Underwater volcano erupts off Oregon causing seafloor to drop more than two metres

An undersea volcano about 480km off Oregon's coast has been spewing lava for the past seven days, giving researchers unique insight into a hidden ocean hot spot.

Researchers know of two previous eruptions by the volcano, dubbed "Axial Seamount" for its location along the axis of an underwater mountain ridge, Oregon State University geologist Bill Chadwick said. But those 1998 and 2011 eruptions were detected months or years afterward, Chadwick added.

Last year, researchers connected monitoring gear to an undersea cable that, for the first time, allowed them to gather live data on the volcano, whose peak is about 1,500 metres below the ocean surface.

"The cable allows us to have more sensors and monitoring instruments than ever before, and it's happening in real time," said Chadwick, who also is affiliated with the US National Oceanic and Atmospheric Administration. In the past, researchers left battery-operated monitoring stations in place for several years, but were able to analyse the data only by retrieving those devices.

Pressure sensors detected an eruption was under way on April 23. After monitoring hundreds, then thousands of small earthquakes each day near Axial Seamount, they detected more than 8,000 tiny quakes over a 24-hour span on April 23, Chadwick said.

As midnight approached, pressure sensors detected the seafloor dropping - a sign that magma was erupting - and the swollen volcano was deflating like an emptying balloon. In total, the seafloor has dropped 2.4 metres in the past week.