Three thousand feet (900 meters) beneath the sea surface, in the middle of the Pacific Ocean, lies one of the world’s largest volcanoes, called Havre. The magnitude of its most recent eruption rivals some of the most memorable eruptions in recent history—for up to 90 days in 2012 lava poured out from a total of 14 vents around the semicircular volcanic opening. It is the largest deep-water eruption ever recorded in modern times, and scientists estimate that the amount of spewed lava, pumice, and ash was similar to what Mount St. Helens produced during its eruption of 1980.
Yet, despite its awesome size and power, the Havre eruption was almost overlooked. A passenger on a commercial flight happened to glance out her window at the right moment and saw an oddly colored patch on the ocean. She thought it might be an oil slick, or a floating pumice raft —a highly porous volcanic rock which is a telltale sign of a recent volcanic eruption. A quick email with an accompanying photo to a geologist prompted an international search for the culprit.
It was also a chance discovery. More than 70 percent of all volcanic eruptions occur underwater and scientists are in the dark when it comes to understanding underwater volcanoes because the eruptions are cloaked from view by thousands of feet of water.
In 2015, a team of scientists aboard the research vessel Roger Revelle sailed to the Havre eruption site 600 miles north of New Zealand with the mission of studying an underwater volcano relatively soon after it erupted. Through the lenses of cameras on the remotely operated vehicle Jason, an astonishing site was unveiled. Large chunks of volcanic pumice, some the size of Volkswagens, littered the seafloor, and a blanket of ash radiated out from the volcano summit. From several of the vents, lava had oozed out, sometimes piling up into domes. The scene was anything but expected.
“It blew our expectations completely out of the water,” says Rebecca Carey, a researcher at the University of Tasmania and co-author on a study that explored the Havre volcano from a remotely operated vehicle. “With any ocean floor investigation, there is room for some really exciting discoveries because we just don’t have the understanding yet of how submarine volcanoes operate underwater.”
At the onset of the expedition, the scientists expected to find layers of pumice and ash of a highly explosive eruption on the seafloor (like Krakatoa 1883 or Mount St Helens 1980). Instead, they found giant meter-sized boulders of pumice that suggest instead that foamy magma oozed out of the vent and broke up on contact with the seawater.
About 75 percent of the lava spewed from the Havre volcano made it to the surface and drifted away in the massive pumice raft first spotted by the passenger and also seen in satellite photos. It consisted of golf ball to soccer ball sized pieces that covered an area of the ocean the size of Philadelphia. The arrangement of the pumice boulders on the seafloor, sometimes precariously stacked four blocks high, suggested that these pieces were also at first buoyant but then gently sank as their pores became saturated with water. Other gas poor dense lavas flowed from the vents similar to lava that slowly flows out of the volcanoes of Hawaii. How is it that these different lava types form? Scientists still aren’t sure.
Carey explains, “In the water, things get really quite complicated.”
In any volcanic eruption, magma (molten rock beneath the Earth’s surface) rises from the depths of the Earth to the surface of the land or the seafloor. The magma contains dissolved gases, which form bubbles as the pressure on the magma is reduced during its ascent. An explosive eruption occurs on land when these dissolved gases are released suddenly—think of the bubbles in a coke bottle spurting out when a shaken bottle is opened and the pressure is released all at once. But underwater the magma still faces the crushing pressure of tons and tons of ocean water once it reaches the seafloor. The Havre volcano, stretching between 3,000 and 4,000 feet below sea level, experiences a pressure between 92 and 122 times that of sea level, which scientists suspect dampened its explosiveness and shaped the various types of lava formations.
Not only does pressure change how lava forms, the interaction between the water and the cooling magma is completely different than when magma interacts with air. When water hits hot magma at 800 degrees Celsius it vaporizes in an instant. Its rapid expansion into steam can be strong enough to break the lava apart. On the flip side, when magma comes in contact with water the temperature change is so dramatic that the magma instantly solidifies in a process called quenching.
Now the scientists’ mission is to determine how these various interactions played a part in forming the various types of lava produced by the Havre. Armed with a collection of samples from their expedition, they hope to figure out exactly what makes an underwater volcano tick.