The 1984 eruption of Mauna Loa was a Hawaiian eruption in the U.S. state of Hawaii that lasted from March 25 to April 15, 1984. It ended a 9-year period of quiescence at the volcano and continued for 22 days, during which time lava flows and lava fountains issued from the summit caldera and fissures along the northeast and southwest rift zones. Although the lava threatened Hilo, the flow stopped before reaching the outskirts of town.
Precursors
Repeated deformation measurements showed that the summit area of Mauna Loa began to inflate shortly after a brief summit eruption on July 5–6, 1975. This was followed by a 3-year period of slowly increasing earthquake activity beneath the volcano that included a swarm of earthquakes deep in mid-September 1983. The earthquakes reached a maximum frequency just after a 6.6 magnitude earthquake took place beneath the southeast flank of Mauna Loa, in the Ka‘ōiki fault system, on November 16, 1983. Following the Ka‘ōiki earthquake, the number of earthquakes over 1.5 magnitude increased gradually as the time of eruption approached. The immediate precursors to the 1984 eruption consisted of an abrupt increase in small earthquakes and volcanic tremor recorded on seismic stations located near Moku‘āweoweo caldera. At 10:55 p.m. on March 24, small earthquakes began at a rate of 2–3 per minute. By 11:30 p.m., the seismic background increased, marking the onset of tremor. Just before 1:00 a.m. on March 25, the tremor amplitude increased to the point that the astronomical telescopes on Mauna Kea, to the northwest, could not be stabilized due to the constant ground vibration.
Eruption
At 1:25 a.m., a military satellite recorded a strong infrared signal from the summit of Mauna Loa, indicating that the eruption was underway. Within just a few minutes, people throughout Hawaii island were reporting an intense red glow above the volcano. Eruptive fissures migrated rapidly down the southwest rift zone to the elevation and across the southern half of Moku‘āweoweo. By 4:00 a.m., lava fountains extended across the northeast half of Moku‘āweoweo and into the upper reaches of the northeast rift zone. At 10:30 a.m., intense steam emissions began along a fracture long farther down the northeast rift zone between the elevation, but no eruptive fissure formed in this area. By mid-afternoon, eruptive activity began to decrease at the uppermost vents between. At 4:41 p.m., a new fissure opened up at the elevation. This fissure rapidly migrated both uprift and downrift, so that by 6:30 p.m., a line of lava fountains slightly longer than was active. Eventually the fissure system condensed to four centers of activity with fountains up to 50 m high. Four parallel flows moved down the northeast flank at speeds as fast as 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, these vents were feeding lava to a fast-moving flow that had advanced 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 to the prison. These flows, however, stopped advancing within 48 hours and never crossed the Powerline Road. geologists board a helicopter as large lava fountains erupt in the background during the 1984 eruption. Helicopters provided access to remote areas of the eruption and were essential for safety. The fast-moving flow advanced as a relatively narrow, channelized ‘a‘ā lava flow. Its rate of advance slowed as it moved downslope, but by March 29 the flow had moved to an elevation of. At this time, the flow front was about from the outskirts of Hilo. Smoke from burning vegetation, loud explosions caused by methane gas along the advancing flow front and the intense glow at night all contributed to a growing concern among Hilo's residents. Early in the morning of March 29, a levee along the lava channel broke at the elevation about upslope from the flow front. The lava was diverted into a new subparallel flow and flow E stagnated, temporarily relieving emergency-response officials and residents of Hilo. This new ‘a‘ā lava flow moved at a comparable rate to the earlier flow E, yet did not extend beyond flow E until April 4. Another significant levee breakout occurred on April 5, forming a third subparallel ‘a‘ā lava flow that moved downslope. Concurrently, lava output at the source vents diminished 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, which moved steadily higher and more distant from the outskirts of Hilo. By April 14, no active ‘a‘ā lava flows extended more than from the vents, and on April 15, the eruption ended.
