Volcano Watch - Mauna Loa Southwest Rift Zone's eruptions stopped traffic—and more

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During the past 148 years, Mauna Loa Southwest Rift Zone eruptions have sent massive lava flows across Hawai‘i Island's main road six times: in 1868, 1887, 1907, 1919, 1926, and 1950.

These flows destroyed villages, displacing residents; burned forests and ranch land, trapping cattle on isolated knolls; blocked roads, disrupting traffic; and torched telephone poles, severing communication.

Last week's "Volcano Watch" article (http://hvo.wr.usgs.gov/volcanowatch/view.php?id=335) focused on the 1868, 1887, and 1907 eruptions. This week, we describe the 1919, 1926, and 1950 lava flows, which were unlike any of the flows observed during the ongoing Kīlauea eruption.

On September 26, 1919, a vent high on Mauna Loa's Southwest Rift Zone erupted for just a few hours. Three days later, a breakout lower on the rift zone erupted fountains of lava up to120 m (400 ft) high and sent a river of lava down the volcano's forested slopes. Within about 20 hours, an ‘a‘ā flow several hundred meters (yards) wide crossed the circle-island "Government Road" (predecessor of Highway 11), burying the small village of ‘Ālika. This flow can be seen today at Highway 11, mile markers 90–91.

road cut through lava flow

Road cut through the 1919 Mauna Loa Southwest Rift Zone lava flow. 

(Public domain.)

The 1919 lava flow advanced 18 km (11 mi) in about 24 hours, reaching the sea north of Ho‘ōpūloa, where it poured into the ocean for 10 days. The eruption then slowly waned until November 5, when all activity ceased.

The 1926 eruption began on April 10 at the summit of Mauna Loa, but fissures soon migrated down the volcano's Southwest Rift Zone. By April 14, three main vents were sending huge ‘a‘ā flows downslope. Two days later, the main flow—4–6-m (15–20-ft) high, 150-m (500-ft) wide, advancing at 2 m (7 ft) per minute—crossed the road. This flow is visible today along Highway 11, at mile markers 87–88.

The 1926 flow thickened and widened as it rapidly advanced beyond the road toward the sea. On April 18, a 9–12-m (30–40-ft) high and 455–610-m (1500–2000-ft) wide ‘a‘ā flow plowed through the Ho‘ōpūloa village and harbor. The destruction was gradual, but complete. The eruption ended on April 26. Today you can see the extent of this flow from the coastal village of Miloli‘i.

lava flow front above village

The 1926 Mauna Loa lava flow overtopped the bluff at back of Ho‘ōpūloa and headed straight for the village. The ‘a‘ā flow front, estimated at 1,000 feet wide eventually widened to about 1,500 feet wide, and was 25-28 feet high.

(Public domain.)

After the 1926 eruption, Mauna Loa's Southwest Rift Zone was quiet for 24 years. That ended in 1950 with one of the volcano's largest historical eruptions.

On June 1, 1950, a 2.4-km- (1.5-mi-) long fissure began erupting high on Mauna Loa's Southwest Rift Zone around 9:00 p.m. Minutes later, the roar of lava fountains could be heard from Highway 11, up to 24 km (15 mi) away. Floods of lava streamed downslope from the rift zone.

Mauna Loa Southwest Rift Zone's eruptions stopped traffic—and more...

This rapidly moving ‘A‘ā lava flow was one of several that advanced down the west flank of Mauna Loa during the volcano's 1950 eruption. The massive flow, tens of meters (yards) high, traveled from the Southwest Rift Zone vent to the ocean, a distance of about 18 km (11 mi), in around 18 hours. Two earlier flows from this eruption reached the ocean in as little as three hours. All three flows crossed Highway 11 as they advanced to the sea. In this black-and-white aerial photo, incandescently hot areas on the flow appear white. Photo credit: Air National Guard, 199th Fighter Squadron.

(Public domain.)

As the fissure extended farther down the rift zone, several flows raced down the west flank of the volcano. Within about two hours, the first of these flows crossed the highway and inundated the village of Pāhoehoe. All villagers reached safety, but for some, who escaped with only the clothes on their backs, it was a close call.

map showing lava flow path and timing

The 1950 Mauna Loa flow crossed the road in three places. The first flow (Honokua) traveled from the Southwest Rift Zone vent to the ocean in about 3 hours, burying the village of pāhoehoe along the way.

(Public domain.)

Thirty-five minutes later, the flow entered the ocean, creating a steam cloud that rose 3,000 m (10,000 ft) into the air. It's noteworthy that, from vent to sea, this massive ‘a‘ā flow traveled a distance of 21 km (13 mi) in only about 3 hours. Two additional flows south of the first one reached the ocean in about 14 and 18 hours.

Before ending on June 23, the 1950 eruption destroyed nearly two dozen structures and cut Highway 11 in three places (visible today between mile markers 92 and 98), burying more than 1.6 km (1 mi) of the road.

If a Mauna Loa Southwest Rift Zone lava flow crossed Highway 11 today, the lives of thousands of residents would be significantly impacted—even if they do not live in the immediate path of the flow. At the very least, travel to homes, schools, and workplaces would be disrupted.

A Mauna Loa eruption is not imminent at this time, but the volcano will erupt again—and chances are that it will occur in your lifetime. So, now is the time to consider how you would cope with the disruptions caused by lava crossing the highway.

Questions about Mauna Loa are answered on the Hawaiian Volcano Observatory website at http://hvo.wr.usgs.gov/maunaloa/FAQ_Maunaloa/.

Volcano Activity Update

Kīlauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 27 m and 36 m (88–118 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, the June 27th lava flow may have stopped, but HVO scientists continue to watch for signs of activity. On the flanks of Pu‘u ‘Ō‘ō, only the eastern breakout remained active. As of June 8, this flow was 2.7 km (1.7 mi) long and continued to advance to the southeast. No lava flows were threatening nearby communities.

Mauna Loa is not erupting. A small earthquake swarm occurred beneath the volcano's west flank on May 28–31. Most of these earthquakes were about 5–8 km (3–5 mi) deep, and the largest magnitude was 2.8. This swarm is expected behavior and does not represent a significant change for Mauna Loa. Seismicity rates have since decreased, but are still elevated above long-term background levels. Deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone continues, with inflation recently occurring mainly in the southwestern part of Mauna Loa's magma storage complex.

No earthquakes were reported felt on the Island of Hawai‘i this past week.