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Remnants of the lost Farallon plate may lie deep beneath California. (Donald Miralle / Getty Images)

Found: A Missing Tectonic Plate

Scientists discover part of an ancient tectonic plate under California

By Jennifer Marino Walters | April 10 , 2013
<p> TOP: The Farallon plate has been sinking deeper into the earth for the last 150 million years. (Karin Sigloch / Science Source/ Photo Researchers) </p><p> BOTTOM: The Isabella anomaly was a mystery until it was identified as a piece of the Farallon plate. (Jim McMahon)</p>

TOP: The Farallon plate has been sinking deeper into the earth for the last 150 million years. (Karin Sigloch / Science Source/ Photo Researchers)

BOTTOM: The Isabella anomaly was a mystery until it was identified as a piece of the Farallon plate. (Jim McMahon)

Millions of years ago, a tectonic plate—a gigantic, slow-moving rock slab—disappeared beneath what is now North America. Called the Farallon plate, it once sat between the Pacific and North American plates. Those plates began converging about 100 million years ago, eventually forming California’s San Andreas Fault.

Tectonic plates cover Earth’s crust, or outermost layer. These enormous masses of rock sit underneath entire continents and oceans. The plates move very slowly, but when they move, they can cause earthquakes, volcanoes, and tsunamis. Over long periods of time, the movement of tectonic plates can make—or destroy—mountains, islands, and even continents.

Millions of years ago, this movement forced the Farallon plate deep inside Earth’s mantle, where it broke into pieces. The mantle is the rocky layer beneath the Earth’s crust.

Many pieces of the Farallon plate went missing. But recently, scientists made a big discovery: They found a large chunk of the Farallon plate 62 to 124 miles beneath California. The scientists published their research in a scientific journal.

A ROCKY DISCOVERY

By locating this plate, scientists solved a big geological mystery known as the Isabella anomaly. (An anomaly is something that doesn’t follow a general rule.) The Isabella anomaly is a large mass of cool, dry material below Earth’s surface in California. Scientists had found it by examining the way seismic waves passed through it. Seismic waves are waves of energy released by the sudden breaking of rock within the earth, often caused by an earthquake.

Seismic waves move slow through soft, hot material and quickly through stiff, cool material. Though there were many theories, no one could figure out exactly what the Isabella anomaly was.

But a few years ago, scientists discovered another anomaly under Mexico’s Baja Peninsula, directly east of some known remains of the Farallon plate. So the scientists took a closer look at the Isabella anomaly. They found that it lined up with the Baja anomaly as well as with other anomalies associated with the Farallon slabs. It was also the same distance below Earth’s surface as these other anomalies, and seismic waves passed through it in the same way.

The scientists concluded that both the Baja and the Isabella anomalies are leftover pieces of the Farallon plate. They had gotten stuck to the North American plate instead of dropping into Earth’s mantle.

“This work has radically changed our understanding of the makeup of the west coast of North America,” says Brian Savage of the University of Rhode Island, one of the scientists who worked on the study. “It will cause a thorough rethinking of the geological history of North America.”

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