(Photo : Forsyth lab/Brown University) An ancient, “lost” tectonic plate that disappeared under North America millions of years ago may have been found again. The Isabella anomaly (IA, above), is at the same depth (ca. 100 km) as other fragments of the Farallon plate under Oregon and Washington, is on a line with fragments off the California coast, and has a similar seismic tomography signature.

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, called 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 remains of the Farallon plate remained at the surface, eventually becoming part of the Pacific plate.

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.

During a survey,scientists 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.

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.

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