California is home to more than 500 volcanic vents and is a hotbed of earthquake activity in the Northwest US. The sunny US state sits on top of a seismically active zone where the North American Plate constantly grinds up against the Pacific Plate. The region is known as the Eastern California shear zone and it is where the so-called San Andreas fault line forms the boundary between the two plates. But just to the northeast of California is another source of seismicity and volcanism, which affects the geology of California and the neighbouring state of Oregon.
The so-called Juan de Fuca plate runs just north of California and is being subducted or driven under North America.
According to researchers at the University of California, Berkley, the oceanic plate has split wide open, triggering activity on the land.
The theory was presented on July 3 in the journal Geophysical Research Letters by UC Berkeley geologists William Hawley and Richard Allen.
The researchers wrote in their study: “A hole in a subducted plate, in the mantle beneath North America, may cause volcanism and earthquakes on the surface of the Earth.
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“Volcanism on the surface of North America appears to have been spatially coincident with a known zone of weakness on the slab for the last 17 million years.
“We suggest that this hole is caused by tearing along the zone of weakness, a feature that created when the plate is formed at the ridge.”
The tearing can lead to volcanic activity in North America but can also distort parts of the oceanic plate offshore.
In time, the researchers said the plate will crumble or “fragment” and the debris will merge with nearby plates.
In essence, Mr Hawley argued “we are witnessing the death of a plate”.
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Mr Hawley, who is a PhD candidate at Berkeley’s Earth and Planetary Science Department, told LiveScience: “Where other people had debated whether or not it was there, we can confidently say that it’s real.”
The Juan de Fuca plate extends for about 600 miles (965km) from Vancouver Island in Canada, down to Cape Mendocino.
The plate is completely submerged beneath the Pacific Ocean.
Professor Allen and Mr Hawley studded the oceanic plate between 2011 and 2015, by dropping seismometers to bottom of the ocean to collect data.
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The collected data found a gap in a particular type of seismic wave under central Oregon.
The researchers concluded the gap was created by a hole at a depth between 60 miles and 155 miles (96.5km to 249.4km).
Approximately 17 million years ago, material pushed out of this hole created the volcanoes dotting Oregon’s so-called High Lava Plains.
Mr Hawley said: “The story links the hole in the tomography with this known weak zone in a plate and with series of volcanic centres in Oregon and with a series of earthquakes and faults off the coast of Northern California.”
