In December 1935, the US Army Air Corps carried out the most unusual bombing mission of its entire history. This was no training flight–the pilots were carrying and delivering live bombs. But the target was not an enemy army, airfield, or fleet. The target was a volcano.
The “Big Island” of Hawaii is home to several active volcanoes, including Kilauea, Mauna Loa, and Hualalai. These have been erupting for thousands of years, causing extensive lava flows, and were, according to the legends of the Hawaiian natives, the home of Pele, the flame-haired goddess.
In September and October 1935, a series of earthquakes shook the island. This was usually an indication that an eruption was imminent, as underground masses of molten rock pushed upward towards the surface. Dr Thomas Jagger, director of the Hawaii Volcano Observatory, began to keep a close watch. On the morning of November 21, there was another large earthquake, and that afternoon red-hot lava began seeping out of cracks on Mauna Loa. The eruption had begun.
Hawaiian lava, because of its chemistry, tends to be fluid and thin; it flows easily and quickly, but also tends to cool rapidly. But as the lava flow cools on the outside, it forms a series of hollow “lava tubes” that can then insulate the molten rock inside from the air, allowing it to travel further. Much of the area around Mauna Loa was criss-crossed by these hollow lava tubes.
The 1935 eruption was rather placid until November 27, when a new volcanic vent opened on the north side of Mauna Loa. The new lava flow gathered into a natural bowl-shaped depression caused by earlier eruptions, where it formed a pool of molten rock. As more and more lava continued to flow, the level of this “lake of fire” continued to rise, until finally, on December 22, the lava overtopped the edges of the natural bowl which held it, and began flowing down the mountain again–directly towards the city of Hilo, some 20 miles away.
This presented a serious danger. Dr Jagger’s measurements indicated that the lava was advancing about 1.5 miles a day, channeling itself through the network of old lava tubes. At that rate, it would reach the city of 28,000 people by January 9, and destroy it completely.
Dr Jagger had already been conducting a series of experiments to prepare for such an eventuality. Since the lava was being channeled through the lava tubes, he hypothesized that, using a series of carefully-placed dynamite explosions, he may be able to collapse particular tubes, dam the flow of lava within them until it cooled, and redirect the molten rock away from specific areas. Using mules to haul TNT up the mountain, he had already successfully collapsed and sealed off old tubes with experimental explosions. Now, as Hilo was threatened by the lava heading its way, Dr Jagger made plans to try out his method to divert the flow away from the city.
He quickly realized, though, that his plan would not work; he simply did not have enough time to gather a sufficient amount of explosives, carry them up the mountain by mule, place them all in specific spots, and detonate them. But then Guido Giacometti, his colleague at the Volcano Observatory came up with an alternative–perhaps the US Army Air Corps bombers, based over on Oahu at Pearl Harbor, would be able to do the job?
Dr Jagger contacted the Army and explained his plan–by dropping aerial bombs on particular parts of the mountain, they might be able to divert the lava away from Hilo and save the city. The Army agreed to give it a try, and Lt Colonel George S Patton was placed in charge of the project. Patton assigned a flight of Keystone heavy bombers from the 23rd Bombardment Squadron and LB-6 light bombers from the 72nd Bombardment Squadron to the mission, and on the morning of December 26 they flew from their base at Luke Field to the auxiliary airfield at Hilo. One of the light bombers carried Dr Jagger over Mauna Loa on a reconnaissance flight, allowing him to select specific targets. That evening, he briefed the bomber crews on his plan.
At 8:30am the next day, the bombers set out on their mission. The Keystones would be loaded with two 600-pound bombs each; the light bombers would trail behind as observers, including Dr Jagger. They would return to Hilo, be refueled and reloaded, then go back to drop more bombs on the mountain.
While one bomb was a dud and failed to explode (it was found by National Park Service employees forty years later) most of the nineteen others were hits. Over the next few days, the flow of lava slowed, then stopped. Hilo had been saved. Dr Jagger would later enthusiastically report, “The smashing of the tunnel had cooled the oncoming liquid so that it dammed itself. This confirmed the theory that the bombing solidified the tunnel lava back into the heart of the mountain….There can be no question whatever that the bombing stopped the flow.” Newsreel footage at the time celebrated the Army flyers as heroes, and even today the insignia patch of the 23rd Bombardment Squadron depicts a rain of bombs falling on a volcano.
But given our current knowledge of vulcanology, it it pretty certain that Dr Jagger’s experiment actually did no good. His ideas were based on some assumptions about the chemistry of the rock that we now know are incorrect, and in any case the 600-pound bombs carried by the Air Corps were mere firecrackers compared to the flow of lava, and likely had no measurable effect on it. The lava had cooled and stopped all on its own.
But the US military never gave up on the idea. In early 1942, when Mauna Loa erupted again, the Air Corps sent a flight of B-18 bombers to drop some ordnance on the lava tubes, apparently without any noticeable effect. In 1974 the Air Force opined that perhaps what was needed were bigger bombs, so in 1975 and 1976, bombers dropped an experimental series of thirty-six 2,000 pound bombs on old lava tubes, to see what effect they could produce. Vulcanologists apparently pronounced the experiment a dud, and today nobody seriously proposes using explosions to put out volcanoes.
Hidden History 
Fascinating story, one that I’d not heard. You have to wonder how Patton felt about this assignment.