into the final moments of a tectonic plate’s life. The Juan de Fuca is one of the few remaining fragments of the once mighty Farallon plate, which North America began languidly consuming some 180 million years ago as the supercontinent Pangea broke apart. What happens when such plates are swallowed up is still unknown—but it’s a fate that awaits all of the planet’s oceanic plates.

“What we are looking at right now is the death of an oceanic plate,” Hawley says. (Find our what might happen when Earth’s tectonic plates grind to a halt.)

Other scientists are greeting the new model with curiosity and excitement, but they also caution that it needs more testing before it can become geologic canon.

“I think there are certainly good ideas,” says Martin Streck, a volcanologist at Portland State University who specializes in the geologic activity of the Pacific Northwest. “But we may discuss this for some years to come.”

Teasing apart a tear

Earth’s crust is fractured into an interlocking network of tectonic plates, whose slow-motion dance has shaped the surface of our planet. They grind against one another, pulling apart at some edges and colliding in others. These collisions often form what are known as subduction zones, where oceanic plates take the plunge while continental plates ride high.

Researchers once thought that at any given subduction zone, the diving plate simply curls down into the deep, dropping in a sheet like a curtain, Hawley explains. But scientists have realized that just isn’t the case, based on the data gathered from an increasing number of instruments imaging Earth’s innards—including the thousands of seismometers of the National Science Foundation’s EarthScope project, as well as the onshore and underwater arrays of the Cascadia Initiative, all of which were used in this latest study.

Now, we know that as one plate dips under another, it can deform, warping and breaking in unexpected ways. How this tortured fate affects the surface, however, is not always clear, particularly as a tectonic plate nears its end. The dying Juan de Fuca plate is a perfect opportunity to study such impacts, in part because it’s not going quietly into the deep, so scientists are closely monitoring the system’s every shiver and burp. (Here’s how we found out that a powerful earthquake split a tectonic plate in two.

To study the plate’s subterranean gap, Hawley and Allen first needed to confirm that it really exists and was not just some artifact hiding in the data. The duo constructed a high-resolution look into the subsurface by mapping out the different speeds of seismic waves that 217 earthquakes sent rippling through the region. The speeds of these waves change depending on the temperature and composition of the rock, so they were able to "see" the colder, denser oceanic plate as it sinks into Earth and confirm that part of it is indeed missing.

With this new high-resolution data on the gap, the duo are effectively saying, “No, really, really. It’s really there,” quips seismologist Lara Wagner of the Carnegie Institution for Science, who has extensively studied the Farallon plate.

Another piece of the puzzle clicked into place when Hawley and Allen discovered that the position of the tear aligns with a previously identified zone of weakness known as a propagator wake, which cuts through the Juan de Fuca plate exposed at the seafloor and likely continues as it descends into the mantle. The researchers posit that the subducted plate is tearing along this zone of weakness as the southern section of the plate rotates in a clockwise direction, slowly splitting from the northern section of the plate. (Find out how a peeling tectonic plate may one day cause the Atlantic Ocean to shrink.)

“You’re essentially unzipping these two plates that used to be together,” Wagner says. “But it was a weak zipper—and that zipper is that propagator wake.”

The motion of this split and twist could also explain the distortion seen—and felt—at the surface off the coast of southern Oregon and northern California, Hawley explains. This southern zone of the Juan de Fuca plate is riddled with earthquake-prone faults—exactly like what you’d expect from the newly proposed model.

Explosive ideas

The final piece of the puzzle is the volcanism. A string of volcanoes called the High Lava Plains stretches across southern Oregon and spouts an odd combination of magmas: Some are silica-rich, forming light-colored rocks known as rhyolite, while others are rich in magnesium and iron, forming jet-black rocks known as basalts. A tiny amount also has a composition in between these magma types.