Lava fountains erupt from a fissure in the southwestern part of Mokuaeoweo, Mauna Loa’s summit caldera, on April 11, 1940 (view looking to the south-southeast). Patches of white snow cling to the caldera walls as fluid pahoehoe lava flows spread across the caldera floor. (Photo courtesy of USGS)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

April 2010 marks the 70th anniversary of Mauna Loa’s third longest summit eruption in recorded history. The 134-day-long eruption in 1940 has been exceeded in duration only by summit eruptions in 1873–74 and in 1949, which lasted 560 days and 147 days, respectively.

The 1940 eruption began around 11 p.m., April 7, as indicated by increased volcanic tremor recorded on Hawaiian Volcano Observatory (HVO) seismographs. Half an hour later, the first glow at the summit of Mauna Loa was observed by people in Kona.

In Hilo, only a faint crimson glow could be seen through overcast skies, but from Hawaii National Park (later renamed Hawaii Volcanoes National Park), the entire western sky was illuminated by the eruption. A park resident also heard a distinct roar but dismissed it as the sound of a distant car motor.

By 3 a.m., military planes from Hickam Field on Oahu were on their way to Hawaii Island. Following a flight over Mauna Loa, the Army Air Corp fliers landed in Hilo and provided the first eyewitness accounts of the eruption, which they described as “spectacular.”

During the first few hours, lava fountains 20–60 m (65–200 ft) high erupted along a nearly continuous line of fissures about 5.5 km (3.5 mi) long. The fissures extended from near the center of Mokuaweoweo (Mauna Loa’s summit caldera), through the southwest caldera rim, to an area down the southwest flank of the volcano.

Within Mokuaweoweo, floods of pahoehoe lava rapidly covered more than two-thirds of the caldera floor and partially filled North Bay (now called North Pit). Fissures outside the caldera erupted aa flows that advanced downslope about 2 km (1.2 mi) and spilled into three pit craters along Mauna Loa’s upper southwest rift zone.

Volcanic gas emissions created a large column of fume that rose more than 3,050 m (10,000 ft) above the summit of Mauna Loa. The lava fountains also produced large quantities of Pele’s hair, thin strands of volcanic glass, which were carried by wind and distributed over the entire southern part of the island.

Soon after receiving reports of the eruption, park rangers began ascending Mauna Loa. By the time they reached Mokuaweoweo on the evening of April 8, active vents were restricted to a fissure in the southwestern part of the caldera and remained there for the rest of the eruption. Activity outside the caldera lasted less than a day.

Lava fountains along the active fissure built a row of cinder-and-spatter cones that gradually coalesced to form a single elongate cone more than 100 m (330 ft) high. In 1984, an eruptive fissure cut through, and lava flows surrounded, the 1940 cone, but it remains a prominent landmark on the floor of Mokuaweoweo.

The 1940 eruption increased in intensity in mid-April, when lava fountains occasionally blasted spatter, pumice, and cinder to heights of 275 m (900 ft). Throughout May, June, and July, the volcanic activity was intermittent, with periods of seeming quiet lava effusion followed by eruptive outbursts. Lava fountains rarely reached heights greater than 30 m (100 ft), but fluid lava continued to cover almost the entire caldera floor. By the end of the eruption, the average depth of 1940 lava flows in Mokuaweoweo was 14 m (45 ft).

Following a brief burst of activity in mid-August, Mauna Loa’s third longest summit eruption ended on the night of Aug. 18.

The 1940 Mauna Loa eruption is also noteworthy in that it was Thomas A. Jaggar’s last as HVO’s Director. He retired from HVO on July 31 but continued his study of Hawaiian volcanoes as a Research Associate in Volcanology at the University of Hawaii.