Mauna Loa’s 1984 eruption | March 25 to April 15

It’s been 30 years since Mauna Loa — the largest active volcano in the world — last erupted sending within 4.5 miles of Hilo.

The 1984 eruption of Mauna Loa ended a nine-year period of inactivity. The eruption began suddenly following a three-year period of slowly increasing earthquake activity beneath the volcano that included a swarm of earthquakes between 5 km and 13 km deep in mid-September 1983.

Before the 1984 event, Mauna Loa last erupted between July 5 and 6, 1975, according to the U.S. Geological Survey. Measurements showed the volcano began to inflate shortly after the eruption and continued until the March 1984 eruption.

At 10:55 p.m. on March 24, 1984, small earthquakes began at a rate of two to three per minute. By 11:30 p.m., the seismic background increased, marking the onset of tremor, according to the USGS.

At 1:25 a.m. March 25, 1984, a military satellite recorded a strong infrared signal from the summit of Mauna Loa, indicating that the eruption was underway, according to the USGS. Within minutes, people all over Hawaii Island were reporting an intense red glow above the volcano.

The eruptive fissures migrated rapidly down the southwest rift zone to the 12,750-foot elevation and across the southern half of Mokuaweoweo. By 4 a.m., lava fountains extended across the northeast half of Mokuaweoweo and into the upper reaches of the northeast rift zone.

By 7 a.m., new lava fountains were erupting on the northeast rift zone between the 12,400- and 12,125-foot elevation. At 9:10 a.m., a new fissure began erupting 4.3 miles east at an elevation of 11,185 feet. This fissure propagated up and down the rift zone, and by 9:30 a.m., the curtain of fountains was more than a mile long. The fountains were 32 to 164 feet high.

By mid-afternoon, eruptive activity began to decrease at the uppermost vents.

At 4:41 p.m., a new fissure opened at the 9,350-foot elevation. This fissure rapidly migrated both uprift and downrift, so that by 6:30 p.m., a line of fountains about one-mile-long was active. Eventually the fissure system condensed to four centers of activity with fountains up to 164 feet high. Four flows moved down the northeast flank at speeds as fast as 300 to 700 feet per hour.

All vents uprift from these new ones quickly became inactive, and the eruptive activity was confined to these vents for the next three weeks. Six large vent structures eventually formed in this area around the active vents.

By daybreak on March 26, 1984, the vents were feeding lava to a fast-moving flow that had advanced 5.5 miles to the northeast and three less active, shorter flows that were advancing east toward Kulani Prison. The prison was put on alert, because the shorter lava flows were as close as 2 miles from the prison. The flows, however, stopped advancing within 48 hours and never crossed the Powerline Road.

The fast-moving flow advanced as a relatively narrow, channelized aa lava flow. By March 29, 1984, the flow had moved 15.5 miles to an elevation of 3,000 feet. At this time, the flow front was about 4 miles from the outskirts of Hilo.

Early in the morning of March 29, 1984, a levee along the lava channel broke at the 5,700-f0ot elevation and diverted lava into a new sub-parallel flow. The reduction in flow from the original channel temporarily relieved emergency-response officials and residents of Hilo. This new lava flow moved at a comparable rate, but did not extend beyond the original flow until April 4, 1984.

On April 5, 1984, another levee break formed a third sub-parallel aa lava flow that moved down slope. At the same time, lava output at the source vents decreased slightly, and the lava became more viscous, resulting in channel blockages and levee collapses that occurred more frequently. The collapses restricted the supply of lava to the flow fronts.

By April 14, no active aa lava flows extended more than 1.2 miles from the vents. On April 15, the eruption ended.