Volcano Watch — The phreatic eruption of 1924

This month marks the 72nd anniversary of the last large explosive eruption of Kīlauea Volcano. The atypical "Hawaiian" eruption occurred in Halema'uma'u Crater when groundwater came into contact with hot rocks surrounding the magma. An intrusion drained magma away from the summit and caused a collapse within Halema'uma'u. This process created cracks within the conduit system, and subsurface water entering these cracks flashed into steam.

The rapid generation of a voluminous quantity of steam blasted the surrounding rocks and formed large, billowing clouds of pyroclastic material. Over a period of 13 days, a series of explosions sent ash columns four miles into the air and hurled eight-ton boulders half a mile away from the crater.

The year began with an active lava lake 165 feet below the rim in Halema'uma'u Crater. By February, the lake had drained away, and the floor of the crater sank 380 feet below the rim.

The subsidence at the summit was followed in March and April by an earthquake swarm that migrated away from the summit as magma intruded into the east rift zone. Ground cracks large enough for a cow to fall in (and did) appeared in the Kapoho area, and sections of the coastline sank as much as 14 feet.

The floor of Halema'uma'u sank again in late April, and, by May 6, had dropped 600 feet below the rim. Heavy rockfall and avalanches characterized this period of subsidence as the walls of Halema'uma'u were shaken loose by numerous earthquakes.

Small explosions began to occur in the crater on May 11, and by May 13, rocks were being hurled 2,000 feet into the air. A loud roar preceding the explosions could be heard two miles away. The eruption cloud produced electrical storms and little pea-sized mud balls, which formed when ash from the plume was moistened by steam. According to Thomas Jaggar, founder and Director of the Observatory, "Dust curdled into rain clouds and came down as mud balls."

The intensity of the explosions increased and peaked on May 18, when the largest ones occurred. A man was fatally injured by a falling boulder when he ventured too close to photograph the crater between bursts, despite warnings of an impending explosion. The explosions were periodic, rather than continuous, and were probably generated by the same process that produces geysers.

The steam explosions continued for 18 days. By the end of the explosive period, Halema'uma'u had doubled in diameter, from 1,400 feet to 3,000 feet, and the rock-strewn crater was 1,320 feet deep. (In comparison, the present floor of Halema'uma'u is only 300 feet from the rim.)

The rocks thrown out by the explosions were old lithic blocks from the walls of the crater, and not new volcanic material. The gases consisted of steam, rather than those characteristic of magma, indicating that the explosions were not magmatic in origin.

The debris from these explosions can still be seen strewn in a half-mile radius around Halema'uma'u Crater—visible reminders of the explosive force of Hawaii's usually gently effusive eruptions.

Volcano Activity Update

The eruption from flank vents on the western side of Pu'u 'O'o continues unabated. Lava enters the ocean primarily at two points—Kamokuna and Lae'apuki. The old ocean entries at Kamoamoa were down to a trickle. During the early morning hours of May 16, a six-acre block of the Kamoamoa bench slid into the ocean.

Surface flow activity was concentrated on the coastal plain inland of the Lae'apuki entry. No surface flows were observed above the 300-ft elevation.

There were no earthquakes reported felt during the past week.