Ancient, 'Lost' Tectonic Plate Discovered Beneath California

An ancient tectonic plate that disappeared under North America millions of years ago may have been found again. Researchers have discovered that the plate is still at the surface in central California and Mexico.

The plate, known as the Farallon oceanic plate, was believed to have sunk deep into the Earth's mantle about 100 million years ago. It used to lay between the Pacific and North American tectonic plates, and scientists believed that it was eventually forced beneath the two as they converged to form the San Andreas fault. Only a few remnants of the Farallon plate remained at the surface, eventually becoming part of the Pacific plate.

The new study, published in the Proceedings of the National Academy of Sciences, shows that this wasn't necessarily the case. Instead, the research reveals that the Farallon plate actually remained attached in pieces to a much larger chunk at the surface. In fact, part of the Baja region of Mexico and even part of central California near the Sierra Nevada mountains currently rest on slabs of this ancient plate.

How did researchers find the remnants of this plate? The scientists used seismic waves recorded from earthquakes or created with dynamic charges and other methods in order to map out the area beneath the Earth's surface. The waves themselves move slower or faster depending on the type of materials they traveled through, which allowed the researchers to get a better image of the world beneath our feet.

During their survey, though, they found something unusual. The seismic waves revealed a large amount of cool, dry material located between 62 to 124 miles beneath the surface. Dubbed the "Isabella anomaly," it took some time before researchers could learn exactly what it was.

Eventually, they found another mass under the Baja Peninsula located directly east of some of the known remains of the Farallon plate. With this in mind, the researchers believed that the anomalies they were detecting may be part of the same plate. Further investigation revealed volcanic rock deposits near the eastern edge of the mass, which are often linked to the melting of oceanic crust. It lined up with the theory that this was the location where the Farallon plate broke off and melted into the mantle.

"Many had assumed that these pieces would have broken off quite close to the surface," said Brown geophysicist Donald Forsyth, who helped lead the research, in a press release. "We're suggesting that they actually broke off fairly deep, leaving these large slabs behind."

The findings could help scientists better understand how the Earth is formed--especially the west coast of North America. It could also give them further insights into the geological history of the continent and other regions.