Andrew Wozniak, a chemical oceanographer at the University of Delaware, struggled to process what his eyes were taking in. Dr. Wozniak was parked on the bottom of the Pacific Ocean beneath nearly 1.6 miles of water in Alvin, a research submersible. As far as he could see lay a mostly barren expanse of jet-black rock.

Just a day before, at this same spot, a vibrant ecosystem had thrived in the sweltering waters of the Tica hydrothermal vent, about 1,300 miles west of Costa Rica. Creatures inhabited every inch of the rocky seafloor, writhing in a patchwork of life. The crimson tips of giant tube worms waggled in the current, tangling around clusters of mussels. Buglike crustaceans scuttled through the scene while ghostly white fish languidly prowled for their next kill.

Now, only a single cluster of tube worms remained in the blackened terrain, all dead. A haze of particulates filled the water as glints of bright orange lava flickered among the rocks.

“My brain was trying to understand what was going on,” Dr. Wozniak said. “Where did things go?”

Eventually it clicked: He and the sub’s other passengers were witnessing the tail end of a submarine volcanic eruption that had entombed the flourishing ecosystem under fresh lava rock.

This was the first time scientists had witnessed a clearly active eruption along the mid-ocean ridge, a volcanic mountain chain that stretches about 40,000 miles around the globe, like the seams of a baseball. The ridge marks the edges of tectonic plates as they pull apart, driving volcanic eruptions and creating fresh crust, or the layer of the Earth we live on, beneath the sea. About