Volcano Watch — Current lava flows contrast with violent eruptions of the past

The 10-year-long eruption along Kīlauea's East Rift Zone continues with little change. Active flows travel underground to the ocean in lava tubes from the eruptive vents on the south and west sides of the Pu`u `O`o cone.

There were three main active lava entries at the ocean in Hawai`i Volcanoes National Park. Through Thursday, the one at Lae`apuki and others east and west of Kamoamoa were all active. On Thursday afternoon, the west lava entry at Kamoamoa and the one at Lae`apuki had slowed significantly, and surface flows had broken out of the tube at numerous locations upslope. These changes appear to reflect small changes in the tube system rather than any change in eruption rate. That rate continues to be about 300,000 cubic meters per day, as it has been since early February.

Kīlauea's eruptions are not always as passive as the current one. Even this eruptive sequence, beginning in January 1983, had numerous periods of high fountaining in contrast to the passive effusion of lava we see today. Other eruptions have been even more explosive. The last of these explosive eruptions took place at this time of year in 1924. The sequence of events that led up to the explosive eruption at Kīlauea's summit in May 1924 is as follows. 

At the beginning of the year, a large and extremely active lava lake filled Halema`uma`u to within 105 feet of the rim. However, the lake level dropped to 370 feet during February and stayed there through March. In April, an enormous earthquake swarm started near the summit and migrated downrift towards Kapoho. 

On April 22 and 23, 200 earthquakes were felt at Kapoho. Fissures opened near Kapoho, and sections sank as much as 14 feet near the coast. No eruption occurred on land, but the sinking ground indicates that magmamigrated farther downrift and presumably erupted along the submarine extension of the East Rift Zone. 

At the summit, the seismographs recorded the ground tremor characteristic of underground magma movement, and, on April 29, the surface of Halema`uma`u once again began to subside. By May 6, the floor of the crater was 600 feet below the rim. 

Beginning on May 11, small explosions hurled rocks from the Halema`uma`u pit. On May 13, five much larger steam explosions blew rocks about 2,500 feet into the air. The situation deteriorated rapidly with larger and larger explosions, until by May 18, ash clouds above the summit reached heights of four miles. [The two photographs, which show an earlier small plume and a dark ash-laden plume from the climax on May 18, are from postal cards sold after the eruption was over.]* Large lava blocks were tossed from Halema`uma`u; many of these blocks still litter the surface near the Halema`uma`u parking area. 

One of the blocks struck a photographer who had approached too close to the crater; he died before he could reach the hospital. This was the last eruption-related death at Kīlauea until a visitor to the National Park was killed two weeks ago, when the lava bench he was standing on next to the lava entry at the ocean collapsed and carried him into the ocean.

As the 1924 explosions continued, the Halema`uma`u pit doubled in diameter from 1,400 feet to 3,000 feet. After the last explosion, on May 24, the Halema`uma`u pit was 1,300 feet deep. Its floor consisted entirely of rockfall debris. Roughly 750,000 cubic meters of rock was excavated from Halema`uma`u, but this amount represented only one percent of the volume lost from the summit. The rest resulted from collapse of the entire summit area as magma withdrew and flowed underground into the rift zone, past Kapoho, and presumably erupted on the sea floor east of Kapoho.

These explosive eruptions threw out incandescent rock fragments, but no new magma was directly involved. The explosions were driven by steam formed from water (probably hydrothermal water that surrounds the zone of magma storage), which entered the conduit as magma migrated out of the summit region. As the water came in contact with the hot rock, it flashed to steam and resulted in the explosive eruptions. Such eruptions are termed "phreatic." Fortunately they occur infrequently at Kīlauea and are not known to have occurred on Mauna Loa. We study these sequences of events to understand how the volcano works. This increased understanding allows us to better predict what may occur before it actually happens.