Like most Hawaiian eruptions, the eruptive activity was immediately preceded by a swarm of earthquakes, followed by tremor. Mauna Loa (“Long Mountain”) began erupting at 6:20 pm on November 21, 1935.
The eruption started with a curtain of fountains near North Pit within the summit caldera, Mokuʻāweoweo. The vents migrated 2-miles down the northeast rift zone.
During the six days of the main event, fissures opened up along the northeast rift zone of the mountain, fountaining lava 200- to 300-feet into the air.
On November 26, the summit eruption died and the northeast rift activity was reduced to a single vent at the 11,400-foot elevation. A small vent also opened up further below on the north flank of the mountain at the 8,600-foot elevation. (USGS)
Lava flows from Mauna Loa were generally fast-moving and voluminous. Lava moved relentlessly at a rate of five-miles each day; it pooled up between Mauna Kea and Mauna Loa at about where the Saddle Road is situated.
The ponded lava eventually began to follow the lay of the land, a natural drainage … Then, things “got interesting.” Lava was heading directly toward Hilo. (USGS)
Dr. Thomas A Jaggar Jr, the government volcanologist, estimated that the flow would reach Hilo by January 9, 1936. He suggested using dynamite to collapse lava tubes near the source of the flow in order to stop or divert it.
Explosives were first suggested as a means to divert lava flows threatening Hilo during the eruption of 1881. However, Jaggar’s plan of mule teams hiking the explosives up the mountain would take far too long – the lava flows were moving a mile a day.
Guido Giacometti, a friend of Jaggar, had suggested using US Army Air Corps bombers to precisely deliver explosives. Jaggar agreed, and the call was made.
The US Army Air Corps approved, and the mission and plans to strategically bomb Mauna Loa were set into motion. Lieutenant Colonel George Smith Patton was called on to oversee the Army operation. (He’s the same Patton who would go on to WWII fame.)
Lava tubes are cooled and hardened outer crusts of lava which provide insulation for the faster-flowing, molten rock inside. Such a conduit enables lava to move faster and farther.
The theory was bombs would destroy the lava tubes, robbing lava of an easy transport channel and exposing more of the lava to the air, slowing and cooling it further. (BBC)
On December 26, 1935, six Keystone B-3A bombers of the 23d Bomb Squadron and four Keystone LB-6A light bombers from the 72d Bomb Squadron joined the rendezvous circle in the predawn darkness off Diamond Head, and then headed to Hilo.
Jaggar briefed the crews on the methods he had in mind to divert the lava flow. He then flew over the volcano to assess the flows and select the right points for bombing.
8:30 am, December 27, 1935, the first five bombers departed on the bombing mission. (A second flight of five aircraft was planned for the afternoon.) Each plane carried two 300-pound practice bombs (for practice and sighting,) as well as two 600-pound Mk I demolition bombs (355 pounds of TNT each.)
The bombers opened formation and fell into a huge circle for a follow-the-leader dummy run over the target area. They were flying at about 12,500-feet, not far above the 8,600-foot altitude of the volcano’s flows.
As the lead pilot tipped the control column forward for his run he lowered the wheels, so that by the time he neared the clump of koa trees which served as reference point his plane would be moving only a little faster than the 65-mph landing speed.
‘OK?’ he called to his bombardier as they began their climb after passing over the flow. Standard radio-voice procedure was unneeded. … ‘OK,’ the bombardier grunted. (Johnson)
Five of the twenty bombs struck molten lava directly, most of the others impacted solidified lava along the flow channel margins; one of them turned out a dud.
“Colonel William C Capp, a pilot who bombed the lower target, reported direct hits on the channel, observing a sheet of red, molten rock that was thrown up to about 200′ elevation and that flying debris made small holes in his lower wing.”
“Bombs that impacted on solidified, vesicular pāhoehoe along the flow margin produced craters averaging 6.7-m diameters and 2.0-m depth….” (Swopes)
“Pilots observed that several bombs collapsed thin lava tube roofs, although in no case was sufficient roof material imploded into the tube to cause blockage.”
Jagger wrote that “the violent release of lava, of gas and of hydrostatic pressures at the source robbed the lower flow of its substance, and of its heat.”
The lava stopped flowing on January 2, 1936. The effectiveness of the lava bombing is disputed by some volcanologist. (USGS)
Here’s a link to a video of the Army bombing runs in 1935. (Lots of information here from Army, USGS, hawaii-gov, 4GFC, Johnson, Lockwood & Torgerson, Swopes and This Day in Aviation History.)
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