Imagine that you’re a fisherman at sea and suddenly your boat is surrounded by dozens of floating pieces of hot, dark rock, hissing and spewing vapor. Some rocks are no bigger than footballs, and some are larger than refrigerators. But just a few minutes later, the mysterious chunks sink below the surface without any hint of where they came from.
That is exactly what happened to a group of fishermen in the Azores, Portugal, in late 1998. It turns out they were witnessing the appearance of lava balloons: floating lumps of hollow, cooled lava burped up from the seafloor after an undersea volcanic eruption.
The fishermen described the balloons as “hot steaming stones whose high temperature caused minor damage to the fishing ropes,” with “fire coming out from the seawater spreading on the air like sparks of fireworks,” according to a report by Portuguese scientists who were called to the scene. Later, the fishermen noticed a “large quantity of dead or injured fish” at the sea surface.
The balloons are a strange and rare phenomenon, but they also serve a scientific purpose: They alert researchers to underwater eruptions that might otherwise go unnoticed, said Ulrich Küppers, a volcanologist at the University of Munich.
Küppers is trying to find out how and why these weird features form. He suspects they are the result of trapped magmatic gas pushing upward through lava during some kinds of undersea eruptions. Küppers presented his theory at the recent AGU Chapman Conference on submarine volcanism in Hobart, Tasmania.
A Rare Thing to Behold
Lava balloons are hollow pieces of cooled basalt, a fine-grained dark lava rock. These rough ellipsoids can be as small as 50 centimeters and as long as 3 meters. They rise to the sea surface during some submarine eruptions, and after a few minutes of bobbing on the surface, they absorb water and sink back down to the seafloor.
“When they’re floating at the sea surface, they’re a bit like icebergs,” Küppers said. “They’re mostly below the surface.”
So far, humans have documented the appearance of lava balloons only five times: off the coast of the island of Hawaii in February 1877; near the Mediterranean island of Pantelleria, Italy, in October 1891; near the Mexican island of Socorro in late 1993 and early 1994; off the coast of the Azores, from 1998 to 2001; and in the Canary Islands, Spain, in October 2011.
But Küppers suspects lava balloons may occur more often than volcanologists think: Because they float on the surface only for a few minutes, it’s hard to catch them in the act. Only for the Socorro, Azores, and Canary Island eruptions have scientists been able to directly observe the balloons while measuring other aspects of the eruption, like seismicity and water temperature.
How Did the Balloons Get There?
After studying data—some collected by him, some collected by others—from the most recent three eruptions, Küppers noticed a few common traits. The balloons didn’t seem to explode or implode as they rose in the water column, and their size didn’t change along their journey to the sea surface. Maps of the seafloor around the eruptions revealed that the balloons all emerged from submarine eruptions in shallow water no deeper than a few hundred meters.
“There’s really a lot of open questions still about how do they form, but we have now a good couple of data sets about eruptive activity ongoing at the ocean floor,” he said.
After gathering balloons from the Azores eruption and analyzing data from the other four eruptions, Küppers has come up with a hypothesis for how the balloons emerge. Every now and then during a submarine eruption, gas accumulates and forms an interface between the magma underneath and water above.
The gas continues to rise because it’s less dense, bringing a coating of magma along with it. The magma that rises above the interface is instantly quenched when it meets cold seawater, creating a thin crust of cooled lava over a gas-filled interior.
The rising gas keeps pushing the shell up; under the right conditions, the balloon becomes buoyant enough that it detaches from the seafloor and rises through the water column. Some are light enough to reach the sea surface, whereas others absorb water and sink back to the seafloor.
What Gas Propels the Balloons?
Lava balloons appeared intermittently during the Azores eruption, which continued until early 2001. At one point, researchers from Portugal’s Research Institute for Volcanology and Risk Assessment explored the eruption area with a remotely operated vehicle.
The team got lucky—they spotted some of these balloons as they rose and filmed them with the vehicle’s camera. Küppers and other researchers studied the footage and found gas bubbles emanating from the balloons that traveled through the water column.
“If that was water steam, in contact with water in the Azores at 20 degrees [Celsius], it would instantly quench, condense, and the bubble would implode, disappear,” he said. “These bubbles survive over several frames, and this is reason for me to believe that these bubbles are primarily filled with carbon dioxide.”
He suspects that the carbon dioxide originates from the magma but separates from the melted rock and accumulates below the lava interface, bulging it outward, perhaps triggering the process of lava balloon formation.
During a research expedition to the Azores in July 2016, Küppers and his colleagues studied the 1998 eruption site in detail. “We have observed many balloon fragments in a heap at the seafloor,” so lava balloons have been launching at the spot for quite some time, he said. “I call it making lava blisters.”
Küppers next hopes to quantify the conditions of blister and balloon formation: How much gas is needed to bulge the lava interface, drive the lava out, and make it detach from the seafloor? He is currently analyzing several hours of video footage of lava balloons and scrutinizing balloon texture in more detail.
—Lauren Lipuma (@Tenacious_She), Contributing Writer