Visitors to Hawai`i Volcanoes National Park (then Hawai`i National Park) pose on the rim of Kilauea Volcano's summit caldera about 2.5 km from an explosive eruption at Halema`uma`u Crater. During 18 days in May 1924, hundreds of steam explosions hurled hot shattered rocks weighing as much as 10 tons more than 1 km from the center of the crater. Dark billowing eruption columns (in distance) of ash and gas rose more than 3 km into the air, at times turning day into night at the town of Pahala, 30 km downwind.
The steam explosions occurred when groundwater flowed into the magma conduit that normally supplied lava to Halema`uma`u. This happened when magma emptied the conduit and moved into the east rift zone.
Before magma drained away, an active lava lake in Halema`uma`u Crater was about 50 m below the crater rim. By February 21, 1924, however, lava had left the crater, the floor of which had sunk to 115 m below the rim. Over the next two months, magma intruded into the lower east rift zone, culminating in mid April in a crisis of earthquakes and ground cracking near Kapoho, 50 km from Halema`uma`u (Volcano Watch for April 4, 1999).
The floor of Halema`uma`u began to sink again on April 29, and by May 7 the crater floor was about 210 m below the rim. Heavy rockfall accompanied this dramatic collapse of Halema`uma`u as he walls of crater were shaken loose by numerous earthquakes. Groundwater poured into the still-hot conduit and quickly heated to steam, but the rockfall debris formed a barrier that kept steam from escaping passively. Instead, steam pressure built up, eventually leading to explosions, starting on the night of May 10-11, that threw rocks and dust derived from the rockfall debris out of the crater. Similar explosions occurred episodically until May 27, peaking on May 18 (Volcano Watch for May 6, 1999).
Future explosive activity from the summit of Kilauea is likely to be preceded by similar earthquake activity and dramatic subsidence of the caldera floor or Halema`uma`u Crater.