In March 2020, a hiker exploring a lava tube on Mauna Loa stumbled across two unexploded bombs, dropped on the active volcano 85 years ago in a failed attempt to divert a lava flow.
One bomb was partially buried in the shattered ground. The other bomb was found protruding from the wall of a lava tube, a natural conduit formed by lava moving beneath the hardened surface of a lava flow.
In November 1935, the Mauna Loa, one of five volcanoes that form the Island of Hawaii, erupted, and a lava river was advancing towards Hilo, counting at the time 15,000 residents.
Volcanologist Thomas Jagger, the founder of the Hawaiian Volcano Observatory, proposed an unusual plan to stop the lava: Bomb the Volcano. More precisely, the plan was to bomb the walls of the lava channels, where the lava was flowing. By creating new openings in the channels it may be possible to redirect the lava away from Hilo.
On December 27, the United States Army Air Corps, stationed at the time in Oahu, sent ten bombers to Mauna Loa. Twenty 600-pound bombs, containing 300 pounds of TNT-explosive, were dropped on the volcano. Five bombs landed in the lava flow. The rest missed, landing hundreds of feet from the target.
Apart from problems with hitting the target, geologist Harold Stearns, on board of one of the planes, also noted that the used bombs were too weak to effectively break the massive walls made of solidified lava.
Jaggar himself watched the bombing through a telescope from the base of Mauna Kea. The effects of the few bombs that hit the lava were not very convincing. The craters created by the explosions were quickly filled by molten rock and the lava continued to advance.
Jagger argued that even if the bombing didn’t stop the flow, the explosions may slow it down. Geologist E.G. Wingate, his superior at the time, proposed to “attack the flow channel by dynamiting” and so be more precise and effective in the operation. This didn’t happen, as the lava flow stopped six days after the first bomb raid as the summit eruption continued to ease.
Despite the first unsatisfying results, bombs were tried on Mauna Loa a second time in 1942. This time the vents feeding the lava flow were targeted, but again the direct hits were few. Three days after the bombing, the vents partially collapsed by natural processes and caused the main lava flow to stop.
In 1975 and 1976 the U.S. Air Force dropped 2,000-pound bombs on old lava deposits to test the effectiveness of the devices. The bombs excavated craters up to 100 feet in diameter. Nowadays the largest ordinary device used by the Air Force is the Massive Ordnance Penetrator (MOP), a 30,000-pound bomb designed to crack underground bunkers. A relatively small nuclear device, like the atomic bomb that destroyed Hiroshima (equivalent to a 300,000,000-pound TNT bomb), would create a crater of almost 650 feet in size. However, no bomb of such magnitude was ever used against a volcano.
Despite the increased firepower, most geologists doubt that bombing a volcano would have any significant effects on its eruptive power. Even if the walls of an active lava channel are successfully hit and destroyed, the path that the lava will take remains uncertain.
The real problem is the full magma chamber of the volcano, continuously pushing out lava from the ground. Even a direct nuclear strike would not be powerful enough to reach and somehow obstruct this underground reservoir of molten rock.
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