On May 24, 1969, Hawaii’s Kīlauea volcano started to erupt—and continued erupting for an incredible 1,774 days. In the first year alone, fountains of lava, some of them nearly 2,000 feet high, spurted out of the volcano on 12 separate occasions. As Laura Geggel reports for Live Science, the United States Geological Survey recently shared an archival photo of a particularly unusual fountaining event, which saw a fiery jet of lava erupt into a symmetrical dome.
The photo, which the USGS posted on Twitter late last month, was taken in October of 1969, during the 10th fountaining episode of the Mauna Ulu eruption—so named because the eruption originated from the Mauna Ulu cone on Kīlauea’s east rift zone. The image shows a red dome of lava on the horizon of what appears to be a body of water. But the scene is actually set on a stretch of cooled lava, Signe Dean explains in Science Alert. The dome is about 65 feet high, but a second image taken at another time shows it grew even larger, possibly to a height of 246 feet.
Lava fountains erupt either from isolated vents in lava lakes, or from lava tubes that are penetrated by water, according to the USGS. The formation and expansion of gas bubbles in molten rock pushes powerful streams of lava into the air—typically in a haphazard fashion, with the fountains spurting every which way. It is rare, the USGS notes, for the fountain to take the shape of a dome, like the one seen at Mauna Ulu.
"The conditions had to be just right to form that dome fountain," Janet Babb, a geologist at the Hawaiian Volcano Observatory, tells Smithsonian.com.
The key to this unique phenomenon is the amount of pressure inside the volcanic vent. “There was enough pressure of the lava coming from the vent that it didn't just ooze out in a lava flow,” Babb explains, “but there wasn't enough pressure to blast it sky high like with the glorious lava fountains that we wish we could all see.”
In a 1979 report, the USGS wrote that the dome fountain appeared frequently during the October event, which "lasted for 74 hours, nearly twice as long as any other fountaining episode of the eruption." The report also notes that the dome had a mottled surface, caused by solidified crust getting mixed with liquid lava. As part of the dome slid away, experts could see a lava core inside, indicating that the dome was "not simply a large bubble."
Over the course of its nearly five-year eruption—with a short break at the end of 1971 and early in 1972—the Mauna Ulu event regaled geologists with many other spectacular sights. In June of 1969, a 772-foot lava fountain came bursting out of the volcano. In August of the same year, Mana Ulu sent streams of lava pouring into the volcano’s ‘Alae Crater in “cascades higher and wider than the American falls at Niagra,” the USGS writes.
At the time, the Mauna Ulu eruption was the largest and longest lasting volcanic event to occur on Kīlauea in more than 2,000 years. But it has since been surpassed by another cone on Kīlauea’s east rift zone: Pu'u 'Ō'ō, which has been erupting since 1983.
Editor's note April 12, 2018: This article was updated to include a comment from Janet Babb, a geologist at the Hawaiian Volcano Observatory.