Asphalt Volcanoes

September 3, 2023
By Damond Benningfield

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When oil percolates through cracks in the ocean floor, it can form asphalt volcanoes. These sometimes split apart to form tar lilies, such as the one seen here. Credit: NOAA Office of Ocean Exploration and Research.

Oil and gas bubble up through the ocean floor all the time. They form oil slicks, create tar balls that wash up on shore, and make pillows of methane ice. And in some rare instances, they form asphalt volcanoes—tall, black mounds with smooth sides.

The first were discovered in 2003, about two miles deep in the Gulf of Mexico. Others have been found in the Gulf since then, along with a few off the coasts of California and western Africa. Some of those in the Gulf of Mexico have split apart, creating shapes that look like the fronds of a palm lily, so they’re called tar lilies.

The volcanoes formed as thick oil percolated through cracks in the ocean floor. When it hit the cold ocean water, it turned solid, building the cones.

There are two fairly tall ones in California, about 10 miles offshore from Santa Barbara. The biggest is six stories tall and as wide as a football field. The volcanoes are about 700 feet below the ocean surface, though, so they weren’t discovered until 2007. They’re dormant today, although small vents of gas bubble around them.

A recent study looked at life around the larger Santa Barbara volcano. Scientists cataloged more than 40 species of fish, plus some corals and sponges. More than half of the fish living along the flanks of the volcano were rockfishes. Flatfish were seen around the structure, but not too close—perhaps because they’d be easy prey against the dark background: a rare asphalt volcano.