Volcano Watch — 1940 was a momentous year for Mauna Loa - and for Thomas A. Jaggar

The 1940 eruption began around 11:00 p.m., H.s.t., on 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 Hawai`i National Park (later renamed Hawai`i 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 3:00 a.m., H.s.t., military planes from Hickam Field on O`ahu were on their way to Hawai`i 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 Moku`aweoweo (Mauna Loa's summit caldera), through the southwest caldera rim, to an area down the southwest flank of the volcano.

Within Moku`aweoweo, 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 `a`a 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 Moku`aweoweo 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 Moku`aweoweo.

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 Moku`aweoweo 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 August 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 Hawai`i.

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Volcano Activity Update

On Kīlauea's east rift zone, small breakouts remain active above Pulama pali and were visible from Kalapana by the beginning of the week. Despite the continued activity, the overall vigor of the surface flows seems lower than usual.

At Kīlauea's summit, a ponded, circulating lava surface deep within the collapse pit inset within the floor of Halema`uma`u Crater was visible much of the time via Webcam during the past week. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Two earthquakes beneath Hawai`i Island, both southwest of Waimea, were reported felt during the past week. A magnitude-2.1 earthquake occurred at 3:13 a.m., H.s.t., on Monday, March 29, 2010, and was located 6 km (4 miles) southwest of Waimea, at a depth of 8 km (5 miles). A magnitude-2.4 earthquake occurred at 1:59 p.m, on Tuesday, March 30, and was located 7 km (4 miles) southwest of Waimea, at a depth of 2 km (2 miles).