New scenario proposed for birth of Pacific Plate

A three-way tectonic tango may have led to the birth of what is now the largest chunk of Earth’s crust.

By scrutinizing what little geologic evidence remains from 190 million years ago, researchers reconstructed the origins of the Pacific tectonic plate, which now covers a fifth of Earth’s surface. The plate formed during the early Jurassic period from a single point where three tectonic plates once met, the work suggests. The plate’s birthplace sat above the gravesite of a section of tectonic plate that sank into the planet’s depths, the researchers report July 27 in Science Advances. The remnants of that sunken plate remain embedded in Earth’s mantle.

This origin story of tectonic life and death is unique in Earth’s known history, says study coauthor Lydian Boschman, a geologist and geodynamicist at Utrecht University in the Netherlands. All of the other modern plates formed by one plate splitting into two, she says. “We’re not sure why this happened, but we now know how it happened.”

A network of shifting tectonic plates covers Earth’s surface. While pieces of continental plates can date back billions of years, the oldest oceanic crust is only about 200 million years old. Anything older has been swallowed into Earth’s interior by subduction.

The oldest part of the Pacific Plate dates back 190 million years and is triangle-shaped. That shape led scientists to postulate that the Pacific Plate formed at a convergence of three other plates. Some scientists proposed that these three plates were in a Y-shape configuration, with the plates spreading away from a central point. As the plates drifted, the ground split and new crust formed as molten rock extruded onto the seafloor and cooled. But such a setup of three spreading ridges wouldn’t create a new plate — the new material would have just made the existing plates bigger.

Reviewing the ages of the seafloor across the Pacific Ocean, Boschman and Douwe van Hinsbergen, a geophysicist also at Utrecht University, instead propose a different story.

The story begins with three tectonic plates that slid together, with one of the plates steamrolling over a section of another, forcing the plate segment into Earth’s depths. That plate segment’s disappearance briefly led to a three-way junction. But instead of the plates moving away from a central point, the three plates bumped and scraped alongside one another in a pinwheel motion. (One plate gliding past another plate is the type of movement seen along California’s San Andreas Fault.)

With all three plates now sliding past one another, a triangular gap opened in the middle. Fresh molten rock from Earth’s interior rushed up to fill the void, creating the nucleus of the Pacific Plate. Spreading ridges formed on each side of this budding plate, adding additional rock to the plate as the gap between the three original plates grew.

“The Pacific Plate is the largest on Earth, but it started out as the smallest,” Boschman says.

Using seismic imaging, scientists previously identified the possible remains of a sunken tectonic plate west of Costa Rica. That lost plate could mark the birthplace of the Pacific Plate, Boschman says.

Verifying this story will be “extremely challenging,” says geophysicist Dietmar Müller of the University of Sydney. No seafloor lingers from before the Pacific Plate formed, and the sunken tectonic plate may have moved over time, making it difficult to concretely tie the plate to the proposed scenario. Boschman says the composition of the Pacific Plate’s oldest rocks may reveal whether they formed over a submerged plate, though collecting such rocks could be expensive and difficult.

Even so, the work provides the first plausible look at the tectonic movements that may have led to the Pacific Plate’s formation, says marine geophysicist William Sager of the University of Houston. “It was a dark room and they turned on the light. It’s a dim light, so we can’t see very far, but it’s something.”