Volcano Watch — The Mauna Loa eruption of 1926

April 10th marks the aniversary of the 1926 eruption of Mauna Loa Volcano. The eruption was preceded by several widely felt earthquakes about one hour before the outbreak of lava, and volcanic tremor was recorded on the few seismigraphs installed on the island about a half hour before the felt earthquakes. 

This eruption, like most Mauna Loa eruptions, began at the summit, but activity then migrated down one of Mauna Loa's two rift zones. In the case of the 1926 eruption, the activity migrated down the southwest rift zone. The initial fissure extended three miles downrift and fed short, silvery pahoehoe flows, mainly towards the southwest above Hilea.

This early phase of the eruption was witnessed by Edward Wingate, a topographical engineer mapping the volcano for the U.S. Geological Survey.

Three days after the start of the eruption, a swarm of earthquakes, including a particularly sharp one, heralded the further migration of activity down the southwest rift zone. On April 14, three main vents formed near 8,000 feet elevation, with the lowest at about 7,400 feet, just east of the source vents for the 1919 eruption. The fissure was several miles long and located on the eastern side of the rift zone; it appeared that the massive 'a'a flows would head towards Waiohinu.

The Kahuku flow advanced slowly towards Waiohinu until about April 24. However, the main flow rapidly advanced westward towards the ocean at Hoopuloa, where it destoyed the small village (12 houses and a church) and harbor on April 18. That flow had stopped by the 19th, and another parallel flow was advancing along the southern edge of the first flow, fed by the eruption along an active fissure still five to six miles long. The entire eruption was apparently over by April 26th, although fume continued to spew from the lower vents until about May 4th. About 13.4 square miles of land were covered by the roughly 150 million cubic yards of lava erupted.

This eruption was one of many on Mauna Loa during the first half of this century and the last half of last century. Lava from eruptions along Mauna Loa's southwest rift reached the ocean in 1868, 1877, 1919, and 1950, in addition to the 1926 eruption outlined here.

Other eruptions in 1916 and 1907 threatened south Kona, and the 1907 flow cut the road. Each of these eruptions, if repeated today, would cause severe disruption of life in south Kona, as transportation and communications would be impaired for an extended period. Such disruptions would be added to any direct damages caused by the flows.

This region of the island has steep slopes which cause rapid advance of the flows. In the case of the 1926 flows, advance of the flows to the ocean took about four days, but one lobe of the 1950 flows reached the ocean within about three hours. Such rapid advance of flows makes eruptions along Mauna Loa's southwest rift particularly dangerous, especially if the flows head towards the west.

Most eruptions of Mauna Loa begin voluminously along fissures, followed by less voluminous activity that forms a few distinct vents along the fissure, and finally by a declining low-volume phase. The initial voluminous phase of eruptions, coupled with the steep slopes in south Kona, makes for an especially dangerous situation. Fortunately, the migration of the eruptive fissures down the rift zone takes a day or more and is accompanied by numerous earthquakes. How far the fissure will finally extend during this early phase as the fissure extends downrift, is not known.

Today, we monitor Mauna Loa with an array of seismometers and take frequent measurements of the change in shape of the surface caused by storage of magma inside the volcano. Magma has been accumulating since the last eruption in 1984. Estimates of the volume of magma that has been stored now exceed the amount stored between the 1975 and the 1984 eruptions but are still less than the volume of lava erupted in 1984. 

Both of the last eruptions of Mauna Loa were preceded by increased seismic activity for roughly one year; that we have not yet begun to see any increase in seismic activity, suggests that any future eruption is most likely at least a year away.

However, the periods of repose between eruptions have exceeded 7.5 years only four times in the historic period: in 1843, 1975, 1984, and the present. Of these, only the 25-year period of repose between 1950 and 1975 exceeds the current repose period. We have expanded our monitoring efforts on Mauna Loa in the past few years and will continue to closely track any changes that will announce the next eruption there.