Aerial view of the pumice and cinder cone (center) built on Mauna Loa’s southwest caldera wall during the 1949 summit eruption. A blanket of light-brown pumice, erupted from high lava fountains and blown west of the cone, is still visible today (top). Lava flowing eastward from the fountains formed the dark lava cone east of the pumice and cinder cone (lower right). (Photo couresty of Hawaiian Volcano Observatory)

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

On the afternoon of Jan. 6, 1949, scientists at the Hawaiian Volcano Observatory, 20 miles from Mauna Loa’s summit, couldn’t see the top of the volcano due to heavy cloud cover but they heard deep rumbling sounds from it. 

Around the same time, island residents with clear views of Mauna Loa saw puffs of fume rising above the volcano’s summit. These sights and sounds signaled the beginning of Mauna Loa’s second longest summit eruption since 1874. 

Lava initially erupted from a series of fissures 3 miles long that extended across the southwest floor of Mokuaweoweo (Mauna Loa’s summit caldera), through the steep caldera wall, and down the  upper southwest rift zone. 

A segment of the caldera fissure also cut through the cinder cone formed during Mauna Loa’s 1940 summit eruption.   

During the first few hours, a nearly continuous line of lava fountains up to 100 feet high gushed from the entire length of fissures. By the morning of Jan. 7, only four short fissures remained active. 

Yet, within 24 hours of the eruption’s onset, most of the southern half of the caldera floor had been buried beneath new pahoehoe flows, leaving the 1940 cone protruding as an island above a sea of molten lava. 

Small fissures outside the caldera — on the uppermost part of Mauna Loa’s southwest rift zone — erupted fluid, fast-moving lava flows that advanced westward about 6 miles. This activity resulted in a short-lived evacuation of Honaunau residents Jan. 7. 

By Jan. 9, the eruption was confined to two lava fountains at the southwest edge of Mokuaweoweo. These fountains grew more vigorous over the next two weeks, reaching heights of more than 500 feet Jan. 23, with some lava bursts estimated to be more than 800 feet high. 

Frothy lava fragments ejected in the high fountains piled up against the caldera wall and on the caldera rim, creating a conspicuous cone of pumice and fine cinder. Much of the ejecta was also blown west of the cone, where it formed a blanket of pumice several yards thick, 0.5 mile wide, and 1 mile long. 

Lava flowing eastward from the fountains breached the east side of the cone, forming a pool of molten lava that fed open channels and lava tubes across the caldera floor. Some south-moving flows cascaded into South Pit, a crater adjoining the south side of the caldera. 

Lava gradually filled South Pit until it overflowed Jan. 25, spilling to the southeast. This flow advanced almost 5.5 miles down the volcano’s flank — to about 6 miles upslope of Wood Valley — before stalling Jan. 31. 

Direct and continuous observation of the eruption was hindered by the difficulty of reaching Mauna Loa’s summit at 13,680 feet above sea level, especially during harsh winter storms, but people flocked to the island to view it from afar. 

Swarms of planes circled Mokuaweoweo while hundreds of vehicles jammed roads leading to the volcano. There was even a special steamer excursion from Honolulu to allow passengers to view the eruption from the sea. 

Eruptive activity began to weaken in early February. As the fountains decreased in size, they erupted denser lava fragments, creating a small, double “conelet” of spatter and coarse cinder within the breached pumice cone. 

On Feb. 5, the lava fountains ceased, and, for a time, the eruption seemed to be over. However, fume rising from the summit was frequently observed through the rest of February and March. 

On several occasions, a distinct glow could also be seen, indicating that magma remained high in the conduit. 

Around April 1, the eruption resumed in Mokuaweoweo and continued through the end of May. During this period, continuous quiet effusion of lava built a steep-sided lava cone east of the pumice cone and sent sluggish aa flows across the eastern caldera floor and into South Pit. 

Exactly when the 1949 summit eruption ended is uncertain, but it’s estimated to be on June 1. If so, it lasted 147 days — second only to the 560-day-long summit eruption in 1873-74. 

To see Webcam images of Mauna Loa’s summit, visit http://hvo.wr.usgs.gov/cams/MLcam.