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Volcano Watch: Mauna Loa rumbled as it awakened from 25-year slumber

Lava fountains up to 20 m (65 ft) high erupted from fissures on the north flank of Mauna Loa early Sunday morning, July 6, 1975. (Photo courtesy of USGS)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Shortly before midnight on July 5, 1975, a hiker sleeping in Puuulaula (Red Hill) Cabin on the northeast flank of Mauna Loa awoke and saw a faint red glow. Attributing it to the camp stove, he rolled over and went back to sleep. But within half an hour, all 10 hikers in the cabin, including Hawaii Volcanoes National Park Ranger Bill Fink, were wide awake, watching in awe as a red-orange glow lit the night sky.

After a 25-year-long slumber, Mauna Loa was erupting! Ranger Fink radioed Park Headquarters with the news, but Park officials, contacted earlier by the Hawaiian Volcano Observatory (HVO), were already aware of the eruption.

The first signs of Mauna Loa’s awakening were detected in 1974. Seismicity beneath the volcano’s summit increased in April and persisted throughout the year, with earthquakes numbering from many hundreds to more than 1,000 per day.

At the same time, geodetic monitoring networks revealed significant expansion across Mokuaweoweo, Mauna Loa’s summit caldera.

With heightened seismicity and summit inflation — both indicators of magma movement toward the surface — the potential for an eruption increased. So, HVO stepped up its surveillance of Mauna Loa and, by June 1975, had installed additional monitoring instruments on the volcano.

Surprisingly, in the week prior to the eruption, daily earthquake counts were relatively low. But around 10:50 p.m. Saturday, July 5, shallow seismicity beneath Mokuaweoweo increased sharply. Soon thereafter, harmonic tremor began and quickly increased to a level that obscured individual earthquakes on the seismic records.

At 11:18 p.m., the sustained high-amplitude seismic activity triggered alarms in the homes of two HVO scientists, who reached HVO in a matter of minutes. Realizing the imminent possibility of a Mauna Loa eruption, they immediately notified Park and Civil Defense authorities.

At 11:42 p.m., scientists at HVO noticed a small glow above Mauna Loa. In less than two minutes, a bright red-orange glow outlined the entire summit region, illuminating a fume cloud that rose thousands of meters (yards) above the volcano.

The source of the glow and fume — lava fountains within Mokuaweoweo — could not be seen by observers at HVO or at Puuulaula. For direct observation of the eruption, aerial reconnaissance was needed, and, by 1:50 a.m., July 6, HVO scientists were flying over the summit in a light plane.

Fissures erupting fountains of lava 20–40 m (65–130 ft) high extended across the length of Mokuaweoweo and about 1 km (0.6 mi) down the volcano’s southwest rift zone. Lava had flooded more than half the caldera floor and was cascading into three pit craters on the upper southwest rift zone.

At 2 a.m., as HVO scientists continued their aerial observations, eruptive fissures opened in Mauna Loa’s northeast rift zone. Voluminous aa flows fed by these fissures moved swiftly down the north and northeast flanks of the volcano, putting the Mauna Loa Observatory access road in jeopardy and downslope residents on alert.

By dawn, the caldera and southwest rift zone fissures had stopped erupting, but the northeast rift zone fissures remained active for another 12 hours. Aa lava eventually overran parts of the summit hiking and jeep trails but did not cut off the Mauna Loa Observatory road. Fortunately, the potentially hazardous flows stagnated and ultimately stopped long before reaching inhabited areas.

The eruption ended around 7:30 p.m. July 6 — less than 24 hours after it began — but for Park Ranger Fink, the drama wasn’t over. He spent an unexpectedly exciting Sunday night alone at Puuulaula, with small earthquakes rumbling through the cabin every few minutes. Except for anxiety and loss of sleep, he was unscathed and was picked up by helicopter the next day.

With the center of seismicity shifting to Mauna Loa’s northeast rift zone, HVO scientists and public safety officials remained on 24-hour watch, concerned that a possible lower-elevation eruption could threaten Hilo. Their worry lessened July 10, when seismic activity noticeably waned. By July 20, seismicity had returned to pre-eruption levels, and the summit had stopped inflating.

Mauna Loa slept once again, but only until March 1984, when it erupted for 22 days. Since then, Mauna Loa has been quiet, exceeding its 25-year-long slumber prior to the 1975 eruption.

On July 6, 1975, eruptive fissures opened along Mauna Loa’s northeast rift zone, feeding voluminous aa lava flows that quickly moved down the north flank of the volcano. (Photo courtesy of USGS)

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Sep 21, 2017 / 5:15 pm



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