Mauna Loa is a shield volcano so massive it covers half the Big Island and rises 13,679 feet above sea level, just slightly shorter than neighboring Mauna Kea. When it erupts, it produces voluminous, fast-moving lava flows rather than explosive blasts. The 2022 event was the volcano's 34th eruption since continuous recording began in 1843, but only its third since 1950. The warning signs built slowly through autumn: daily earthquake counts beneath the summit caldera, Moku'aweoweo, rose from a dozen or so to 40 or 50 by mid-September. The National Park Service closed the summit in early October. By late October, civil defense officials warned of a "state of heightened unrest" -- but still saw no signs of imminent eruption. The mountain, it turned out, was not finished building pressure.

At 11:30 p.m. on November 27, lava broke through the floor of the summit caldera. The aviation color code jumped from Yellow to Red. Within three hours, the glow was visible from Kailua-Kona, across the island. By 6:30 the next morning, the eruption had migrated to the Northeast Rift Zone, where four fissures opened in succession. Lava fountains along the fissures reached 100 feet. Spectators along Saddle Road noted whirlwinds spinning near the flows, which the National Weather Service identified not as tornadoes but as a kind of dust devil generated by intense surface heating -- a phenomenon seen in previous Hawaiian eruptions. On November 28 alone, sulfur dioxide emissions hit 250,000 tonnes per day, and USGS recorded at least 249 earthquakes above magnitude 1.2.

One of the eruption's most unexpected casualties was the Keeling Curve. A lava flow from the third fissure crossed the access road to Mauna Loa Observatory on the evening of November 28, cutting power to the instruments that have tracked atmospheric carbon dioxide since 1958. That dataset, one of the most important in climate science, went offline for the first time in its history. NOAA and the University of Hawaii scrambled to arrange backup measurements from the UH88 telescope facility on Mauna Kea while the observatory remained inaccessible. For scientists, the eruption gave with one hand and took with the other: the lava flow interrupted climate monitoring but offered a trove of fresh volcanic data from instruments far more sophisticated than those available during the last eruption in 1984.

As lava crept toward Saddle Road -- the Daniel K. Inouye Highway, the only route connecting the island's east and west sides -- Hawaii's governor activated 20 National Guard members and the Department of Transportation prepared to close the highway between mile markers 8.8 and 21. The closure never came. The lava traveled 16 miles from the fissures and stopped 1.7 miles short of the road on December 10, when the supply of magma to the fissure 3 vent simply cut off. Total damage to private infrastructure came to $1.5 million, with another $600,000 spent on traffic enforcement and the viewing area. No injuries or fatalities were recorded. The civil defense administrator called it "the best situation we could have asked for from Mauna Loa."

For Native Hawaiians, the eruption's timing on La Ku'oko'a was an event of cultural and spiritual resonance, connecting the volcanic forces of the land with a day already charged with meaning. Tourism surged as well: hotels sold out across the island, helicopter tour operators ran nonstop flights, and thousands of spectators clogged Saddle Road before authorities established a designated viewing area. An estimated 100,000 people visited that viewing area over the eruption's two-week span. By December 13, the Hawaiian Volcano Observatory downgraded the alert level, and both Mauna Loa and neighboring Kilauea fell quiet. Kilauea would resume erupting on January 5, 2023, but Mauna Loa had said what it needed to say -- in 200 to 250 million cubic meters of lava, and 13 days of fire.