California is home to more than 500 volcanic vents and a stronghold of earthquake activity in the northwestern United States. The sunny US state is located on a seismically active zone, where the North American plate constantly strikes against the Pacific plate. The region is known as the eastern California shear zone, where the so-called San Andreas fault line forms the boundary between the two plates. But especially in northeastern California, there is another source of seismicity and volcanism that affects the geology of California and neighboring Oregon.

The so-called Juan de Fuca plate runs north of California and is subtracted or driven under North America.

According to researchers from the University of California at Berkley, the oceanic plate has opened wide and triggered activity in the countryside.

The theory was presented on July 3 in the journal Geophysical Research Letters by geologists William Hawley and Richard Allen of UC Berkeley.

The researchers wrote in their study: "A hole in a lowered plate in the Earth's mantle under North America can cause volcanism and earthquake on the earth's surface.

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"The volcanism on the surface of North America seems to have spatially coincided with a known zone of weakness on the plate in the last 17 million years.

"We suggest that this hole is caused by tearing along the weakening zone, a feature that arises when the plate forms on the ridge."

The rupture can lead to volcanic activity in North America, but also deform parts of the oceanic plate off the coast.

Over time, the researchers said that the plate crumbled or "splintered" and the debris merged with the nearby plates.

In essence, Hawley argued, "we are experiencing the death of a plate."

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Mr. Hawley, a Ph.D. student at the Berkeley Earth and Planetary Science Department, told LiveScience, "Where other people discussed whether it was there or not, we can confidently say it's real."

The Juan de Fuca plate stretches about 965 km from Vancouver Island in Canada to Cape Mendocino.

The plate is completely submerged under the Pacific Ocean.

Professor Allen and Mr. Hawley studied the oceanic plate between 2011 and 2015 by dropping seismometers to the bottom of the ocean to collect data.

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The data collected revealed a gap in a particular type of seismic wave in central Oregon.

The researchers concluded that the gap had formed through a hole at a depth of between 96.5 km and 249.4 km.

About 17 million years ago, material pushed out of this hole formed the volcanoes that littered the so-called High Lava Plains in Oregon.

Mr. Hawley said, "The story combines the hole in tomography with this known weak zone in a plate and with a number of Oregon volcano centers and with a series of earthquakes and faults off the Northern California coast."