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April 23, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Shhh! Don't tell there is an eruption - Eruption of 1942

The eruption of 1942 was noteworthy for several reasons. (1) It was declared a secret so the press was not allowed to publicize the event. (2) This was the second time lava diversion was tried on an eruption of Mauna Loa. (3) The volcanologists were able to predict the timing and place of the eruption.

World War II was four months old when the eruption began. Hawai`i was observing war-time restrictions that imposed a night-time blackout. Pele was having none of this, and the eruptions began on April 26, 1942, at 5:05 p.m. Government officials felt that advertising the eruption would permit the Japanese military to use the bright glow as a means to guide their warplanes under cover of darkness to wreak havoc upon Hawai`i.

The eruption, heralded by three months of elevated seismic activity, started when fissures opened along the western rim of Moku`aweoweo, and lava cascaded into the caldera. By early morning April 27, the summit activity began to wane. Eventually, seismic activity migrated toward the northeast rift zone. New fissures then opened at the junction between the summit caldera and northeast rift on April 27. Here the dike stopped advancing at the 3,658-m (12,000 ft) level. A line of spatter ramparts issued an `a`a flow that traveled in a north-northeasterly direction 7 km (4 mi) away from the rift. all this activity had ended by the early morning of April 28.

However On April 28, activity jumped far down the northeast rift zone. This new vent system opened at 4:40 a.m., creating a 1-km-long (0.6 mi) fissure at an elevation of 2,850 m (9,350 ft), 14.5 km (9 mi) from Mauna Loa's summit. Lava fountains propelled 70 to 90 meters (230 to 295 ft) high, producing small flows. Meanwhile the summit activity continued to fade as the flank activity expanded. Eventually, the summit activity ceased altogether.

By 3 p.m., the 1-km-long (0.6 mi) fissure lengthened an additional 0.5 km (0.3 mi). Fountains then were visible only from the distal ends of the fissure system; the central vents were drowned by lava moving in from the margins.

At 8 p.m., the eruption appeared to have begun in earnest as lava fountains reached heights of 150 m (500 ft). The fountains fed a rapidly moving `a`a lava flow headed in the general direction of Waiakea Uka. Gradually, eruptive activity condensed to a few spatter ramparts at the center of this fissure system. These vents were responsible for the bulk of the lava production.

A few hours later, the dike propagated another 4.6 km (2.9 mi) northeast. Here the dike changed orientation from northeast to north. Lava issuing from these lower elevation vents appeared to be gas-poor and more dense in character. The character of the lava caused scientists to speculate that these "new vents" were really sites where upslope lava entered an old tube or fracture system, only to reemerge at this lower elevation.

On May 1, the flow front was moving at 100 to 150 m (328 to 492 ft) per hour in the direction of Waiakea Uka. The advancing flow threatened the `Ola`a flume, Mountain View's water source, and would ultimately endanger the circum-island road. It was at this time that the decision was made to utilize aerial bombing to influence the course of the lava flows.

Aerial bombing was selected as the most efficient means of emplacing explosives at the optimal sites. The points of contact were lava tubes and channels that fed the flow front. The Army Air Force conducted the bombing missions and selected the sites. In all, 16 bombs weighing between 300 to 600 pounds each were dropped. Though most appeared to hit their mark, the bombs had little or no impact on the eruption or on the flow's direction.

On May 4, mauka activity became more restricted to a single, large cone edifice with vents producing lava at its center. The cone grew to a height of 30 meters (100 ft) and lava fountains stretched to heights of 200 m (650 ft). Later in the day, lava flows accomplished what bombs could not. These lava flows caused the central cone to collapse, resulting in the creation of secondary flows. These secondary flows diverted the lava from the flow front, slowing its advance, and resulted in the stagnation of the main flow on May 7. When the eruption ended on May 9, the lava flow extended 26 km from the main vents at 2,850 m (9,350 ft) and came within 11 km (6.8 mi) of Upper Waiakea Uka.

Eruption and Earthquake Update

Activity within Pu`u `O`o increased during the past week, and lava covered the floor of the crater. The light from the lava cast a bright glow onto the fume clouds. Lava continued to flow through a network of tubes down to the seacoast, where it entered the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

There were no earthquakes reported felt during the past week.

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Updated: 27 April 1998