1942 Eruption of Mauna Loa

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14 minutes, 29 seconds

Historic Volcano Observatory Footage of 1942 eruption of Mauna Loa. The eruption of 1942 was noteworthy because volcanologist were able to predict the timing and place of the eruption based on geologic instrumentation.


On April 26, 1942, the summit crater Moku‘āweoweo on Mauna Loa began to erupt, and within less than a day its flow spread along the northeast rift. This eruption lasted for 14 days, covered over 12.3 square miles (34 sq km) of land, and became known as Mauna Loa’s “secret” eruption.

Large volcanic gas plume on the northeast flank of Mauna Loa.

Mauna Loa Eruption, 1942. Photo National Park Service, Hawai‘i Volcanoes National Park, Historic Photo Collection, Accession HAVO-554.

By 1939 most of the world was engulfed in war. Military impacts at Hawaiʻi Volcanoes National Park, formerly known as Hawaii National Park, started even before the U.S. involvement. The park was used for an Army headquarters, a bombing and training area, and a camp for Japanese internees. These events were only exasperated by another monumental event - the April 26, 1942 eruption of Mauna Loa.

Four months after Pearl Harbor was attacked, Mauna Loa began to erupt from its summit caldera Mokuʻāweoweo. A fissure erupted along its northeast flank producing fountains as high as 290 feet (90 meters) high. Joe Castro, an army medic from the 105th Infantry Regiment stationed in Hilo town wrote,

“My second night on the island the volcano suddenly erupted, the sky and everything lit up red everywhere, it was beautiful. Everyone was scared, not knowing what to do.”

- Joe Castro's account of witnessing the eruption just one night into his stay on Hawai‘i island.

The military government feared the Japanese would use the glow from the eruption as a beacon to guide their warplanes at night. Park Superintendent Wingate noted, “Almost everyone expected the Japanese to follow up on their Pearl Harbor successes by an invasion attempt which could easily have been effective." War-time restrictions continued with nightly curfew, blackouts, rationing, and censorship.

American officials declared the eruption a “secret” and did not allow the news media to cover it. In addition, the lava flows from the eruption threatened to engulf the water flume from Mauna Loa to the Army’s medical center and destroy ammunition depots in the town of Hilo. This would have had tremendous impacts on war operations on the island.

With Superintendent Wingate’s approval, Hawaiʻi District’s military governor Pennell made a decision to have the Army stop the advancing flow through targeted bombing. The Army Air Force, with limited success, deployed 16 bombs weighing between 300 to 600 pounds (136 - 272 kg) at selected target sites where lava tubes and channels fed the flows. Fourteen days after the eruption, lava flows crept to a stop almost seven miles away from Upper Waiākea Uka. Although the 1942 eruption was spectacular, it was also an ill-timed event.


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Last updated: April 25, 2022

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