Mauna Loa is one of Hawaii's five volcanoes. According to the United States Geological Survey, it has erupted repeatedly during the 20th century. Hawaiian volcanic eruptions are not explosive, like Mt. St. Helens, but instead feature large volumes of fast moving lava flows. The 1984 eruption was typical with floods of molten basalt streaming down the slopes of the 13,677-foot tall volcanic mountain.
Mauna Loa is the Hawaiian Island's iconic volcano, shooting streams, fountains and spattercones of glowing lava into the air. According to the United States Geological Survey, it's the world's biggest active volcano. Its last major eruption was in 1984. Mauna Loa is a shield volcano, meaning that it is composed almost entirely of hardened basaltic lava flows. It features a bubbling, red-hot summit caldera crater sitting atop a magma reservoir of molten rock melted by the heat of the Earth's core.
The 1984 Mauna Loa three-week-long eruption began at about 1:30 a.m. on March 25. It started in the southwest corner of the volcano's Moku'aweoweo summit crater. Inside of three hours, 80% of the crater was covered with lava, but it never breached the rim. The eruption then shifted to the opposite side of the caldera. Other high-volume basalt floods flowed out of rift zones on Mauna Loa's flanks. The eruption triggered landslides of basaltic rock on the volcanic mountain's shoulders and slopes.
The primary hazardous effect of Mauna Loa's eruption in 1984 was that of unrestrained high-volume lava flows. The walls of burning rock envelop and destroy anything in their way. The flows can burn houses down and cover the ground with a thick impenetrable layer of black rock. About 16 square miles of land were paved over by the 1984 flows. The fluid, high-velocity flows of 1984 stopped just four miles short of the coastal town of Hilo as residents packed to evacuate.
In 1984, in addition to flood basalts, Mauna Loa's caldera emitted a noxious mixture of fumes called volcanic smog (or "vog"). This smelly gas is a combination of sulphur dioxide, carbon dioxide and hydrogen sulphide. The vog reacted with atmospheric moisture, sunlight and oxygen to form acidic aerosols that contributed to the creation of acute acid rainfall. The aerosol pollutants caused probable lung degradation and threatened respiratory function, especially in children and asthmatics.