May 6, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Explosions from Halema`uma`u hot on heels of ground cracking in Puna in 1924

The shaking was finally over. Lower Puna returned to normal following the calamitous episode of ground cracking and subsidence in April 1924. But it was only the calm before the storm.

Lava had drained from Halema`uma`u Crater on February 21, resulting in a pit 115 m (380 ft) deep. The crater was unaffected by the Puna crisis, but its floor began to collapse on April 29, as soon as Puna stopped shaking. Large clouds rose from rock falls and mixed with fume to make viewing conditions dreadful. The falling rocks built piles of talus on the sinking floor. The crater deepened to more than 150 m (500 ft) on May 1 and nearly 210 m (700 ft) on May 7. The pace of sinking even increased during the next three days, judging from the huge dust clouds.

During the night of May 10-11, an explosion blew rocks out of the crater, the first at Kilauea since about AD 1790. One rock weighing 180 kg (400 lb.) was thrown 60 m (200 ft) from the rim, and another weighing 45 kg (100 lb.) about 225 m (750 ft). More small explosions took place on May 11-12, and observations showed that the bottom of Halema`uma`u continued to drop.

A series of violent explosions began on May 13 and lasted 10 days. A burst at 4:00 p.m. threw rocks 750 m (2500 ft) into the air; one weighing 135 kg (300 lb.) landed on a road 425 m (1400 ft) from the rim of the crater. By May 14 nearly all sectors of the caldera floor around Halema`uma`u were covered with rocks ejected from the crater. Avalanches and landslides continued tearing rocks from the crater wall. Numerous small earthquakes, felt particularly at Uwekahuna but also at Volcano House, accompanied the explosions and avalanches. Lightning flashes illuminated the eruption clouds, and thunder rolled across the summit area. Heavy rainstorms caused by the eruption were common.

The explosive intensity peaked on May 18, 1924. Explosion after explosion took place that day, with black cauliflower clouds of dust that reached more than 6,500 m (21,000 ft) high. One explosion, at 11:09 a.m., hurled a 10-ton block about 1,050 m (3,500 ft) from the center of the crater. This block is visible today along Crater Rim Drive southeast of Halema`uma`u. This particular explosion also took the life of a photographer standing about 550 m (1,800 ft) from the crater.

The explosive activity slowly tapered off and ended on May 27. By then Halema`uma`u was 1,035 by 915 m (3,400 by 3,000 ft) wide and 410 m (1,335 ft) deep---double its former width and more than triple its depth. The caldera floor near Halema`uma`u was littered with rocks and sand as much as 60 cm (24 inches) thick. You can see these deposits, as well as impact pits made by falling rocks, along the Halema`uma`u trail today.

Everything far downwind from the crater was coated in ash, commonly muddy. In Pahala, about 7 mm (1/4 inch) of fine ash accumulated---about 450 kg per hectare (400 lb. per acre) every 24 hours. In Wood Valley on May 25, fine sand showered down, stinging skin and funneling down wrinkles of clothing in continuous streams.

The explosions were driven by steam made by heating groundwater within the volcano. Groundwater entered the hot conduit below Halema`uma`u after lava withdrew. Rocks and sand falling into the deepening pit choked off the water and steam, allowing pressure to build. Alternating rock falls and explosions occurred in a pulsating rhythm that allowed HVO scientists to crudely anticipate the timing of the next explosion.

The events 75 years ago will be long remembered---not as flukes but as examples of what might occur the next time the floor of Halema`uma`u, or of the surrounding caldera, collapses substantially.

Eruption Update

There was a pause in eruptive activity from 1:00 p.m. on May 4 to 10:00 p.m. on May 5. Rapid inflationary tilt at the summit of Kilauea signaled the onset of the pause and indicated that magma was accumulating in the summit storage area. Field observations of no lava flowing in the tube system, on the surface, or into the ocean at Kamokuna confirmed the pause in activity. Resumption of activity was detected as a thermal anomaly by the GOES satellite on Wednesday night and confirmed by an HVO field crew on Thursday morning. Lava was flowing sluggishly in the old tube system, and two surface flows were sighted.

There was one felt earthquake for the week ending on May 6. Residents of Kurtistown and Glenwood reported feeling an earthquake shortly after midnight on Sunday, May 2. The 12:26 a.m. temblor (mag.-3.4) was located 18 km (10.8 mi) southeast of the summit of Kilauea Volcano at a depth of 4.5 km (2.7 mi).