Mauna Loa - Historic Eruptions

Historic Eruptions

Although Ancient Hawaiians had witnessed eruptions for many centuries, written records exist only for eruptions that have occurred since the early 19th century. The first historical eruption occurred in 1832, and since then 32 eruptions have been documented.

The typical pattern is for an eruption to start at Mokuʻāweoweo, then move to a focal vent lower on one of the rift zones. Often, a major rift eruption is preceded by a smaller event that is limited to Mokuʻāweoweo, a year or more before the larger one. The latter occurred in 1880, 1940, and 1949, before the massive flank eruptions of 1881, 1942, and 1950. In total, these eruptions have covered over 310 square miles (800 km2) of the volcano's flanks with lava flows. Typically, eruptions have been brief but intense, with 0.06 to 0.12 cubic miles (0.25–0.5 km³) of lava erupted over a few weeks.

The most prominent historic eruptions of Mauna Loa, with extensive lava flows in forest and agricultural areas, were in 1855, 1881, 1935, 1942, and 1950 on the northeast rift zone; 1887, 1907, 1916, 1919, 1926, and 1950 on the southwest rift zone; and the 1859 radial vent eruption.

An eruption in 1935 that headed toward the city of Hilo led to an unusual employment of air power. Five bombers of the 23d and 72d Bombardment Squadrons of the United States Air Force dropped bombs in the path of the lava in order to divert it away from Hilo. The eruption stopped six days later. The first flow extended 24 kilometers (15 mi) from the vent at 2,800 meters (9,200 ft) elevation to the ocean in only three hours during the night, and a second flow before dawn cut off the escape route for people in the village of Hoʻokena. The eruption eventually focused on a flow further to the south, which also reached the ocean.

On March 25, 1984 an eruption started: a small summit eruption was followed by a larger flank lava flow that threatened Hilo. Fissures opened to the northwest and southeast, from the summit down to 9,500 feet (2,900 m) above sea level. Flows from this eruption headed rapidly towards Hilo again, but stopped about 2.5 miles (4.0 km) from the outskirts when the eruption ended after three weeks.

As of 2009, Mauna Loa has been inactive for over 25 years, its longest quiet period in recorded history.