May 17, 1996
A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.
The Phreatic Eruption of 1924
This month marks the 72nd anniversary of the last large explosive
eruption of Kilauea 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.
Eruption 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.
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