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This update current as of April 17, 1998. Eruption updates are posted monthly; more frequent updates will accompany drastic changes in activity or increased threat to residential areas.

[Previous eruption updates may be accessed through our archive index.]

A typical month for Episode 55

Episode 55 of Kilauea's east rift zone eruption continues. When viewed over a period of a month, the eruption shows several elements of the pattern that characterizes its long-term history.

On most days lava issues quietly from an area near the south flank of Pu`u `O`o and travels in tubes to the coast. The Pu`u `O`o cinder-and-spatter cone itself has been shrouded in fume lately, and we rarely see activity within the crater of the cone. The tube system is about 12 km in length, from the eruption site at the 2600-ft elevation to the ocean entry at sea level. Tube discharge has been roughly 300,000 to 400,000 cubic meters of lava per day.

Changes at the cone

Pu`u `O`o crater has a major vent within it, a feature we call crater vent. Additionally, three small pits have formed elsewhere across the crater floor. These pits periodically overflow and spill small lava flows across the crater floor. All these flows remain contained within the crater of Pu`u `O`o.

Aerial oblique view of Pu`u `O`o cinder-and-spatter cone, looking west-southwest. Visible in foreground are crater overflows that occurred chiefly in 1997. Main crater floor lies 30-40 m below the east rim. The feature known as "crater vent" is the heavily fuming area in upper left of crater floor; it forms a shallow depression about 100 m long within the crater floor. Only scant fresh lava was visible in the crater vent when this photo (p0910) was taken February 12, 1998. Distance across crater is about 200 m (left to right). Rain clouds and dense fuming have frustrated our attempts to gain a more recent photograph of the cone,

Within the past week a new collapse pit about 50 m in diameter has formed on the outer south flank of Pu`u `O`o. This pit and an adjacent pit 70 m to the southwest are adjacent to a large collapse pit that formed in December 1997. Pits such as these form by undermining. Magma that issues from beneath Pu`u `O`o is able to erode blocks from its conduit and drag them into the tube system. Where the conduit is shallow, collapse of the ground surface occurs.

South flank of Pu`u `O`o cinder-and-spatter cone, showing newly formed pits. Right-hand pit is about 50 m in diameter. A larger pit that formed in December 1997 is obscured by fume.

A visit to the surf zone

At the coast, the tube system discharges lava at two sites, Waha`ula and Kamokuna. In past months these two sites received roughly equal rates of lava supply, but lately the Kamokuna site has been taking the lion's share of the tube-bound flow.

As the lava enters the sea it builds new land along the ocean edge. This land proves unstable, owing to the steep submarine slope along the south coast of the Big Island. The lava builds a low shelf known as a bench, but periodically the bench and its underpinnings slide seaward, a process called bench collapse. These collapses are life endangering; the land itself is destroyed and numerous explosions ensue as the hot lava reacts violently with the ocean water.

Kamokuna bench, looking west. Area of bench extant prior to collapse in mid-April lies between white line and sea cliff. Bicuspate shape results from two entry points building continguous benches. Combined bench length was about 400 m; greatest breadth about 80 m. Photo p01306, taken Thursday, April 16, 1998.

The Kamokuna bench underwent such a collapse on Monday or early Tuesday, April 13 or 14. The photo shows the approximate extent of bench that existed seaward of the cliff prior to collapse; area of destroyed bench was 1.5 hectares (3.6 acres).

Blockage and surge

On Saturday, April 4, magma supply to the eruption site was obstructed. Consequently, the tubes drained of their lava and the plumes at the coastal entry dissipated. Coincident with the drainout, the summit area of Kilauea inflated gradually, gaining about 3 microradians of tilt as measured at the Uwekahuna tiltmeter site. (Tilt calculation involves both east-west and north-south components; only north-south tilt record shown in accompanying figure.)

Tiltmeter records from Uwekahuna vault, showing gradual summit inflation. Inflation ended roughly coincident with M-4.1 earthquake at 12:43 a.m. Sunday, April 5. Surge of magma from Pu`u `O`o vent overwhelmed tube system and led to new surface flows on flow field.

A magnitude-4.1 earthquake from the upper east rift zone at 12:43 a.m. Sunday morning, April 5, marked the end of summit inflation. Magma stored temporarily then moved down the east rift zone in an amount sufficient to overwhelm the tube system. New lava flows issued from skylights at the top of Pulama pali and were active until about noon Sunday. By Monday, April 6, the tube system had returned to normal, and lava was again entering the sea at the Waha`ula and Kamokuna sites.

These short-lived pauses of magma supply to the vent site occur irregularly. Sixteen such pauses have occurred since episode 55 began in March 1997.

Eruption-viewing opportunities change constantly, so those readers planning a visit to the volcano should contact Hawai`i Volcanoes National Park for the most current eruption information (ph. 808-985-6000). Additional photographs and descriptions of east rift eruptive activity may be found on the University of Hawai`i's web site.

Flow-field map showing lava emplaced since Jan. 14 (red), other lava of episode 55 (orange), and lava from previous episodes of the 15-year-long ongoing eruption on Kilauea Volcano's east rift zone. This map was compiled on February 20, 1998; little change has occurred since then.

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Updated: 17 April 1998