# Volcano Watch — The 1950 eruption of Mauna Loa: a nightmare that could reoccur

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On the night of June 1, 1950, after many residents of Hookena-mauka village in South Kona had already gone to bed, Mauna Loa began to erupt. Soon the roar of the lava fountains could be heard from Highway 11, 24 km (15 mi) away, as molten lava poured from fissures high on the volcano's southwest rift zone. In only three hours, an aa flow reached the highway and invaded the village.

The streets were lit by flames as lava consumed several houses and the post office. Thirty-five minutes later, the flow entered the ocean. By daybreak, lava flows had crossed Highway 11 in two places, cutting off the only escape route. The villagers all reached safety unharmed, but for some it was a close call.

Mauna Loa has erupted twice since 1950, with a one-day outbreak at the summit in 1975 and a three-week eruption on the northeast rift zone in April 1984.

Most of Mauna Loa's eruptions in the last 150 years began at vents near the summit. About half of these summit eruptions quickly developed into flank eruptions along one of two rift zones that extend down its northeast and southwest slopes. A few eruptions have also originated at isolated vents on the volcano's northern slope.

The 1984 eruption followed the typical pattern, beginning at the summit and quickly migrating down the northeast rift zone. Lava flows came within 6 km (4 mi) of the outskirts of Hilo before the eruption ended. This eruption paved 41 square kilometers (16 sq mi) of land with lava in just three weeks, whereas the ongoing eruption of Kīlauea that began in 1983 took three years to cover a comparable area. Fortunately, most of the property buried by lava in 1984 was uninhabited land owned by the state.

Eruptions on the southwest rift zone present a much greater threat to life and property. The slopes are steep, and residential areas extend from Highway 11 right up to the rift zone. Although the population has increased greatly since 1950, the two-lane highway remains the only escape route.

The good news is that our ability to monitor, and possibly forecast, the next eruption of Mauna Loa has been greatly enhanced by better instrumentation. Since 1984, the Hawaiian Volcano Observatory installed two more seismometers on the southwest rift zone. In the summer of 2000, three dilatometers, instruments that measure expansion and compression, were cemented into boreholes 130 m (425 ft) deep on Mauna Loa's flanks.

From 1975 through 1983, measurements of ground deformation near Mauna Loa's summit indicated slow but persistent inflation. For over a year prior to the 1984 eruption, the number of earthquakes beneath the summit of Mauna Loa gradually increased.

If Mauna Loa follows a similar pattern of deformation and seismicity before the next eruption, we will have a year or so of warning. Since the rate of inflation has slowed considerably over the past several years and the seismicity has not increased, we don't think that the next Mauna Loa eruption is right around the corner. But that doesn't mean we can forget about it. It means that if we act now, residents and county officials still have time to prepare for the inevitable. Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Puu Oo vent during the past week. Breakouts from the tube system occur above Pulama pali and feed multiple flows that are usually tubed over as they descend Pulama pali. Surface flows are seen throughout the coastal flats, and the most active area has been beyond the eastern boundary of Hawaii Volcanoes National Park near Kapaahu. Lava reentered the ocean on May 5 in an area east of Kupapa`u. As of May 10, lava was entering the ocean at three separate locations spanning about 450 meters (1,500 ft) of shoreline.

One earthquake was reported felt during the week ending on May 10. Residents of South Kohala felt an earthquake at 4:33 a.m. on the morning of May 9. The magnitude-3.5 earthquake was located 14 km (8.4 mi) southwest of Kawaihae at a depth of 21 km (12.6 mi).