Mauna Loa’s 1984 lava flow came within 4.5 miles of Hilo

Lava from Mauna Loa’s 1984 eruption pushed down the volcano’s northeast rift zone and stopped within 4.5 miles of Hilo, Hawaii’s second-largest city, before the flow ran out of momentum in early April. The U.S. Geological Survey’s Hawaiian Volcano Observatory has tracked the volcano ever since as one of the two most active in the world, and it now rates Mauna Loa’s threat potential as Very High.

The 1984 eruption remains the closest call Hilo has faced from Mauna Loa in more than a century, and it is still the event the observatory points to when explaining why the city sits inside the volcano’s hazard zone at all.

Lava Advanced Nearly 16 Miles in Four Days

The eruption began in the early hours of March 25, 1984, with a glow at Mauna Loa’s summit caldera, according to the National Park Service’s account of the eruption. Activity shifted within hours to a vent that opened at about 9,350 feet elevation on the northeast rift zone, and by the next day fast-moving channelized lava rivers were racing downslope, advancing nearly 16 miles in just four days. That speed is what turned a summit eruption into a threat to a populated coastline in under a week.

The flow’s approach toward Hilo unfolded over roughly three weeks before it stalled. A Volcano Watch retrospective by HVO geologist Katie Mulliken notes that the observatory had actually forecast, in June 1983, that an eruption could occur within the following year, an early example of the kind of long-range monitoring HVO now relies on routinely. Parallel breakouts near the vent in early April diverted lava away from the advancing front, and as the eruption’s supply diminished the flows retreated rather than pushing further toward the city.

The Flow Stopped Within 4.5 Miles of Hilo

According to the USGS’s current hazard page for Mauna Loa, the 1984 lava flow “came within 7.2 km (4.5 mi) of Hilo, the largest population center on the island,” before the eruption ended on April 15. It was not the closest Mauna Loa has ever come. An 1881 eruption sent a flow to within 1.1 miles of Hilo’s bayfront, close enough that residents at the time reportedly prepared to evacuate, a comparison the National Park Service’s history page uses to show how much variation exists between individual Mauna Loa eruptions.

Hilo’s exposure comes from geography as much as eruption size. The city sits at the base of Mauna Loa’s northeast rift zone, one of the volcano’s two main rift systems, which means any sufficiently large and fast-moving flow from that side of the mountain has a relatively direct path toward the coast.

The Volcano Has Erupted 34 Times Since 1843

Mauna Loa has erupted 34 times since its first documented historical eruption in 1843, according to the USGS’s frequently-asked-questions page on the volcano, averaging roughly one eruption every 3.5 years before 1950. The pace has slowed sharply since then: only three eruptions have occurred since 1950, in 1975, 1984 and most recently beginning November 27, 2022, when lava came within 1.7 miles of the Daniel K. Inouye Highway before that eruption, too, ended without reaching a population center. Over the past 150 years, new lava has covered roughly 14% of the volcano’s surface.

That eruption record is why the National Volcano Early Warning System places Mauna Loa in the Very High threat-potential category alongside a small number of other U.S. volcanoes, a ranking driven as much by the size of the population living on and around the mountain as by how often it erupts.

Monitoring Has Changed More Than the Volcano Has

What has changed most since 1984 is not Mauna Loa’s behavior but the observatory’s ability to watch it. HVO now runs a network of webcams that deliver near-real-time imagery of the remote summit and rift zones, produces detailed lava-flow maps from stitched thermal and visual overflight images within hours, and uses digital elevation models to forecast where a new flow is likely to travel, according to the Volcano Watch retrospective. A notification service also emails alerts directly to the public when activity picks up, a tool that did not exist during the 1984 eruption.

HVO geologist Frank Trusdell, marking the eruption’s 25th anniversary in a USGS press release, put the volcano’s long-term outlook plainly: “Mauna Loa will erupt again, and there’s a good chance that it will be during [a resident’s] lifetime.” That same release stressed community education for newcomers to the island who may not know the mountain’s history, since Mauna Loa has erupted about once every six years on average over the past 3,000 years, far more often than its recent, quieter pace suggests. Whether the next flow stops as far from Hilo as the one in 1984 did is not something any monitoring network can promise in advance.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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