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 80 percent of Earth’s volcanism happens on the seafloor, with the vast majority occurring along the mid-ocean ridge. Before this latest sighting, only two underwater eruptions had been caught in action, and neither was along a mid-ocean ridge, said Bill Chadwick, a volcanologist at Oregon State University who was not on the research team.

“That’s a super exciting first,” he said.

Observing such an event live offers a unique opportunity for scientists to study one of our planet’s most fundamental processes: the birth of new seafloor, and its dynamic effects on ocean chemistry, ecosystems, microbial life and more.

“Being there in real time is just this absolutely phenomenal gift — I’m really jealous,” said Deborah Kelley, a marine geologist at the University of Washington who was not part of the research team.

Dr. Wozniak and colleagues sailed on a ship, the R/V Atlantis, before setting out in the Alvin sub. Their original goal was to study carbon flowing from the Tica vent, funded by the National Science Foundation. Hydrothermal vents are like a planetary plumbing system, expelling seawater that’s heated as it seeps through the ocean floor. The process transports both heat and chemicals from Earth’s interior, helping regulate ocean chemistry and feeding a unique community of deep marine life.

The dive on Tuesday morning started like any other. Alyssa Wentzel, an undergraduate at the University of Delaware who joined Dr. Wozniak aboard Alvin, described the enchantment of sinking into the darkness of the ocean depths on the 70-minute journey to the seafloor. As the light vanished, bioluminescent jellies and tiny zooplankton drifted by.

“It was magical,” she said. “It really takes your words away.”

But as they approached the site, a darker magic set in as temperatures slowly ticked upward and particles filled the water. The usual dull gray-brown of the seafloor was capped by tendrils of inky rock that glimmered with an abundance of glass — the result of rapid quenching when lava hits chilly water.

As particulates clouded the view from Alvin, Kaitlyn Beardshear of the Woods Hole Oceanographic Institution, the pilot in command of the day’s journey, slowed the sub, keeping close watch on the temperatures. As they ticked up, so too did concerns for safety of the submersible and the crew. Eventually, the pilot made the call to retreat.

“It was an incredible sight to see,” they said. “All the life and features that I had seen just a few days before, wiped away. I can’t believe we were so lucky to have been there within a few hours of eruption.”

The team learned after returning to the ship that sensitive microphones, called hydrophones, aboard the Atlantis had detected the volcanic eruption earlier in the day. It registered as a series of low frequency booms and campfire-like crackle.

This was the third known eruption at the Tica vent since its discovery in the 1980s. Over the decades, Dan Fornari, a marine geologist at Woods Hole, and his colleagues have closely monitored the site, tracking changes in temperature, water chemistry and more. Combining these analyses with modeling of seafloor spreading, they realized the site seemed poised for an eruption, proposing it would happen either sometime this year or last.

In 1991, he and his colleagues had arrived at Tica within days of an eruption’s start. It might even have still been active, he said, but they saw no flashes of lava to confirm. This time, he said, there’s no doubt of what the Alvin crew saw. “This has been the closest that we ever come to witnessing the initiation of an eruption” along the mid-ocean ridge, he said.

The team is continuing to study the volcanic activity. Given safety concerns, they’re collecting data and taking photographs remotely from the Atlantis.

The data will help researchers unravel the mysteries of deep-sea volcanism and the role it plays in marine ecosystems. “All of this has to do with understanding this holistic system that is Earth and ocean,” Dr. Fornari said. “It’s so intertwined, and it’s both complex and beautiful.”