First discovered in 2007, the seven mounds sit about 700 feet (213 meters) beneath the ocean, roughly 10 miles (16 kilometers) offshore from Santa Barbara (see map).
Recent explorations using a submarine robot named Alvin have revealed that the largest dome is about as wide as two football fields laid side by side and is as tall as a six-story building. (Related: "Iceland Volcano Creates 27-Story 'Mountain.'")
The domes are made mostly of petroleum, or crude oil—essentially the same stuff used to pave highways and parking lots and the raw material for gasoline.
"If I could convert all the asphalt in the largest volcano to gasoline, it would be enough to fuel my Honda Civic for about half a billion miles," said lead study author David Valentine, an earth scientist at the University of California, Santa Barbara.
But there's little point in harvesting the asphalt mounds for fuel, he added, as "the quality of the material is very poor. ... It's not worth something like light sweet crude."
Analysis of samples taken from the mounds suggests they required several decades or even centuries to build up to their current size, and that the undersea volcanoes last erupted about 35,000 years ago.
"To me, as an oil-spill chemist, this was very exciting," study co-author Christopher Reddy, director of the Woods Hole Oceanographic Institution in Massachusetts, said in a statement. "I got to find out what oil looks like after ... 35,000 years."
Asphalt Eruptions Spurred Dead Zone, Oil Slick
The processes that drives asphalt volcanoes is not well understood, Valentine said. Scientists know only that such eruptions occur where natural oil and gas deposits are leaking from Earth's interior.
The team thinks the Santa Barbara asphalt volcanoes could account for a mysterious spike in ocean methane known to have occurred around 35,000 years ago.
"Methane is typically formed along with petroleum, so they are naturally associated," Valentine said. In addition, methane gas is still bubbling up in the vicinity of the asphalt mounds, even though the petroleum flow stopped long ago.
Eruptions of the California mounds might have once spewed enough methane to dramatically boost populations of methane-eating marine bacteria.
These bacteria depleted the water's oxygen, creating a giant "dead zone" in the Santa Barbara basin that was lethal to most marine life. (See "World's Largest Dead Zone Suffocating Sea.")
To make matters worse, the eruptions were also likely accompanied by enormous oil slicks on the ocean surface, as lighter petroleum molecules floated to top of the water column. (Related: "Undersea Volcano Erupts, May Form New Island.")
The asphalt domes themselves are made from heavier petroleum molecules that form when crude oil has not had enough time to "bake" in the Earth interior, Valentine said. As a result, the petroleum was more viscous than usual.
This heavier form of petroleum remained on the seafloor, congealing into swaying pillars of crude oil that gradually became polluted with microscopic creatures, sand, and other pieces of debris floating in the water.
"When we take that material and dissolve away all the oil ... about 20 percent of the mass is accumulated junk," Valentine explained. "As those things accumulated in the [petroleum], it became heavier and settled back down to the seafloor."
Over time, the successive layering of hardened asphalt created the giant mounds, which are described in the current issue of the journal Nature Geoscience.
Asphalt Domes Younger Than Thought?
Underwater asphalt mounds like those discovered off Santa Barbara are known elsewhere around the world, but they are relatively rare, Tom Lorenson, a geologist with the U.S. Geological Survey who was not involved in the study, said in an email.
Some, like ones recently discovered in the Gulf of Mexico, are even active. "I think there are [also] some in the Caspian Sea, the eastern Mediterranean Sea, and perhaps the Persian Gulf," Lorenson said.
Lorenson isn't convinced that the age estimates for the Santa Barbara mounds are correct, because their surfaces are relatively bare. The surfaces of other known mounds are heavily colonized by marine life, which suggests that the newfound domes are younger than thought.
However, it may be difficult to date asphalt mounds based on the abundance of sea life on their surfaces, because variations in petroleum composition could affect their habitability, said Ian MacDonald, a biological oceanographer at Florida State University, who also did not participate in the research.
One thing is certain: Asphalt mounds in general help create environments for marine life that might not otherwise exist.
"Processes that produce hard substrates in the deep ocean are rare. ... Generally speaking, the deep ocean is a muddy place," MacDonald said.
"I think it's really cool that there's this other process that we didn't really know about before that, at least in some places, is making pretty extensive hard bottoms for animals to colonize."