Volcano Watch — Mauna Loa poses most serious threat to South Kona

This week marks the anniversary of one of the largest of Mauna Loa's historic eruptions. On June 1, 1950, the last great eruption of Mauna Loa began just after 9:00 p.m. The volume of lava produced was nearly three times that of the 1984 eruption. In all, about 600,000 cubic yards of lava were erupted in 23 days, with about one-third to one-half of the volume being erupted in the first 36 hours. In contrast, the previous great eruption, which occurred in 1859, took nearly 10 months to produce a similar total volume.

At 9:04 p.m., seismographs began to record tremor, a harmonic vibration of the ground associated with magmamovement. The first fissures opened on the southwest rift zone at an elevation of about 12,600 feet, but within 15 minutes, the fissures extended 2.5 miles toward the southwest along the rift zone. The initial fountains were several hundred feet tall and fed rapidly advancing flows that moved about five miles toward the west.

At 10:15 p.m., another series of fissures began erupting much lower down the rift zone, at an elevation of 8,250 feet. These fissures, about eight miles from the initial fissure, extended both up and down the rift zone, so that by 4:00 a.m. the following morning, the active fissure was eight miles long. The fountains were as high as 1,000 feet tall and fed a flood of lava that surged down the mountain toward both the south and the west. By dawn on June 2, the flow towards the south was 10 miles long, but the flow to the west advanced even faster. It had crossed the road and entered the ocean by 1:05 a.m. This flow had advanced 15 miles in little more than 2.5 hours, giving an average rate of advance of 5.8 miles per hour.

The flows were inactive by noon on June 2, but two new flows were streaming down the west side of the mountain. One of these reached the ocean about noon, whereas the third flow did not reach the ocean until 3:30 p.m. This final one became the one that continued to flow for the next three weeks.

The rapidity with which these flows advanced down the steep, west flank of Mauna Loa was startling, and it was fortunate that no one was killed or injured. The 1950 eruption occurred at a time when few houses existed in the south Kona area, so the damage was minimal. Today, these flows seem benign as we drive over them where they originally crossed the highway. However, the population has grown rapidly, and the next southwest rift eruption of Mauna Loa poses the most serious hazard from lava flows on Hawai`i, mainly because of the extremely short warning time for flows that advance so quickly.

Magma continues to accumulate inside Mauna Loa Volcano, causing the mountain to inflate like a balloon. This inflation has continued since the last eruption ended in 1984. We have very little instrumental record for Mauna Loa eruptions because only a few instruments existed in 1950, and the volcano has only erupted twice since, in 1975 and 1984.

This lack of background data makes Mauna Loa eruptions more difficult to predict than those of Kīlauea, which have been very active during the time period when instruments have been installed. The last two Mauna Loa eruptions were preceded by increased seismicity for period of at least one year. To date, the seismicity is unchanged from levels of the past nine years. We interpret this lack of change to indicate that we are at least one year from a Mauna Loa eruption. On the other hand, the continuing magma accumulation within the volcano indicated that the total volume of magma erupted in 1984 will have again accumulated in another five years. Based on these scant observations, we think it most likely that the next Mauna Loa eruption will be in one to five years.

In order to better monitor Mauna Loa, we have been adding seismic stations on the southwest rift and on the northwest flank on Mauna Loa. We are also determining some background gas emission levels to use as a base to evaluate whether the gas emissions increase before eruptions begin. Finally, we will be acquiring the instruments to define the deformation on Mauna Loa using satellite technology. We have been making these measurements for a number of years (on borrowed equipment).

These new Global Positioning System (GPS) instruments allow us to determine the location of any point within a few centimeters, using signals from a group of orbiting satellites. When Mauna Loa next erupts, we will have been collecting a dense data set to evaluate precursors to its eruptions for the first time.