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1935 Eruption Threatened Hilo

Aerial view by the Naval Air Service of the 1933 Mauna Loa eruption...
Aerial view by the Naval Air Service of the 1933 Mauna Loa eruption from a fissure across the rim and floor of Moku‘āweoweo Crater. (Public domain.)

On November 21, 1935, an eruption began in Mauna Loa's summit caldera and quickly migrated down its Northeast Rift Zone. On November 27, another vent erupted on the volcano's north flank, well outside the rift zone, sending flows of pāhoehoe lava (smooth, billowy, or ropey) into the saddle area between Mauna Loa and Mauna Kea. Lava flows then turned east toward Hilo, advancing a mile per day, which alarmed residents. In an attempt to divert the flows, then Lieutenant Colonel George S. Patton (later a famed general in World War II) was called on to oversee a U.S. Army operation, suggested by HVO's Thomas Jaggar and Ruy Finch, in which military planes dropped bombs near the eruptive vent on December 27. Jaggar thought the operation was a success, but because the eruption ended just 6 days later, the efficacy of disrupting lava channels with bombs or other explosives remains disputed.

See the table summarizing Mauna Loa activity over the past ~200 years here.