# Nobody's Talking About the 800-pound Gorilla of Hawaiian Volcanoes

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March 25 marks the 25th anniversary of the most recent eruption of Mauna Loa volcano.

Mauna Loa 1984 summit eruption.

(Public domain.)

Although Kīlauea, Mauna Loa's younger and smaller sibling, gets all the attention these days, we would like to remind readers that Mauna Loa is still an active volcano. An eruption from Mauna Loa is more likely to pose a significant threat to life and property than a Kīlauea eruption. Furthermore, when this sleeping giant erupts, it has the capability of disrupting commerce island-wide.

Mauna Loa comprises slightly more than half of the surface area of the island. The volcano has erupted 33 times in the last 150 years, producing lava flows that have covered extensive areas on the flanks of the volcano and reached the ocean eight times along the south, west, and northwest coasts of the island.

From our fieldwork and research, we have found that all historical eruptive activity commenced with a fissure eruption in Mokuaweoweo, the summit caldera. The eruption may consist solely of this summit activity or may progress to the flank when magma intrudes into the rift zone and opens additional vents.

Preceding the outbreak in 1984, HVO observed 18 months of higher-than-average shallow- and intermediate-depth seismicity. The number of larger earthquakes increased in a steady progression as the eruption approached. Likewise, summit-crossing survey lines showing signs of extension and outward tilt indicated that magma was accumulating in the volcano. We were in a heightened state of alert preceding the eruption; yet, in the last days before the outbreak, there was no clear change in gas emissions and deformation.

Signs that an eruption was imminent came in the form of seismic precursors: a swarm of earthquakes (hundreds per hour) and volcanic tremor late in the evening of March 24. At 10:55 p.m., H.s.t., the earthquake flurry commenced followed a half hour later by the onset of tremor. The eruption began on March 25 at 1:30 a.m. with the opening of a fissure vent across the summit caldera.

By 4:00 a.m., the eruption migrated into the upper northeast rift zone at around the 12,000-ft (3,658-m) elevation, sending flows parallel to the rift. Concurrently, summit activity began to wane. The dike continued to propagate downrift to the 9,300-ft (2,835-m) elevation. Here the eruption stabilized, and this region became the focal point of the eruption.

Initially, three fast-moving flows advanced toward Kulani Correctional Facility. To the north, a fourth flow pirated lava from the "Kulani flows" as it swept rapidly down the mountain. It advanced 15 miles in 3 days, down to the 3,000-ft (914-m) elevation. Intense glow, fires, and occasional methane explosions contributed to Hilo's anxiety.

In the early hours of March 29, the first flow advancing to Hilo was thwarted by a channel breakout and the formation of a new flow north of, and parallel to, the first. This process repeated itself over and over in the next few weeks, sabotaging any chances that the flows would reach Hilo.

Thereafter, the lava production began to dwindle, and the lavas became pasty, resulting in more blockages and breakdown in supply to the flow fronts. Throughout April, the supply continued to decrease, and flow lengths correspondingly shortened. The eruption lasted for 22 days, ending on April 15.

Mauna Loa has great volcano-hazard potential for the Island of Hawaii. The population on the slopes of the volcano is growing rapidly increasing the risk of significant damage during the next eruption. In recognition of this fact, we are planning a series of public outreach events. The first event on March 17 and 24 is a pair of public "After Dark in the Park" lectures in Hawaii Volcanoes National Park,.

On March 28, in conjunction with our colleagues at the University of Hawaii at Hilo (UHH) and Hawaii County Civil Defense, we plan to hold an informational forum on the UHH campus at UCB100. Then, on April 4, we will be at the Konawaena Intermediate School cafeteria. Come join us in March and April to learn more about Mauna Loa: its history and current status. Watch your local papers or visit the HVO website.

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### Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halemaumau Crater is emitting elevated amounts of sulfur dioxide gas and producing small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the south part of Kīlauea caldera and produced occasional air quality alerts in more distant areas, such as Pahala and communities adjacent to Hawaii Volcanoes National Park, during kona wind periods. Views inside the vent early in the week with a thermal camera showed a small lava lake, approximately 100 yards below the vent rim, undergoing cycles of filling and drainback. Heavy fume precludes views of the lava lake with the naked eye.

Puu Ōō continues to produce sulfur dioxide at even higher rates than the vent in Halemaumau Crater. Trade winds tend to pool these emissions along the West Hawaii coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

Lava erupting from the Thanksgiving Eve Breakout (TEB) vent at the eastern base of Puu Ōō continues to flow to the ocean at Waikupanaha through a well-established lava tube. Breakouts from a western branch of the lava tube have been intermittently active on the coastal plain near the National Park boundary. The new entry fed by these flows at Wahaula ceased activity early in the week. Several deflation-inflation events at Kīlauea's summit over the past week have led to fluctuations in lava supply and brief episodes of bubble-bursting and other littoral explosions at Waikupanaha.

Be aware that active lava deltas can collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions. The Waikupanaha delta has collapsed many times over the last several months, with three of the collapses resulting in rock blasts that tossed television-sized rocks up onto the sea-cliff and threw fist-sized rocks more than 200 yards inland.

Do not approach the ocean entry or venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Call Hawaii County Civil Defense at 961-8093 for viewing hours.

Mauna Loa is not erupting. One earthquake was located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano, combined with slow eastward slippage of its east flank.

No earthquakes beneath Hawaii Island were reported felt within the past week.

Visit our Web site for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov. skip past bottom navigational bar