Geologists: Mysterious Ocean Floor "Potholes" Caused By Lava Bubbles

November 5, 2003

GAINESVILLE, Fla. — Thousands of feet below the surface of the Pacific, along a massive volcanic mountain range called the mid-ocean ridge, much of the ocean floor is pockmarked with jagged-edged semi-circular holes and collapsed pits of varying sizes.

Geologists had a theory about how this unusual topography occurred, but some believed it didn’t add up. Now, in research to be reported this week in the journal Nature, a group of scientists headed by a University of Florida geologist offers a new explanation: The pothole-like structures, they say, are collapsed lava bubbles – bubbles that occur when lava seeps up from underground volcanoes on to the ocean floor. Sloshing along the bottom like blobs in a huge lava lamp, the molten mass vaporizes water confined beneath it, creating a briny steam that gets trapped under the lava’s surface and forces it to bubble up and outward. Under the pressure of tons of near-freezing seawater, these bubbles quickly condense, then implode and harden, causing the collapses, the geologists say.

The research helps explain the long-observed porosity of much of the upper ocean floor and may offer fresh insights into the mysteries behind the abundant microbial life inhabiting the “deep biosphere,” said UF geology professor and lead researcher Mike Perfit.

“The bacteria and other life forms that reside on the deep, dark, cold ocean floor – they need heat, water and space,” he said. “These pits and cavities may provide a kind of habitat.”

No one has ever witnessed an underwater volcanic eruption in the deep ocean thousands of feet down. For the National Science Foundation-funded research, Perfit, along with Dan Fornari from the Woods Hole Oceanographic Institute and colleagues, relied on evidence from hardened lava samples they retrieved from dives in the submersible ALVIN along the mid-ocean ridge a few hundred miles from Mexico’s west coast. The area, about 8,000 feet deep, is the site of recent underwater volcanic eruptions and is replete with hydrothermal vents, deep-sea animal communities and collapse pits ranging in size from several inches to dozens of yards.

Conventional wisdom holds that the collapse pits form as the lava erupts, then pools and hardens on the surface as its molten interior drains away, Perfit said. This hypothesis had several problems, however, including that the potholes were observed relatively far from the volcanic vents, he said. Geologists didn’t think the lava, despite reaching temperatures well over 2,000 degrees Fahrenheit, could boil the seawater because of the intense pressure at such great depths, he said.

Perfit and his colleagues focused on samples collected from the underside, or ocean bottom-facing side of the hardened lava. There, they found several features that suggested the lava didn’t harden immediately, including curious lava drips that look like crystalline stalactites. These drips, the scientists believe, form between the cooling lava and the superheated vapor.

“The bubble buoys up the lava flow and allows the lava inside it to move around, but once the vapor cools it creates negative pressure and it collapses,” Perfit said. “The process also allows the lavas to flow over greater distances of the seafloor.”

The finding offers an alternative explanation for the unusual chemical composition of some lavas – compositions that suggested they encountered salty brine beneath the ocean floor. In fact, Perfit said, this composition may result from interaction not with underground brine but rather with the steamed seawater on the surface.

David Clague, a geologist at the Monterey Bay Aquarium Research Institute in California, said the researchers’ description of the samples they retrieved “provides important new data that future models for submarine eruptions along ridges must take into account.”

He disagreed with Perfit and his colleagues’ interpretation of their observations, however, saying he believed that the undersides of the lava were exposed not to steam but rather to gases from underground lava, or magma. “I happen to think that the vapor phase is more likely to be magmatic, and evidence for more extensive magmatic gas discharges during submarine mid-ocean ridge eruptions,” he said.