1984 Eruption of Mauna Loa

The latest eruption of Mauna Loa— the last before its longest period of quiet in recorded history— began in March 1984. In dramatic fashion, lava descended to the doorstep of Hilo, the island's population center.

 
Volcanic fissure fountaining lava
Fountaining lava on Mauna Loa, March 26th, 1984 (USGS/JD Griggs)
 
 
Map showing lava flows from 1984 eruption of Mauna Loa
Map showing lava flows from 1984 eruption of Mauna Loa from the USGS Hawaiian Volcano Observatory

Early in the morning of March 25, 1984, swarms of earthquakes underneath Mauna Loa began to swell. The tremors soon became so constant that nearby telescopes on Mauna Kea could not be stabilized due to continuous vibrations. By 1:30 a.m., satellites picked up indications of an eruption and people around the island reported seeing a glow above the volcano.

The 1984 eruption, like most on Mauna Loa, started in the Moku‘āweoweo summit caldera (flow "A" on the USGS map) and soon expanded to the upper reaches of the northeast rift zone (flow "B").

 

By late afternoon on that first day, the fissure that would become the eruption's main vent opened up at 9,350 feet in elevation (2,850 m). Within hours, it expanded both mauka (toward the mountain) and makai (toward the ocean), creating a curtain of fire, a solid line of lava fountains over a mile long.

The fountaining later condensed into several areas of activity, with fountains reaching heights of 165 feet (50 m). On the eruption's second day, March 26th, flows channelized into fast-moving rivers of lava that came within two miles of Kulani Prison. The flow eventually slowed and spared the prison, but by March 29th it was encroaching on the city of Hilo. The lava was now at an elevation of 3,000 feet and only four miles away from the city. It had moved nearly 16 miles in four days.

 
Glowing lava flow descending to city at night
Lava flows loom above Hilo on April 4th, 1984, as seen from near the Hilo airport. (Photo by David Little)

The lava was visible from Hilo, unnerving citizens who were all too aware that the city itself is built on comparatively young lava flows. An 1881 eruption of Mauna Loa had come within 1.1 miles of the city's bayfront. Two other eruptions had also threatened the site of the city in the last millenium, so the danger was by no means unprecedented.

In the early days of April, parallel flows broke out (flow "F"), lessening the momentum of the flow towards Hilo. The output of the vents also slowed. As lava supply diminished, the active flow fronts retreated closer and closer to the vents. The last full day of eruption was April 13th. By April 15th, the eruption was officially declared over. The forces of Pele had spared the city of Hilo yet again.

 

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Last updated: March 11, 2021

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