Scientists find underwater asphalt volcanoes off Santa Barbara

Scientists have discovered a cluster of underwater asphalt volcanoes rising from the seafloor just off Santa Barbara.

The seven volcanoes, about 65 feet tall, probably last disgorged petroleum and natural gas into the sea 30,000 to 40,000 years ago, during the most recent ice age, according to geochemist David Valentine of UC Santa Barbara.

Valentine and his fellow researchers first came across the asphalt behemoths in 2007 while exploring in a white, three-seat U.S. Navy submarine operated by the Woods Hole Oceanographic Institution.

Bundled in warm, nonflammable clothes and crammed against the damp submarine walls, they had been trying unsuccessfully to find methane seeps.

Frustrated, they took a detour, curious about a place where earlier surveys had indicated some kind of mound rising from the seafloor to 726 feet below the surface. They peered through portholes into the gloom.

The vehicle’s lights illuminated only a few feet. “We were coming across very boring, very silty seafloor with not a lot of life on it. Suddenly we saw a cliff looming,” Valentine said. “Then there was a giant black wall teeming with life.”

The nearly vertical black wall seethed with basket stars, wolf eels and fish. It seemed to be a deep-water reef of sorts, but it wasn’t coral. At first, the submarine occupants thought the black cliff face was basalt.

The submarine’s robotic arm gripped a chunk of it. To the researchers’ surprise, the piece snapped off like hard candy: It was asphalt, left behind from the last greasy eruption.

When they got to the surface, the researchers examined the piece. It was lightweight, only a little more dense than seawater, and peppered with holes bored by marine animals. Critters crawled out before their eyes.

Over the next two years, funded by the National Science Foundation, Valentine and his team explored the big mounds in the tiny, battery-powered submarine, whirring through cold depths beyond the reach of scuba divers, dodging the junk that litters the seafloor: old nets, discarded Christmas trees, the occasional torpedo.

They found brittle cascades of hardened asphalt rippling down the volcanoes’ flanks. Though dormant, two still burped bubbles of methane here and there. Some were steep and craggy, looming in total darkness. Some had shallow craters at the top. All were made of solid asphalt.

The volcanoes are in an area of the seafloor where layers of sediment provide a kind of geophysical archive, like the bands of a tree trunk. There are also tar deposits filled with remnants of ancient marine life: a marine version of the La Brea tar pits.

Researchers studying these sites had previously found evidence of several cataclysmic methane-gas releases about 40,000 years ago. The methane had fostered bacteria that leeched oxygen out of the water and created a dead zone. They wondered what had happened.

Now “we found a candidate,” said Valentine, who worked with colleagues from the Woods Hole group, UC Davis, the University of Sydney and the University of Rhode Island.

The volcanoes are on a fault. If an eruption happens again, it will probably occur somewhere else on the fault line, Valentine speculated.

Like a 15th century explorer, Valentine got to name the volcanoes. The depth surveys reminded him of pictures he had taken in Florence, Italy. The largest, which stands alone about a mile from the others, he dubbed Il Duomo, after the Florentine cathedral. The next largest is Il Duomito, and the smaller ones, Los Duomitos.